We are seeing many reports of yellowing and/or wilting squash, pumpkin and other cucurbits in commercial fields and gardens this month. Chances are that the cucurbits have been affected by cucurbit yellow vine decline (CYVD) or bacterial wilt. Both of these diseases are caused by bacteria transmitted to plants during the feeding of their insect vectors. Once infected, the plants cannot recover; these diseases must be managed preventatively by controlling the insect vectors, ideally early in the season. At this point, insecticides may be applied to prevent the diseases from spreading to healthy plants. It is also useful to remove and destroy symptomatic plants that serve as sources of bacterial inoculum.

Sticky bacterial ooze from a cucumber vine with bacterial wilt

Bacterial wilt of melon

Bacterial wilt affects many cucurbits including cucumber, melon, pumpkin and squash. The causal agent, Erwinia tracheiphilia, overwinters in the digestive system of spotted and striped cucumber beetles.  When these beetles emerge in the spring, the pathogen is spread from beetle feces to healthy cucurbits mainly via wounds caused by insect feeding.  Bacterial wilt occurs almost every summer in Ohio, but is less severe after very cold winters that reduce overwintering beetle populations. Symptoms begin as discoloration and wilting of individual leaves.  As the disease progresses, the entire plant begins to wilt and collapse as the bacteria clog the xylem vessels.  When the stem is cut along the base, clear to white elastic strands comprised of the bacteria and “gum” are visible when cut ends of stems are slowly pulled apart. More details can be found here. Insecticides labeled for cucurbits and effective against the beetles can be found in the Midwest Vegetable Production Guide for Commercial Growers. Covering plants with netting or floating row covers until flowering can also protect plants from early infection.

Cucurbit yellow vine decline in summer squash

Cucurbit yellow vine decline in pumpkins.

Cucurbit yellow vine decline (CYVD) is caused by the bacterial pathogen Serratia marcescens, transmitted by squash bugs. It is uncommon in some years but in others can do a lot of damage. Bright yellowing of leaves, followed by by wilting and death of plants is indicative of possible CYVD. If squash bug adults, nymphs or eggs are found on the underside of leaves, this is a good clue that the symptoms are caused by CYVD. A cross-section of the vine may show a light tan discoloration of the vascular tissues. CYVD is managed by applying insecticides (see Guide link above). For both CYVD and bacterial wilt, fields and gardens should be scouted regularly for the insect vectors beginning soon after transplanting or seedling emergence.

The OSU Plant Disease Diagnostic Clinic in Wooster can provide a definitive diagnosis of CYVD using a PCR assay.  Testing for this and other diseases and pests is free of charge for Ohio commercial vegetable growers thanks to financial support from the Ohio Produce Growers and Marketers Association.

Eggs of the squash bug

Cross-section of squash vine with light browning of the vascular system caused by the bacteria

Reports of cucurbit downy mildew as of July 21, 2022. cdm.ipmpipe.org

A new report came in this week of cucumber downy mildew in Fulton County, as well as another report from Medina County, this time from a different farm in the Homerville area.  Downy mildew is favored by cooler conditions so despite many hot days, nights were cool enough for the pathogen to get a foothold in northern Ohio. In addition, we have seen a number of storms across the area; spores of the pathogen travel by air and clouds protect them from UV light, while rain drops them to earth.

Cucumber and melon growers in northern Ohio should scout fields for downy mildew and apply appropriate fungicides on a 7-10 day schedule, depending on the fungicide. See this post for fungicide recommendations.

Many thanks to Bill Holdsworth and Frank Becker for reporting these outbreaks.

Cucurbit downy mildew incidence report, July 20, 2022. https:/cdm.ipmpipe.org

Downy mildew was reported on cucumbers in NE Ohio (Medina and Wayne counties) and southern Ontario last week, suggesting widespread occurrence possible in northern Ohio. This morning Marty Hofbauer, CCA and Agronomist at Luckey Farmers, Inc., discovered downy mildew in fresh market cucumbers near Tiffin, Ohio in Seneca County. Marty sent me excellent photos of leaf lesions taken with his smartphone that allowed me to confirm downy mildew without the need to send in a sample. By taking the pictures early in the morning, the sporulation on the undersides of leaves was clearly visible in young lesions. Sometime about mid-late morning the sporangia will be discharged. Older lesions tend to dry out and are not necessarily diagnostic for downy mildew. Several of his pictures are included here as examples of what to look for.  Angular leaf spot, caused by a bacterium, also makes angular lesions, but the grey fuzzy growth and black dots (sporangia) on the lower side of the lesion is diagnostic for downy mildew.

Cucumber downy mildew – young lesions. Photo by Marty Hofbauer.

