New diseases or pathogens are rare these days. But scientists found one carried by a common pest. It is called Fusarium dieback, and it can kill your trees. [See a map of its presence in SoCal here.]
Fusarium dieback is a fungal disease carried by invasive borers. As borers burrow into trees, they spread three different fungal pathogens. These fungi form colonies within a tree’s vascular system, blocking the flow of water and nutrients. Trees infected with Fusarium dieback must be removed and disposed of by professional arborists. Symptoms of Fusarium dieback Since this disease affects many different types of trees and has multiple pathogens, it is no wonder that there are a variety of symptoms. Infected avocado trees, for example, will exhibit sawdust-like frass (bug poop), gumming, and sugar volcanoes. Sugar volcanoes are white discharges of sugary sap. On other host trees, you may see dark, greasy-looking areas on the bark, withered leaves and stem tips, and white mycelium under the bark. Mycelia are the vegetative growths of fungi. As the infestation progresses, perfectly round, tiny borer entry and exit holes may become visible. These holes are only 0.03 inches in diameter, so look very closely, usually just below areas showing disease symptoms. Eventually, branches will start to die. If you cut into an infected limb, you will see that the wood is discolored, brown, or black. If you scrape the bark away from entry or exit holes, you will also see discoloration. After each cut, disinfect your tools with a household cleaner to avoid spreading the disease. Disease background Fusarium dieback originated in Israel and spread to Southern California by 2003. Botanists are not sure how far the disease will spread from there. At first, scientists thought the tea shot hole borer was the carrier, a common pest of tea plants in Sri Lanka. DNA testing, however, showed that this was an entirely new species, now named the polyphagous shot hole borer. By 2010, this borer, and the disease it carries, had become a serious threat to Southern California’s box elder trees, palm trees, black locust, and our beloved avocado trees. In 2015, a second carrier, the Kuroshio shot hole borer, joined the party and started infecting the popular California palms (Washingtonia filifera). This disease currently threatens 25% of all the trees lining Southern California’s streets. Beetle description The two borer species responsible for Fusarium dieback look identical at first. Female beetles are tiny and black, only 0.07 to 0.1 inches long. Males are brown and even smaller, at only 0.06 inches long. Female beetles can fly and will leave their birthplace to find other host trees to use as nurseries, carrying the disease with them when they go. Males do not fly and generally stay in the tree of their birth. These beetles are most active during summer and fall. Fusarium dieback hosts Shot hole borers feed on and breed in over 200 species of woody plants. That’s a lot of potential hosts. While this disease prefers gums, palms, pines, and privets, it is also a threat to apples, bay laurel, carob, chestnut, elderberries, figs, grapes, olive, persimmons, pineapple guava, pistachios, pomegranate, stone fruit, and stone pine are also vulnerable to Fusarium dieback. How infection occurs This disease starts when female beetles bore into tree trunks and branches, creating galleries of tunnels. Within the tunnels, they build egg chambers. Female beetles have developed a symbiotic relationship with three different fungi, which they carry around in their mouths, much the way we have bacteria in our gut. [The fungal pathogens of Fusarium dieback are Fusarium euwallacea, Graphium euwallaceae, and Paracremonium pembeum if you enjoy the Latin.] Those fungi set up housekeeping within the tree and become food for the newly hatched beetle larvae. By eating the fungi, the larvae then become carriers of the disease. These fungal colonies develop rapidly once inside a tree, and there is no known treatment at the time of publication. Not all infected trees will show signs of infection. Some infected trees serve as breeding grounds without showing any signs of disease. We don’t yet know why. Preventing disease Healthy trees are far better able to protect themselves against borers. So select plants appropriate to your microclimate, irrigate and fertilize them properly, and provide healthy soil. Also, monitor your trees regularly. If you suspect Fusarium dieback on a tree, contact your local County Extension Office. Together, we may be able to slow or stop the spread of this disease. Summer sunburns hurt, but winter sunscalds are worse (for trees). You might not think sunscald, or sunburn, could occur in winter, but it can. [Ask any of your skier friends. They know.] Unlike summer sunburns, like the sunburns we get, winter sunscald occurs when it gets warm enough in the daytime for plant cells just under the bark to break dormancy. Breaking dormancy Several chemical changes occur as plants enter dormancy. These changes protect plants against the cold. Warm, sunny days can trick overeager cells into breaking dormancy and losing those protections. Then they die, leaving a sunken area below the bark.
If there are any fruits present in winter, they, too, can be affected by winter sunscald. Going from protective shade to direct sunlight can damage the outer skin enough to allow insects and diseases to enter. In other cases, the fruit exterior may look normal, but the underlying tissue becomes damaged and is usually inedible. Winter sunscald is rarely fatal, except to one- or two-year-old trees. But it can weaken a tree and make it susceptible to wood-boring pests and canker diseases. Nearly all fruit trees, and all newly planted trees, are vulnerable to winter sunscald. Other thin-barked trees, such as ash, beech, birch, maple, and willow, often develop winter scald if unprotected. Preventing winter sunscald There are several ways you can prevent winter sunscald from damaging your trees. First, irrigate your trees properly. A water-stressed tree is far more likely to develop winter sunscald. Also, avoid damaging the trunk with lawnmowers, car doors, and weedwackers. Damaged tree trunks have a hard time protecting themselves. You can prevent mechanical injury and stabilize soil temperatures with a thick layer of mulch around the tree. Just keep the mulch from touching the trunk, which can set the stage for fungal disease. If watering and mulching aren’t enough, whitewashing the trunk and exposed surfaces of major branches can help prevent winter sunscald. It will protect against summer sunburn, too. Just don’t whitewash your trees with enamel paint. They will suffocate. Instead, combine equal parts water and white latex paint (preferably one without extra chemical additives) and use that to paint your trees. Traditionally, people wrapped their trees to prevent winter sunscald, but now we know that those wraps can harm trees. Tree wraps allow moisture to collect, setting the stage for rot, and they provide an excellent hiding place for pests and fungal diseases. Also, cracks that occur under the wraps do not heal as well. Bottom line: don’t wrap your trees.
