Posts Tagged: leaf spot
Common leaf spot in alfalfa is common this year
Why do my alfalfa leaves look spotty?? Common leaf spot (Pseudopeziza medicaginis) has been very...
A Few Notes on Angular Leaf Spot
Angular leaf spot (or as some insiders refer to it, "ALS") caused by the bacterial pathogen Xanthomonas fragariae, has been showing up a bit more than expected this year in several strawberry varieties, including Cabrillo and Petaluma.
I'd like to demonstrate several descriptions of this disease from the UC IPM guidelines for strawberry with pictures just taken this morning on a field of Cabrillo variety strawberry. Picture 1 shows the water soaked spots on the leaves which as a matter of fact when seen in the low angle of light in the early morning show the angular formation of infection bounded by the venation of the leaf. Further, picture 2 shows the viscous exudates (reminiscent of pus) matched to these areas on the underside of the leaf. These exudates tend to be moist and lustrous in the morning, and dry down to a more scaly appearance as the day progresses.
These exudates come from the breakdown of the plant cell walls that this pathogen engenders, and the exudates are of course also filled with Xanthomonas bacteria cells. Given the size and proliferation of these disease propagule filled exudates, it doesn't take much imagination to realize how easily it is to move disease around, especially via rain and overhead irrigation.
The Guidelines state that cool, moist days (think rain) followed by cold nights near freezing are what favor this disease, so hopefully by now we are through most of the woods on this issue for this year.
Note on the leaf on the right the stark, angular shaped infected areas backlit by the low angle sun of the morning.
Impressive amount of oozing on the underside of a bacteria infected strawberry leaf. Photo shot with my iPhone 5 at 830 am, March 17.
Protecting California’s parsley crop
California parsley is produced typically in high volumes and with high quality. However in the past few years, growers began to observe unfamiliar disease issues in their parsley fields. Leaf spots, blighted foliage and yellowed plants contributed to loss of quality and reduced yields. Steven Koike and Oleg Daugovish, UC Cooperative Extension advisors in Monterey and Ventura counties respectively, stepped in to investigate the new parsley problems. They collaborated with farmers and pest control advisers to understand the extent of the problems and to obtain samples of the diseased crops.
The UC Cooperative Extension plant pathology diagnostic lab in Salinas was successful in isolating and identifying several pathogens that were responsible for causing the disease symptoms. Working with USDA, they found that three new diseases were present in California parsley crops: bacterial leaf spot, Stemphylium leaf spot, and Apium virus Y disease.
Two of these problems are seedborne, so future management will include the use of pathogen-free seeds. The Apium virus Y pathogen is found in weeds, so growers will need to remove poison hemlock, among others.
Previous to this research, some growers were spraying symptomatic fields because they believed that a disease called late blight was responsible for the disease symptoms. Growers have now ceased making these sprays, eliminating the use of unnecessary chemicals and saving costs.
Click here for more on this research.
Redbud Leaf Spots
Client Question:
The client brought a leaf sample into the Help Desk. The leaf was from a Redbud tree. It was covered with spots. The client wondered if we could help with diagnosis and treatment of the problem.
CCMG Help Desk Response:
The spots on the leaf are most likely caused by a fungus, Cercospora. The web page at http://bit.ly/ZePPMa was generated from a Google search of “cercospora leaf spot redbud” It has photos which you could check to see if they look like your problem.
The good news is that most fungal diseases, including Cercospora, will not seriously damage the tree, but it will not look its best at times, including the loss of some leaves. The best way to manage this is to give the tree good cultural care, as stressed trees are more susceptible to problems. Eastern Redbuds, being originally from the Eastern U.S., need regular water and good drainage, so you should water when the top 2 inches of soil are dry, but not so much that the soil stays wet. A several inch layer of mulch out to several feet past the drip line of the tree will help, but make sure the soil close to the trunk is clear of mulch. Also using drip or soaker type irrigation near the drip line will help; to keep the leaves dry, you should not use overhead watering. If the foliage is thick you can lightly prune out the tree to improve air flow, and you can prune off and discard any dead wood and badly affected leaves to remove some of the fungal material. Clean up fallen leaves and debris under the tree as the fungus can survive in fallen leaves and twigs. You should avoid over fertilizing, as new growth is more susceptible to diseases. Anti-fungal sprays are not usually recommended in the home garden and are not usually helpful.
Like most landscape trees, they sometimes have their minor problems, but with some minimal care, the Eastern Redbud is a beautiful and useful garden tree that can add significantly to your home landscape.