Underside of cucumber downy mildew lesion showing sporulation of the pathogen. Photo by Marty Bauer.

I continue to be amazed at the increasingly high quality and resolution of smartphone images. For a disease like downy mildew, where quick diagnosis and treatment is crucial, taking high high quality pictures that can be sent to us for confirmation saves both time and money. Ohio cucurbit growers, consultants and others may send me pictures such as these by text (330-466-5249) or email (miller.769@osu.edu) for diagnosis. Physical samples may be sent to Dr. Francesca Rotondo in the OSU Vegetable Pathology Lab – see submission instructions here. Vegetable and fruit diagnostics are free to Ohio growers due to financial support from the Ohio Produce Growers and Marketers Association’s Ohio Vegetable and Small Fruit Research and Development Program.

Cucumber downy mildew July 13, 2022, Wayne County, OH. Photo by OSU diagnostics intern Vansh Khatri.

Quick on the heels of our report of cucumber downy mildew on Monday in Medina County, we have diagnosed the disease on cucumbers in the Fredericksburg area of Wayne County. With thunderstorms rolling across northern Ohio this afternoon, we expect the disease to spread to cucumbers and melons throughout this part of the state. See my post on July 11 for fungicide recommendations.

Reported cucurbit downy mildew, July 13, 2022. http://cdm.ipmpipe.org

Cucurbit downy mildew map, July 11, 2022. cdm.ipmpipe.org

Today OSU plant diagnostician Francesca Rotondo diagnosed the first cucumber sample of the 2022 growing season with downy mildew. Downy mildew has been a bit slow to appear, likely due to the high temperatures and often sunny and dry conditions. Last year our first report of cucumber downy mildew in Ohio was on July 12. I expect that conditions last week – rain and overcast skies in northern Ohio -promoted spore transport, deposition and infection. The sample came from an organic farm in the Homerville area, in Medina County – it was just getting started in the field and disease incidence and severity were low. Cucumber and melon growers in northern Ohio should ramp up their spray programs to include highly effective fungicides against downy mildew such as Orondis Opti, Ranman, Omega, Previcur Flex, and Elumin (see efficacy table below). Curzate was effective in our 2021 tests but has been variable in efficacy over the years. It is recommended to tank mix these products with chlorothalanil. Check the labels carefully for use instructions and restrictions. Remember to alternate products in different FRAC groups. Fungicides must  be applied preventatively – they  are far less effective if applied after  infection.

Growers in central Ohio should intensify scouting of cucumbers and melons and apply a protectant fungicide. Look for yellow or tan angular lesions delimited by veins on the top surface of leaves, and fuzzy grey/brown growth on the undersides of the lesions. With a good hand lens or a smartphone camera with high magnification you may be able to see small dark brown/purple spots within the fuzzy growth. These are the spores of the downy mildew pathogen.

Smartphone image of a downy mildew lesion with the pathogen sporulating on the underside of a cucumber leaf.

If you suspect downy mildew in cucumber or melon please text or email pictures to Sally Miller (330-466-5249; miller.769@osu.edu) of both sides of lesions, with the underside in the highest possible magnification. I can often confirm downy mildew from photos, but if not will ask you to send a sample to the OSU Vegetable Disease Diagnostic Lab for confirmation. Instructions for sample submission are here. Thanks to financial support from the Ohio Produce Growers and Marketers Association’s Ohio Vegetable and Small Fruit Research and Development Program, there is no fee for this service for Ohio vegetable growers.

An old post updated:

Septoria leaf spot has been found in Ohio recently, not surprisingly after heavy rains in many areas. This excellent photo taken by Dr. Francesca Rotondo is diagnostic for Septoria leaf spot: round tan to brown spots on the leaves and leaf yellowing. In the more mature spots, margins are dark brown and small round black dots can be seen in them through a hand lens or the lens of your smartphone camera. The tiny black dots are called pycnidia, the fruiting bodies of this fungus. Pycnidia are flask-shaped structures with a small hole at the top, partially submerged in leaf tissue. Pycnidia contain large numbers of spores held in a gelatinous matrix; when humidity is high or free water is on the leaf surface, the spores ooze out of the pycnidia like toothpaste being squeezed out of a tube. The spores are dispersed by rain or irrigation water to other leaves on the same plant and to other nearby tomato plants.

Septoria lycopersici is seedborne and also survives at least 1-2 years in soil. Septoria leaf spot is favored by moderate temperatures, high humidity and rain or overhead irrigation. While Septoria does not cause spots on tomato fruit, it can rapidly defoliate the plant. If this happens early the plant is likely to die. Later on, defoliation leads to small fruit, poor ripening and problems with sunscald. Even large, previously healthy, vigorous plants can be completely defoliated.