If your trees tend to get winter sunscald, you can block or shade the southwest side of your trees to provide some protection against winter sunscald. Winter sunscald care Once the damage has occurred, you can speed up the healing process by smoothing the edge of the wound with a sharp, sterilized knife. Do not remove more than 1/2” of the bark. Also, resist the urge to paint tar or sealant over the wound. Sealants trap moisture in the wound, increasing the chance of disease and decay. Finally, you know that adage about getting lost in the woods and looking for moss on the north side of trees? Well, that piece of advice is useless. Moss grows on the side of the tree with the most moisture and sun protection. What you can do, however, is look for winter sunscald, which nearly always occurs on the south or southwest sides of trees in the northern hemisphere. Now you know. Fusarium crown and root rot, or crown and foot rot, means death for asparagus.
Fusarium causes several crown and root diseases. This one attacks asparagus. Heavy soil, poor drainage, over-harvesting, and insect feeding create the perfect habitat for this ubiquitous fungi. Disease pathogens Three different forms of the Fusarium fungi cause this fungal disease of asparagus. This trio of Bad Guys (Fusarium oxysporum f. sp. asparagi, F. verticilliodes, and F. proliferatum) colonize the roots, crown, and xylem tissue. Symptoms of Fusarium crown and root rot Asparagus plants infected with the Fusarium fungi decline over time. At first, you may see one or more stunted, bright yellow ferns. Do not ignore this bright yellow warning if you want to save your asparagus patch. Wilting is also common. If you look at the crown area, you will see reddish-brown discoloration. At this point, pull the plant out of the ground for a closer inspection. Cut open the crown or below-ground area of the plant to see if sunken reddish-brown lesions are visible. Feeder roots will probably rot off completely, though any remaining tendrils will have the same reddish-brown discoloration seen elsewhere. Disease lifecycle Unfortunately, these fungi can survive in the soil indefinitely and are everywhere. The disease can move around on equipment, seeds, shoes, and tools. Choose seeds and seedlings from reliable sources. This disease can occur anywhere underground. Very often, insect feeding creates points of entry for these fungi. Asparagus miners are a common culprit. Controlling Fusarium crown and root rot Environmental conditions that keep plants healthy also improve their ability to prevent these fungi from entering, so avoid water stress and feed plants regularly with top dressings of aged compost. [Asparagus plants are very heavy feeders.] Infected plants should be dug up, roots, soil, and all, and disposed of in the trash. Remember, perennial asparagus can provide you with many years of delicious spring and autumn spears, so don’t let these pathogens stop you from trying to grow your own! You’ve probably seen it in your garden. Instead of developing a tightly packed head of green florets, your broccoli looks a little loose and floppy. Then, the florets turn brown. This die-off of unopened flowers is called brown bud or brown bead.
As a farmer, brown bud can make a crop unsellable, devastating a family’s financial situation. As a home gardener, things aren't nearly so dire. Now, we are not talking about the tiny black and brown fungal spots that occur on broccoli stored too long. You can cut those spots out and eat the rest, but that particular broccoli is probably past its prime. Brown bud is not a disease. Instead, it is a physiological disorder. Symptoms of brown bud Unlike your standard grocery store variety of broccoli, with its tightly packed head of unopened flower buds, heads affected with brown bud have a loose arrangement. The unopened florets at the center of the head first turn yellow (chlorotic) and then brown. This browning can spread across the head. These dead florets break off, providing easy access for pathogens. Causes of brown bud This poorly understood condition most frequently occurs when temperatures are higher than is ideal for this cool-weather crop. Heavy clay soil is another contributing factor. Brown bud is more likely to occur when nitrogen is deficient and during periods of low relative humidity. Some believe insufficient calcium causes brown bud, but research does not support those claims. Controlling brown bud If brown bud has been a problem for you in the past, try starting your broccoli when temperatures are lower and the weather is wetter. And provide plenty of nitrogen since fast-growing heads are less likely to develop brown bud. Regular irrigation can also help prevent this disorder. Some cultivars, such as ‘Skiff’, are more likely to develop brown bud. While others, such as ‘Coaster’ and ‘Shotgun’, are less likely. To avoid brown bud in your garden, feed those heads plenty of water and nitrogen! Black rot may sound like the perfect name for your next Halloween character, but this bacterial disease can wreak havoc on plants in the cabbage family (Brassicaceae). Popular brassicas, or cole crops, include broccoli, cauliflower, kohlrabi, horseradish, Napa and Chinese cabbages, collards, turnips, rutabagas, Brussels sprouts, watercress, kale, radishes, bok choy, mustard. Rapeseed (canola) is also a member of this family. And all of these plants are susceptible to black rot, a close cousin to bacterial spot. Black rot symptoms The early symptoms of black rot (Xanthomonas campestris pv. campestris) don’t look like anything serious. You will see some chlorosis (yellowing) along leaf edges (margins) and some V-shaped lesions pointing toward the center of the plant. Then, those lesions may dry up and fall away, lulling you into thinking the problem has resolved itself. Instead, black rot bacteria have gained a foothold in the phloem and xylem of your cabbage. Wilting and dieback occur as they populate and block these vascular tissues. If you cut an infected stem longitudinally, you will see blackening of the vascular tissue. [Just be sure to disinfect your cutting tool afterward so that you don’t spread the disease to other plants.]
Managing black rot in the garden Since this disease is most likely to occur in warm, humid weather, planting your cole crops after any chance of Indian summer has passed can help prevent it. The moisture left behind from overhead watering can also create the perfect conditions for black rot to take hold, so water your cabbages at ground level. Resistant varieties are available, so shop for those if you have already had problems with black rot. Also, it’s a good idea to use crop rotation with cabbages and cauliflower in particular, as these two crops are the most likely to be affected. Since this bacteria can survive on cruciferous weeds, try keeping your cabbage patch weed free. Bacterial leaf scorch is a collection of diseases that can affect a wide variety of plants. Bacterial leaf scorch (BLS), mistakenly known as bacterial leaf spot, is a tricky disease, because it is actually several diseases caused by different strains of a single pathogen. That pathogen, Xylella fastidiosa, causes different diseases in different plants. And sometimes those bacteria strains overlap their feeding habits, making classification and control difficult.