Contra Costa Master Gardeners Help Desk
Editor's Note: The Contra Costa Master Gardener Help Desk is available year-round to answer your gardening questions. Except for a few holidays, we're open every week, Monday through Thursday from 9:00 am to Noon at 75 Santa Barbara Road, 2d Floor, Pleasant Hill, CA 94523.
We can also be reached via telephone: (925) 646-6586, email: ccmg@ucanr.edu, and we are on the web at http://ccmg.ucanr.edu/
/span>/span>
Bacterial Leaf Spot of Lettuce: Request for Samples
Chronic problem. Bacterial leaf spot of lettuce has been affecting coastal California crops for many years and has become a chronic problem. The disease was first noted in California in 1964 and became an economic concern in the 1990s. Bacterial leaf spot now occurs to some degree every season. In addition, it is possible that new strains of the pathogen may cause disease in previously resistant lettuce cultivars. For these reasons researchers are continuing to study the problem and are now requesting samples from cases that occur in 2013.
Symptoms. Early symptoms of bacterial leaf spot are small (1/8 to 1/4 inch), water-soaked spots that usually occur only on the older, outer leaves of the plant. Lesions are typically angular in shape because the pathogen does not penetrate or cross the veins in the leaf. Lesions quickly turn black—this is the diagnostic feature of this disease. If disease is severe, numerous lesions may coalesce, resulting in the collapse of the leaf. Older lesions dry up and become papery in texture, but retain the black color. Lesions rarely occur on newly developing leaves. If disease is severe, secondary decay organisms (bacteria, Botrytis cinerea) can colonize the leaves and result in a messy soft rot of the plant. Bacterial leaf spot can occur on all types of lettuce: iceberg, romaine, leaf, and butterhead. See photos below.
Pathogen. Bacterial leaf spot is caused by Xanthomonas campestris pv. vitians. The taxonomy of this pathogen is unsettled and the name is likely to change in the next few years. This bacterium is a pathogen mostly limited to lettuce, though under greenhouse conditions several weeds in the same plant family can develop bacterial leaf spot disease when inoculated. We have not yet found naturally infected weeds showing leaf spot symptoms in the field. Some researchers indicate that X. campestris pv. vitians from lettuce can infect very different crops such as pepper and tomato when these plants are artificially inoculated; however, naturally infected pepper and tomato have never been found in California. Bacterial leaf spot disease of lettuce should not be confused with other Xanthomonas diseases. For example, bacterial spot disease of tomato and pepper is caused by a distinct pathovar (Xanthomonas campestris pv. vesicatoria); this pathogen will not infect lettuce. However, a related pathogen caused bacterial leaf spot on radicchio in California.
Disease cycle. The pathogen is highly dependent on wet, cool conditions for infection and disease development. Splashing water from overhead irrigation and rain disperses the pathogen in the field and enables the pathogen to infect significant numbers of plants. The pathogen can be seedborne, though the extent and frequency of seedborne inoculum is not currently known. If lettuce transplants are grown from infested seed, the pathogen may become established on plants during the greenhouse phase of growth. The bacterium can survive for up to five months in the soil. Therefore, infected lettuce plants and residues, once disked into the soil, can supply bacterial inoculum that can infect a subsequent lettuce planting. The bacterium has also been found surviving epiphytically on weed plants, though the significance of this factor is not known. In terms of time of year, a very consistent pattern of bacterial leaf spot outbreaks is documented for the Salinas Valley. There is almost an annual pattern in which severe bacterial leaf spot occurs in August and September. Researchers have not clearly documented why the disease consistently occurs at severe levels in this late summer period.
Control. Clearly the elimination or reduction of the use of overhead sprinkler irrigation will significantly curtail this disease in all situations, except when rains occur. Some resistant lettuce lines have been identified, though resistance is not widely available in currently used cultivars. Residual bacterial inoculum, left in the soil following an infected lettuce crop, will potentially cause problems for the next lettuce planting unless that planting is delayed for five months or longer. Therefore, crop rotation schemes will need to be evaluated if bacterial leaf spot is a chronic problem in fields heavily planted to lettuce. Effective foliar sprays have not been identified for this disease. Lettuce seed should be free of the pathogen.
Samples needed. Differences in pathogen genotypes have been demonstrated and correlated to disease responses on resistant and susceptible lettuce cultivars. In California the deployed lettuce germplasm is resistant to the strains of the pathogen collected many years ago in California. We therefore request samples of bacterial leaf spot disease so as to determine if novel, resistance-breaking strains are found in California. If you encounter this disease, samples can be submitted to the Cooperative Extension Diagnostic Lab in Salinas (1432 Abbott Street, Salinas).
LettuceXanthomonas2010a
LettXcv07d
Lettuce Xcv 2b