Commercial growers can manage Septoria leaf spot by including a strobilurin fungicide such as Quadris or Cabrio in a fungicide program that also includes a protectant such as chlorothalanil or mancozeb. A list of labeled fungicides to manage Septoria leaf spot can be found in the Midwest Vegetable Production Guide for Commercial Growers. Cultural practices to minimize Septoria leaf spot should be undertaken, including crop rotation of 3 years or more, planting tomatoes on raised beds, using adequate spacing, using drip irrigation, pruning foliage to allow good air movement through the canopy, and removing diseased plants from the field. Septoria leaf spot is rarely a problem in high tunnels, greenhouses and other structures that protect plants from rain.

Organic growers need to follow the cultural practices described above and may consider growing tomatoes in protected culture. Copper-based fungicides formulated for organic production can suppress disease development if applied soon after initial symptoms appear.

Home gardeners should adopt the cultural practices described above and should also remove and destroy leaves with symptoms. This is really only effective when symptoms first appear. Always avoid the foliage when watering plants. Fungicides containing chlorothalanil or copper can be applied to slow disease spread.

Blossom end rot of bell pepper

The very hot temperatures and dry periods over the last month have been stressful for peppers and tomatoes, resulting in the appearance of blossom end rot. While blossom end rot, as the name implies, typically occurs on the blossom end of tomato fruits, in some cases (see figure) it occurs on the shoulder and sides of the tomato fruit. In peppers it occurs commonly on the sides of the fruit but can appear anywhere on the fruit surface.

Blossom end rot is the result of plant stress brought on by periods of dry vs moist soil.  Calcium deficiency in the plant is the cause but applying calcium to the foliage won’t help.  Calcium is relatively insoluble and plants under stress can’t move it to flowers and developing fruit.  It is a vital component of plant cell walls and the matrix that holds the cells together. When fruits start to form without sufficient calcium the tissues soften and die.  Secondary molds often colonize the dead tissue.

Severe case of blossom end rot of a green tomato, starting at the blossom end.

Blossom end rot of a green tomato.

The disorder is especially disappointing to home gardeners, because fruits with blossom end rot are often the first to ripen but usually inedible. I don’t recommend cutting off the affected parts and consuming the rest due to the risk of mold growth and presence of potential toxins to which some people may be sensitive.

Blossom end rot becomes less problematic with more consistent soil moisture and as the plants grow and develop their root systems.

Thanks to Carri Jagger for the blossom end rot of pepper photo.

Powdery mildew colonies on the underside of a pumpkin leaf. Fungicide applications should start when these colonies are first observed during scouting. It is important to check both surfaces of the leaves. Photo by Josh Amrhein.

Powdery mildew will soon be appearing on squash, pumpkins, and other cucurbits.  Organic growers should always start with varieties with some degree of resistance to powdery mildew – seed catalogues often call partial resistance “tolerance”.  Although resistance will generally not be complete, efforts to manage powdery mildew with organic-acceptable products will be more productive if growers start with a variety that can put up a fight on its own than one that is highly susceptible.

In 2021 we evaluated  OMRI-approved fungicides for efficacy against powdery  mildew  in our standard bioassay. Young pumpkin plants were produced in a greenhouse, one plant per pot, sprayed with a fungicide, and taken to a pumpkin field with active powdery mildew.  After about  24 hours of exposure, the plants were returned to the greenhouse and evaluated for disease development 7 and 10 days after exposure.

Seven days after exposure (DAE) to powdery mildew in the field, disease severity was moderate (24.2%) on non-treated control bioassay plants. All of the OMRI-listed products significantly reduced powdery mildew severity compared to the non-treated control. Regalia, Milstop, and Microthiol Disperss reduced disease severity to <1%, providing 96-100% control, significantly lower than severity on plants treated with Badge X2 (7.8%; 68% control) or Serifel (9.5%; 61% control). Powdery mildew severity on pumpkin plants treated with Sonata (4.2%; 83% control) was statistically similar to that of plants treated with Milstop (1%; 96% control), Badge X2 and Serifel. Ten DAE to powdery mildew, disease severity on non-treated control plants increased to 42.1%. Powdery mildew severity was lowest on plants treated with Regalia (0.6%) and Microthiol Disperss (0%), followed by Milstop (9.8%), and Sonata (23%), corresponding to 99, 100, 77 and 45% control, respectively. Treatment with Serifel or Badge X2 did not significantly reduce powdery mildew on pumpkin plants 10 DAE compared to the non-treated control.

Best results are obtained when these products are used preventatively or at the very first signs of powdery mildew (see photo above), usually in mid-July in Ohio.  If you wait until powdery mildew has progressed significantly, it will probably be too late to get it under control.