Lifecycle of Xylella fastidiosa This particular bacterium is what’s called a fastidious mollicute, which means it must live within a plant’s xylem to be able to reproduce. The diseases they cause occur because they get so overcrowded that they block the flow of water and nutrients through the xylem. Not all plants are negatively affected by this bacterium. Clover, blackberry, goldenrod, and many grasses can host this pathogen, acting as a way station without suffering any consequences. Unfortunately, when a sharpshooter feeds on one of these plants and then moves to your garden, trouble can start. Symptoms of bacterial leaf scorch Scorch diseases are characterized by the same symptoms you would see as a result of environmental conditions, such as herbicide overspray or too much fertilizer, or other diseases, such as verticillium wilt. Initially, you will see wilting and/or chlorosis. Leaf edges look, well, they look scorched! Then leaves start dropping. Fast. Before you know what happened, the plant dies. Except when it doesn’t, because sometimes it won’t. Scientists are still trying to sort it all out. Disease vectors
The bacteria that cause scorch diseases are carried into your garden by leafhoppers and spittlebugs. Actually, it’s in their saliva. Sharpshooters are the biggest carriers of the disease, as far as we know. These insect pests have a wide host range of their own. As sapsuckers, every bite they take infects the plant on which they are feeding. Because their host range is so large, they are spreading diseases to plants that have never been exposed before, so they have no defenses in place. There are no known chemical treatments for scorch diseases, so controlling the disease carriers is your best bet. If the disease appears, remove the infected plant completely and put it in the trash. Mulching and proper irrigation can help your plants protect themselves. Curly top may sound like a cute little redheaded kid, but it’s a viral disease of many garden plants. Curly top host plants This viral disease can infect watermelon, horseradish, pumpkins, beans, squash, spinach, melons, peppers, chickpeas, groundcherries, tomatoes, and more. Scientists have identified different species of viruses that cause similar symptoms. For lack of better ideas, those viruses are called beet curly top geminivirus (BCTV), beet mild curly top virus (BMCTV), and beet severe curly top virus (BSCTV).
This condition is often fatal in young plants. Infected older plants may only turn yellow. The telltale symptom of curly top occurs when the top of the plant turns into a rosette or tiny bouquet. If any fruit develops, it will taste bad, the skin will be dull rather than shiny, and it will ripen before it reaches full size.
Curly top virus lifecycle The virus overwinters in annual and perennial weeds. From there, beet leafhoppers (Circulifer tenellus) carry the disease to your garden plants. Symptoms don’t start to appear until long after the leafhoppers are gone. Since they are the disease vector, controlling leafhoppers goes a long way toward preventing this disease. Unfortunately, insecticides are generally not effective against leafhoppers. Leafhoppers have many natural enemies, so make your garden hospitable to beneficial insects. You can do this by avoiding broad-spectrum insecticides, planting a variety of umbellifers, such as dill, carrot, and fennel, and providing a water source. The symptoms and host plants of curly top look too much like several other viral diseases, such as spotted tomato wilt, to be identified by the casual gardener. Laboratory tests are needed to know for sure. In the case of viral disease, it is simpler to yank the plant and toss it in the trash rather than spreading the infection to other plants. Plants may not have an active immune system, but that doesn’t mean they passively roll over and take whatever hits them.
When pathogens strike, plants can respond in two ways to prevent infection: they use pre-existing structures and chemicals and activate responses triggered by the presence of a pathogen. Pre-existing defenses Just as our skin blocks many pathogens from entry, a plant’s skin, or epidermis, does the same thing. That’s why insect feeding and mechanical damage can increase the chance of a disease taking hold - something has already breached the plant’s first line of defense. Plant cell walls also block viruses, bacteria, and fungi. Unlike our human immune system, which uses white blood cells to hunt down and destroy invaders, plant cells have antimicrobial defenses built right in. These defenses take the form of saponins, glucosides, and antimicrobial proteins. Enzyme inhibitors can also stop some pathogens from feeding on the plant. Plants also have chemicals that can neutralize toxins created by a pathogen. Finally, receptors can recognize a pathogen and alert the plant to take further action. Inducible plant defenses Once a pathogen is recognized, plants reinforce their cell walls and produce defensive chemicals such as hydrogen peroxide, antimicrobial enzymes, and proteins. In some cases, plants have evolved a ‘hypersensitive response’ that kills the healthy tissue surrounding an infection to block further infection to neighboring cells. Disease resistant varieties Rather than treating a disease after it occurs, you can grow plants that defend themselves. Installing resistant plants reduces the need for pesticides and fungicides. When shopping for plants, look at the plant label to see if that particular plant is resistant to diseases that tend to appear in your garden. Plant labels use the following codes to designate specific disease resistances:
For example, a plant label with VFN displayed tells you that the plant is resistant to Verticillium wilt, Fusarium wilt, and nematodes. While not a guarantee against disease, it does mean the chances are significantly lower. You can help your plants fight disease by selecting resistant varieties, spacing plants properly, employing crop rotation, and providing adequate water and nutrients. Also, be sure to sanitize your tools regularly, to halt the spread of disease from one plant to another. Olive knot is a bacterial disease of olive and oleander caused by Pseudomonas savastanoi pathovar (pv.) nerii and P. syringae pv. savastanoi. These bacteria are like evil twins that don’t cause serious harm but are still a pain to be around.
Symptoms of olive knot The knobby bit you see is called a gall. Galls are swollen, distorted, woody growths that can girdle stems and twigs. The P. savastanoi pathovar (pv.) nerii bacteria is the one that causes the galls on both oleander and olive twigs. It also forms galls on the leaves and flower buds of oleander. The P. savastanoi pv. savastanoi twin only affects olive, causing galls on twigs, stems, and leaf nodes. Olive knot lifecycle The bacteria responsible for olive knot enter the tree through pruning wounds, leaf scars, frost cracks, and sun scald wounds after traveling on wind and rain. Infection occurs in autumn and winter, but symptoms do not appear until late spring or early summer. The bacteria live in the galls they create and on the surface of twigs, leaves, and fruit. To prevent olive knot in your garden or foodscape, plant disease-free rootstock, avoid overhead watering, and prune off any infected material during the dry season, disposing of infected material in the trash. Be sure to sanitize your tools afterward. Fixed copper sprays (one in the autumn and two in spring) can help prevent and treat olive knot infections, but there is no sure cure. While this bacterial disease is not dire, it is spreading and becoming more of a problem. Due, in part, to the popularity of a particularly susceptible cultivar, the Manzanillo olive. White mold, also known as lettuce drop, tomato timber rot, and Sclerotinia stem and crown rot, is a collection of infections that can affect several garden plants. And they kill Crown rot is also known as white mold in some regions, but that is something else entirely. White mold (Sclerotinia spp.) can remain dormant in the soil for years. It takes a significant amount of cool, moist temperatures to wake these fungi up from their dormancy. But the underside of a head of lettuce, or a cabbage, provides the perfect humidity needed to trigger an awakening and an ensuing infection. Symptoms of white mold Wilting is often the first symptom of white mold. You may see pale brown, woody lesions that girdle lower stems, causing wilt. And which may ultimately kill the plant. Affected plant tissue develops watery lesions as cottony white mycelia form on the surface. Mycelia are the vegetative part of a fungus, made up of threadlike hypha. These white fungal growths appear on outer leaves, lower stems, and bean pods. White mold starts at the base of a plant and spreads, causing outer leaves to wilt and fall away while remaining attached. Garbanzo beans are particularly likely to become infected in the crown area. As damaged tissue dies and dries up, it will turn white and look bleached. Tiny (0.25–0.5 inch) irregularly shaped black flecks called sclerotia can be seen on and inside dead stems. Sclerotia are the resting body of the fungi, made up of a cluster of hyphal threads. These structures can remain dormant for a surprisingly long time.
White mold host plants Along with lettuce and tomatoes, the legume, nightshade, and cabbage family members are susceptible to white mold. Other broad-leafed plants and many weeds can also become infected. How to avoid white mold in the garden Since prolonged moisture is needed for this fungus to come among us [sorry, I couldn’t resist], keeping things dry is a good defensive plan. These specific steps can help you avoid a white mold problem in your garden:
Severe infestations may require fungicides or soil solarization. As prevalent as white mold is, it’s a good idea to know what to look for ahead of time. Johnson spot is a fungal disease in rice, wheat, barley, rye, and millet. It also attacks your lawn. Other names for this disease include rice blast fungus, pitting disease, and ryegrass blast. As a threat to your lawn, Johnson spot can infect kikuyugrass, fescues, ryegrasses, and St. Augustine grass. The fungal pathogen The fungus that causes Johnson spot is called Magnaporthe grisea (previously known as Pyriculria grisea). It is a highly effective fungus. Spores attach themselves to plant surfaces. They can reproduce both sexually and asexually, and they are prolific. A single spore can complete its reproductive cycle in one week, though it can live for 20 days. And thousands of new spores are generated each night. With numbers like that, it's no surprise that this disease can overwhelm a plant. Johnson spot symptoms Early signs of Johnson spot infection include white to grayish-green spots with dark borders. As they age, the lesions take on a more elliptical shape. Eventually, affected leaves die and fall off. These symptoms can be seen on many parts of the plant, including the leaf collar, stems (culms), and flowers (panicles). And seed production nearly stops, which is a problem since crops such as barley, millet, or rye are seeds. How to prevent and control Johnson spot
Moisture is the key ingredient to this fungal growth. If leaves are wet and temperatures are between 77 and 82°F, Johnson spot can quickly take hold. Use these tips to prevent Johnson spot:
The Johnson spot fungus is resistant to chemical treatments, so these cultural practices are your only option. Johnson spot is the most significant disease of rice in the world. Experts estimate that this disease destroys enough rice to feed 60 million people annually. Beans are easy to grow, help improve soil structure, and add nitrogen to the ground. They can also become infected with bean yellow mosaic. Three different bean mosaic diseases commonly occur in North America: bean common mosaic, cucumber mosaic, and bean yellow mosaic. These are all viral diseases that cause downward cupping and wrinkling of leaves, especially as leaves get older, along with the telltale mosaic pattern. Bean leaves that develop a bright yellow mosaic pattern may have bean yellow mosaic virus. There are several strains of bean yellow mosaic (BYM). In addition to beans, bean yellow mosaic can infect peas, peanuts, soybeans, black locust, and fenugreek.
Bean yellow mosaic symptoms You can differentiate between bean yellow and the other mosaic infections because bean yellow has a yellow mosaic rather than a light or dark green mosaic. Bean yellow mosaic also exhibits bright yellow spots on leaves. Plants infected at an early stage of development can become severely stunted. Remove them from the garden and toss them in the trash. Bean yellow mosaic lifecycle The bean yellow mosaic virus, or BYMV, commonly overwinters in legumes, such as alfalfa, clovers, fava beans, gladiolus, and vetch. The virus moves from plant to plant in aphids. When an aphid feeds on an infected plant, it becomes a carrier, transporting the disease to every plant it feeds on from that point forward. Since resistant cultivars are not yet available, these tips may help prevent bean yellow mosaic in your garden:
Finally, if you see an infected plant, trash it. Onions, chives, and garlic plants with pink roots are not happy. This disease rarely causes significant problems in garlic but can shrink your onion and chive harvests by quite a bit. The pink root pathogen
Pink root is a fungal disease caused by Phoma terrestris. Phoma terrestris is nearly always present in the soil and is relatively eternal. It causes no serious problems most of the time. But, if your onion plants are experiencing drought, insufficient or excessive fertilizer, water stress, insect feeding, compacted soil, or any other less-than-ideal circumstances, they may become susceptible. This pathogen thrives in temperatures between 75°F and 85°F. It can move around the garden by splashing rain and irrigation water, on tools and shoes. Symptoms of pink root Aside from the pink roots, plants infected with this fungal disease also exhibit roots that darken to red, purple, and, eventually, black. These roots shrivel up and die. These discolorations may move up into the bulb. This infection leads to stunting, but it rarely kills the plant. This disease looks a lot like fusarium wilt. Preventing and controlling pink root Keeping plants healthy and employing crop rotation are the two best ways to prevent pink root from causing too many problems. A note on crop rotation: do not follow a cereal crop with onions, as it creates conditions that promote this particular pathogen. Soil solarization is the only option in severe cases, but that’s a pretty drastic measure for the home gardener. If you see pink, purple, or black shriveled roots on your onions, try growing them in a different area, in fresh soil. If the lower limbs of your almond tree are turning brown, you have a problem.
While it is normal for some leaves on lower limbs to turn yellow because of being shaded by the growth above, lower limb dieback (LLDB) goes much farther and can kill your almond trees Upper canopy or crown leaf loss is shade tree decline, a potentially fatal disorder caused by drought. Symptoms of lower limb dieback First seen in 2005, LLDB consistently appears in spring and on 7- or 8-year-old trees when leaves on lower branches turn yellow and brown, cankers develop, and brown discolorations are visible under the bark of infected branches. Cankers can girdle a limb and kill it. Susceptible varieties Some almond varieties are more susceptible to lower limb dieback than others. Padre almonds are the most likely to get this disorder, with Butte being a close second. Install resistant varieties, such as Aldrich, Carmel, Fritz, Mission, NePlus Ultra, Nonpareil, Sonora, and Wood Colony, when possible. While research is underway, we still don't know if lower limb dieback is a fungal disease or an environmental disorder. Two fungi have been found in trees exhibiting lower limb dieback (Botryosphaeria spp. and Phomopsis spp.). They may be causing the problem or are simply opportunists. Preventing lower limb dieback Low light levels, chemical overspray, soggy soil, and excessive fertilizer weaken trees, making them vulnerable to whatever causes this problem. Specific conditions that seem to lead to LLDB include excessive water early in the growing season and compacted soil. Hull rot seems to make trees more prone to develop lower limb dieback, and scale insects may be involved. Keeping your trees healthy is the best way to prevent lower limb dieback. This means proper irrigation, good drainage, the right amount of fertilizer (after a soil test shows a need), and control of scale insects. Fixed copper and sulfur treatments are not effective. Fungicides may provide a cure, but we don't know yet. Pierce’s disease poses a significant threat to grape vines and citrus trees. The bacteria responsible for Pierce’s disease, Xylella fastidiosa, was first seen on grapes in Southern California in the late 1800s when it was called Anaheim vine disease. By the 1930s and 1940s, it had spread to California’s Central Valley. By the late 1990s, the disease had spread to several California counties. This increase appears to be a function of warmer temperatures allowing more bacteria to survive the winter and increasing their range. According to CABI, Pierce’s disease now occurs throughout the Americas, Italy, Iran, and Taiwan. Disease vectors Pierce’s disease is carried by sap-feeding insects. Most commonly, this means sharpshooters, such as blue-green and glassy-winged sharpshooters. [Did you know that sharpshooters can consume hundreds, or even thousands, of times their body weight in sap in their short lives?] Spittlebugs also carry this disease. Whichever insect is chewing on your grape vines injects the bacteria into the vine’s vascular bundle as they feed, making them a disease vector. These bacteria then live and reproduce in the xylem, clogging the flow of nutrients and water through the plant. Host plants
Pierce’s disease can occur in several weedy and ornamental crops, such as wild grape, California blackberry, periwinkle, stinging nettle, eucalyptus, live oaks, blue elderberry, and mugwort. These plants are not affected by the bacteria that cause disease in grapes. But they provide a transitionary location for the insects that do carry the disease. Symptoms of Pierce’s disease Infected plants exhibit leaf scorching and stunting. These symptoms start as slightly yellow or red leaf margins (edges) of white or red grape varieties, respectively. Concentric areas of infected leaves may dry up. You may also see ‘matchstick’ petioles, ‘green islands’ on mature brown stems, shriveled clusters of fruit, and dieback. These symptoms do not appear until spring after temperatures are above 65°F. There is no cure for Pierce’s disease. Sometimes it will disappear on its own. We don’t yet know how or why. It seems to be a function of temperature, the timing of the initial infection, and the variety of plants. Generally speaking, late-season infections have a 95% chance of recovery. Water-stressed plants are more likely to succumb. If a plant becomes infected early in the season, the bacteria will take over, and the vine is doomed, so remove it. Pierce’s disease control and prevention This disease triangle consists of the host plant, the feeding insect carrier, and the disease-causing bacteria. Break the connection between any of those three, and you reduce the chances of disease. The easiest way to prevent Pierce’s disease is to keep host weeds out of the area and control sap-sucking insect pests. Since insect-eating birds, such as bluebirds, and several predatory insects, love to eat sharpshooters, keep your garden welcoming to these natural helpers. Monitor your plants for signs of Pierce’s disease so that you can act quickly, reducing the spread of the disease. Most vector insects are low fliers, so physical barriers can be used to quarantine potentially infected plants. During the dormant season, remove any vines infected for more than one year. They will not recover. As vector insects feed on them and move to nearby plants for more feeding, they spread the disease to healthy plants. There is no cure for Pierce’s disease. In some cases, the disease will disappear on its own and we don’t yet know how or why. It seems to be a function of temperature, the timing of the initial infection, and the variety of plant being infected. Generally speaking, a late season infection, one that occurs after June 1st, has a 95% chance of recovery. Water stressed plants are more likely to succumb to the infection. If a plant becomes infected early in the season, the bacteria have time to become firmly established. Once that happens, you will ultimately have to remove the vine completely. Pierce’s disease control and prevention This disease triangle consists of the host plant, the feeding insect carrier, and the disease-causing bacteria. Break the connection between any one of those three and you can reduce the chances of disease. The easiest ways to prevent Pierce’s disease is to keep host weeds out of the area and treat for the sap-sucking insect pests. Since insect-eating birds, such as bluebirds, along with several predatory insects, love to eat sharpshooters, keep your garden welcoming to these natural helpers. Monitor your plants for signs of Pierce’s disease so that you can act quickly, reducing the spread of the disease. Most of the vector insects are low fliers, so physical barriers can be used to quarantine potentially infected plants. During the dormant season, remove any vines that have been infected for more than one year. They will not recover and they will spread the disease to other plants as vector insects feed on them and then move to nearby plants for more feeding. Square watermelons, portrait gourds, heart-shaped oranges, and Buddha pears are purposefully distorted fruits that can be a fun way to play with your plants. These distortions are kin to the method of tree training known as pleaching. When it occurs without human intervention, however, fruit distortions warrant a closer look. Affectionately known as ugly fruit, naturally occurring fruit distortions can be nothing more than cosmetic. Or, they may indicate the presence of pests, disease, nutrient deficiencies, or chemical misuse. It can also be from stress. Fruit distortions caused by stress Stressed plants (and people) do not perform as well as they might otherwise. If you were a plant, those stresses might be drought, insect damage, extreme temperatures, herbivore feeding, mechanical injury, excessive salt, insufficient nitrogen, severe weed competition, or water stress. If you were a stressed-out member of the cabbage family, you might surround yourself with a protective layer of bronzed leaves. Or you might shrink your head in a response called buttoning. Stress-induced distortions also include stunting, misshapen flowers, and reduced leaf size. Mechanical injury or blockage of cucumbers and other cucurbits can cause crooking. Low temperatures during pollination can cause uneven fruit development in strawberries. But what if it isn’t stress that is causing fruit distortion? Fruit distortions caused by nutrient deficiency Plant nutrients are critical to the proper development of fruit. Distorted fruits often occur in boron-deficient soil. Of course, without a soil test from a reputable lab, you won’t know what’s in your soil. Unfortunately, those cute, over-the-counter soil tests are not [yet] accurate enough to be helpful. Insects that cause fruit distortion Citrus bud mite feeding can cause some dramatic distortions, especially in citrus. While there isn’t anything you can do to get rid of citrus bud mites, their feeding can create points of entry for other pests and diseases, so you will want to monitor infested trees. Fungal disease and fruit distortion
Fungal diseases, such as apple scab, can also cause fruit distortion. Unfortunately, in this case, you won’t want to eat the fruit, as it will be mushy and rotten. One easy way to break the fungal disease triangle is to remove fallen leaves under infected trees and toss them in the trash. Chemicals and distorted fruit Since many herbicides are plant hormones (auxins) that force plants to grow so fast that they die, chemical misuse or overspray from a neighbor’s yard can cause fruit distortion. In this case, if in doubt, don’t eat it. If the suspected chemical is systemic, you won’t be able to wash it off. Unless fruit distortions are from chemicals or fungal disease, taste and texture are rarely affected. These fruits are simply funny looking. We can enjoy them for their uniqueness. With a name like bottom rot, you know it won't end well. Bottom rot is a fungal disease of lettuce and other leaf vegetables caused by the Rhizoctonia solani fungi. This is the same fungus that causes damping off. It lives in the soil and can be a big problem in warm, moist conditions. Plants affected by bottom rot In addition to lettuce, bottom rot can wipe out your Chinese cabbage, escarole, broccoli, radish, collards, Brussels sprouts, cauliflower, and turnip crops. The Rhizoctonia solani fungus also attacks potatoes, onions, beans, and corn. Symptoms of bottom rot
At first, all you may see is some wilting of outer leaves. Closer inspection will show reddish-brown, sunken lesions on the midribs of leaves touching the soil. Brown or white fungal tissue may be visible, and lesions may discharge a light brown ooze. Leaf spots and brown lumpy bits may also be present on the plant. The fungus grows inward, toward the center of the head or body of the plant. The damaged plant tissue then becomes susceptible to other soft rots, causing a total collapse of the plant. Controlling bottom rot Fungicides are ineffective against bottom rot, so prevention is your only option. These tips will help protect your leafy bottoms:
As with all bottoms, keep them dry, and they will be happy. Young sunflowers track the sun across the sky, reaching new heights with every passing day - except, sometimes they don’t. Sometimes, a small injury can become infected and a black rot spreads around the base of the flower, finally engulfing it in a black goo that dries and hardens into a smelly shadow of what might have been. What causes this, and can it be prevented?
Head rot, also known as pin rot, is a disease of sunflowers, lettuce, and broccoli, caused by the Pectobacterium carotovorum, subsp. carotovorum and P. atrosepticum bacteria. Symptoms of bacterial head rot The first symptom of bacterial head rot is nothing more than a small, brown, greasy or water soaked looking area on the surface of a cluster of unopened flowers or leaves. These lesions are usually seen at the sight of mechanical injury caused by bird and insect feeding, hail, or falling twigs. Bacteria enter the damaged tissue and that’s where the infection begins. Affected areas turn from brown to black as the infection spreads into surrounding plant tissue. There is a distinctly bad smell, similar to rotten potatoes, but it is rare for secondary fungal growths to occur when head rot is present. If the bad smell is absent and other bacterial and fungal infections are present, the infection is more likely to be caused by Alternaria fungi. Bacterial head rot prevention and control Cool winter and spring temperatures combined with prolonged periods of rain, fog, and dew provide the perfect medium for bacterial head rot pathogens. This means good air circulation between plants can go a long way toward preventing this disease. That’s a good thing, because chemical sprays and other treatments have not been consistently effective in preventing bacterial head rot. The best way to prevent this problem in your garden is to start with resistant cultivars, such as broccoli with dome-shaped heads, space plants properly, and avoid overhead watering. It is normal and healthy for the pruned tip of a twig to dry up and seal itself off from pests and disease. When that death keeps moving inward, there’s a problem. This creeping death is called dieback. Dieback can be from environmental conditions, insect feeding, or disease. Physical dieback It is not understood why, but delayed leafing out seems to be associated with dieback. The same environmental conditions that cause delayed leafing out and dieback include winter drought, extreme cold, or insufficient chilling hours. This form of dieback is common in blackberries and raspberries. Other causes of dieback include poor irrigation and hot, dry winds, potassium or zinc deficiencies, phosphorus toxicity, insect feeding by shot hole borers, black scale, wooly aphids, and mealybugs. Also, when the raspberry horntail, a tiny wasp, lays its eggs in a raspberry or blackberry cane. Dieback by nematodes Nematodes are microscopic, eel-like roundworms that live in the soil. Some nematodes are beneficial predators, and some are plant-eating, disease-carrying parasites. Nematode feeding can cause reduced plant vigor, wilting, smaller fruits and leaves, and twig dieback. Fungal dieback Several different fungi can cause dieback. Each pathogen has its own set of symptoms and host plants:
Viral dieback Lettuce is susceptible to viral dieback caused by the lettuce necrotic stunt virus. This pathogen causes stunting, leathery, dark inner leaves, and rotted areas on outer leaves. Bacterial dieback Apple, citrus, pear, and stone fruits are susceptible to bacterial blast, blight, and cankers, all caused by Pseudomonas syringae. This pathogen kills flower clusters and nearby leaves, along with twig tips. Fireblight is another bacterial infection that causes twig dieback. This disease is easy to spot because the dead twigs curl into a shepherd’s crook shape. Watch for fireblight in June. Huanglongbing, a deadly citrus disease, includes twig dieback as one of its early symptoms. How to prevent dieback
Healthy plants can often protect themselves from dieback. These tips can help reduce the risk of dieback in your garden and landscape:
There are many causes of dieback. Determine the reason behind dieback to find an effective treatment. Too much water can result in a lack of vigor or sudden death by Phytophthora root and crown rot. Phytophthora is Greek phytón (plant) and phthorá (destruction), so the name means the plant-destroyer. What is Phytophthora root and crown rot? Phytophthora [Fie-TOF-ther-uh] is a family of water molds called oomycetes. Oomycetes fall somewhere between fungi and algae in the web of life. There are many different types of Phytophthora molds. They generally attack stems and roots. Stem damage occurs at or just above the crown, at the soil line, though it can appear elsewhere on a plant. These molds cause many plant diseases, including sudden oak death, potato blight, damping-off disease, and crown rot. Phytophthora root and crown rot can kill a tree or shrub if the soil remains wet for too long or when planted too deeply. [Moist soil around the trunk is never a good idea.] Host plants Nearly all fruit and nut trees, including cherries and kiwifruit, are susceptible to Phytophthora root and crown rot. But so are members of the nightshade and cabbage families. So, tomatoes, eggplant, and potatoes are vulnerable, as are cauliflower, Brussels sprouts, and cabbage. And all because of too much water. Symptoms of Phytophthora root and crown rot Plants affected by Phytophthora root and crown rot look drought stressed. Unfortunately, because the natural response is to provide more water - the last thing you want to do when Phytophthora is present. Symptoms start in just one branch or area of the affected plant before spreading. Leaves may turn purple or reddish. Plants may die suddenly or linger poorly for years before dying. Symptoms can vary greatly, depending on the type and age of the plant, the plant’s genetic resistance to infection, overall health, soil temperatures, and moisture levels. The bark around the crown and upper roots of infected plants is dark. A dark sap or gum may ooze from damaged areas. Using a sharp knife, cut away an area of bark. Infected plants will show reddish-brown streaks or patches. Water-soaked areas on roots may also be visible. If white threads are visible between the bark and the inner layer or around the root system, it is Armillaria root rot. Preventing Phytophthora root and crown rot infestation Proper water management is the best way to prevent and control Phytophthora root and crown rot. Never allow standing water to remain around tree and shrub trunks. Also, don’t let sprinklers hit tree trunks. These other tips can help you manage Phytophthora in your garden or landscape:
You may be able to maintain an infected plant with proper irrigation and good cultural practices, but it will never be the same. Phytophthora can stay in the soil for many years, so prevention is far easier than control. NOTE: One new-to-us variety, Phytophthora tentaculata, is on the Dept. of Agriculture’s watch list. If it appears in your garden or landscape, contact your local Cooperative Extension Office. They may have helpful advice to protect your plants and need to know where this disease is spreading.
Seeing unripe fruit or nuts on the ground, under your tree, can be normal or may indicate a problem. Fruit drop, or June drop, is a natural process that allows a tree to get rid of more fruit than it can support. Fruit drop is common to citrus, apple, avocado, almond, tomatoes, and many other crops. Earlier in the growing season, some trees will rid themselves of unwanted blooms (blossom drop) for the same reason. Some trees, such as loquat, can be messy during this time. Manual fruit thinning works the same way, reducing the quantity of fruit but improving its quality. Fruit drop can also indicate insect pests, disease, or adverse environmental conditions.
Fruit drop caused by insects Black scale feeding weakens the tree, leading to wilting, twig dieback, stunting, and early fruit drop. Mealybug feeding can cause early fruit drop, chlorosis, and sooty mold. Feeding by mites can also reduce a tree’s ability to support a crop, causing fruit drop. Finally, while weevils are better known for burrowing into beans, cotton bolls, and cereal grains, they will also feed on roots, stems, buds, flowers, leaves, and fruit. Often, the first sign of a weevil infestation is leaf wilting, scalloped leaf edges, and early fruit drop. Fruit drop caused by disease Trees will frequently abort diseased or malformed fruit rather than invest water and nutrient resources in fruit that won’t reach maturity. Fruit drop caused by environmental conditions Sudden cold or extreme heat can cause fruit drop, especially in young trees. Strong winds can blow unripe fruit from trees. The most common environmental cause of excessive fruit drop is insufficient irrigation or unbalanced soil nutrients. Almonds and tomatoes are particularly sensitive to feeding and irrigation fluctuations. Pollination and fruit drop Fruit drop can be the result of insufficient pollination. Some trees need genetically compatible neighboring trees they can use for cross-pollination. It can also mean there are not enough pollinators in your area. You can attract more pollinators to your garden by avoiding the use of broad-spectrum pesticides and by installing a wide variety of flowering plants. Or, you can start raising honey bees. Honey bees take up surprisingly little space, boost pollination of nearly all your crops, plus you get honey! Fruit drop and pruning Heavy pruning can leave a tree unable to support the initial crop, resulting in fruit drop. Unless necessary, it is better to leave pruning and tree training for the dormant season. Fruit drop and the soil Low magnesium (Mg) levels in the soil can cause fruit drop, as can high potassium (K) or boron (B) levels. You can’t know what your soil’s nutrient levels are without a soil test from a local, reputable lab. While they look convenient and appealing, over-the-counter soil tests are not yet accurate enough to be useful. The type of soil can also have an impact on fruit drop. Sandy soils are far more prone to fruit drop than heavy clay soil. Don’t panic if your orange tree drops dozens or hundreds of tiny green fruits in May or June. It is normal. Just pick them up and add them to your compost pile. If you notice heavy insect infestations, signs of disease, chlorosis, or wilting, track down the cause and correct it. Remove fallen fruit and mummies to avoid creating a disease triangle or a hotel for pests. If older leaves on cucumber, melon, or squash turn yellow and leathery, they may have cucurbit aphid-borne yellows.
The cucurbit aphid-borne yellows luteovirus (CABYV) causes this viral disease. Luteoviruses are a genus of viruses that use plants as hosts. Aphids carry this disease and spread it to plants as they feed. Symptoms of aphid-borne yellow virus Early symptoms are chlorotic (yellow) areas on lower leaves. These spots expand to include the entire leaf, leaving the large veins bright green. The affected areas become leathery and brittle. Stunting and fruit drop are common symptoms as the plant struggles. Before genetic testing, gardeners thought this was plant aging (senescence), nutrient deficiencies, or other diseases, like cucurbit yellow stunting disorder. How the disease spreads As the name suggests, aphids carry this disease. As aphids pierce plant tissue to feed on sap in the xylem, they spread infection. Once infected, the aphid will continue to spread the disease as it feeds on squash family plants. Lettuces, beets, and many weeds can also become infected. Controlling aphid-borne yellows There is no way to control the virus, but you can reduce the presence of aphids in your garden with these tips:
Remove and destroy infected plants to prevent the disease from spreading to nearby plants Yesterday, I harvested my first sun-warmed, tangy-sweet raspberry of the season. For me, raspberries rank right up there with tomatoes for good reasons to grow your own. Store-bought raspberries rarely have the flavor or plump texture that fresh, homegrown raspberries provide. That being said, cane blight can take all the fun out of growing raspberries at home. If you know what to look for, you can prevent this disease from spreading. Cane blight is a fungal disease. The fungus responsible for cane blight goes by the name Leptosphaeria coniothyrium. It is also known as Paraconiothyrium fuckelii. [Apparently, someone hates this disease even more than I do!] Cane blight can also attack blackberries. The same fungus also causes a rose canker and root rot on strawberries. Unaffected host plants include bamboo, juniper, stone fruits, blueberries, and currants, so you may want to keep them at a distance from your blackberries and raspberries. Symptoms of cane blight This disease starts by looking similar to fireblight, spur blight, and anthracnose, but it progresses differently. Canes infected with Leptosphaeria coniothyrium will develop purplish-black lesions at wound sites. These lesions will develop into black or brown cankers that encircle the cane, causing wilt and twig dieback. Infected canes may also twist around other stems rather than grow normally. The bark may split, and the wood becomes very brittle. Leaves and fruit on those canes will wither and die but stay attached. If you look closely, you can see tiny, black fungal bodies on the wood. [Sadly, I was unable to find any photos.] Preventing cane blight
Fungal spores can blow in on the wind and be splashed onto plants by rain, sprinklers, and overhead watering. Spores enter healthy canes through injury sites. Jagged pruning cuts, growth cracks, and damaged roots can all provide points of entry, so keeping plants healthy and protected is the best way to prevent this disease. Use these tips to help prevent cane blight on your raspberries:
How to control cane blight The only control is the removal of infected canes. The cane blight fungus overwinters on the canes, so infected canes should be removed and destroyed. Be sure to dip your pruners in a household cleaner between each cut. Remove severely infected plants. Fungicides are not effective against cane blight. Keep your raspberry plants healthy for many years of summer deliciousness! If you see yellow spots that spread to fill the spaces between leaf veins, your melon or squash plants may have cucurbit yellow stunting disorder. Cucurbit yellow stunting disorder (CYSD or CTSDV) is a viral disease of cucurbits carried by whiteflies.
Cucurbits are popular garden plants. They include summer squash, winter squash, watermelon, muskmelons, pumpkins, cucumbers, and more. Since some whitefly species can travel relatively long distances, this disease can find your garden throughout the growing season. To reduce the chances of the infection spreading from one plant to another, it is a good idea to learn how to recognize this disease in its earliest stages. CYSD Symptoms The first symptoms of CTSD look much like water stress, as the virus begins spreading throughout the plant. Then, yellow spotting on older leaves will develop. Or, this yellow spotting may mean the plant has a molybdenum deficiency or has cucurbit aphid-borne yellow virus (CABYV). You can take a leaf sample to your nearest county farm advisor, where they will conduct a molecular test to verify the disease. If the chlorotic areas spread to the point that the entire leaf is yellow, except for the veins, it is likely to be CYSD. Small green patches may appear in the yellow areas. Other symptoms include leaves curling upwards and becoming brittle. As the virus spreads through the plant's vascular system, older leaves start to fall off, and fruit development slows or stops. Fruits that do occur are smaller, less flavorful, and more prone to pests and other diseases. Also, fruits from infected plants do not store as well as fruits from healthy plants. Controlling CYSD There are no chemical or biological treatments for this disease. Controlling whiteflies is critical to halting the spread of cucurbit yellow stunting disorder. These tips can help reduce the chances of CYSD spreading through your garden: • Only use high-quality, disease-free seeds. • Quarantine and carefully inspect new plants for signs of disease or whiteflies. • Avoid using broad-spectrum pesticides, which kill whitefly predators. • Use floating row covers over newly planted seeds and seedlings. • Remove and destroy infected plants. • Fertilize and irrigate established plants properly to keep them healthy. • Employ crop rotation. Protect your cucurbits from whiteflies for an abundant harvest this summer and fall! Who wants scabby apples? Yuck! If you see pale yellow spots on your apple leaves, fruit, and stems, that turn brown, your tree may be infected with apple scab. Apple scab is a fungal disease that can defoliate your tree and destroy your crop. This disease is most common in areas with cool, moist weather in spring and early summer. This disease can also affect ornamental crabapple. Before losing your apples to the Venturia inaequalis fungus, let’s see what we’re up against. Apple scab symptoms Apple scab starts out as chlorotic (yellow) spots on leaves. Then dark, olive-colored spots appear on leaves and fruit. In severe cases, these spots may also show up on stems. If you look on the underside of infected leaves, the surface will look velvety. That ‘velvet’ is fungal growth. Affected leaves may also pucker or twist. Severe infestations cause leaves to turn yellow and fall from the tree. Infected stems can lead to blossom drop. Finally, later in the growing season, sooty or greasy grayish-black areas appear on the fruit. These lesions often have a red halo. As the fungi develop, the area becomes sunken, tan colored, and scabby. The fruit may also become distorted and crack, providing other pests and diseases with a point of entry. Apple scab lifecycle When infected leaves fall from the tree, they take fungal spores with them. These spores then overwinter on these leaves. When rain or sprinkler water hits these leaves, the spores can be bounced back into your tree, where they will germinate and start the whole process over again.
If your apple tree(s) become severely infected, you can use fixed copper, Bordeaux mixture, sulfur, or neem oil from the time green tips appear, until the tree is in full bloom. Avoid using these treatments after that time as they can cause fruit russetting. And be sure to follow the directions for any of these treatments carefully. For example, applying sulfur within 3 weeks of using horticultural oils, or on days when temperatures are above 90°F can lead to phytotoxicity.
As you conduct a weekly inspection of your apple tree each spring, be sure to enjoy the emerging blossoms! |
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