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.

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 usually appears on pumpkins and other cucurbits in Ohio beginning in early July. The pathogen does not overwinter in Ohio; infections result from spores blown into the area on the wind.  Powdery mildew is favored by moderate to high temperatures and high humidity. However, unlike most other fungal plant pathogens, it is inhibited by free moisture on the leaf surface.

Signs of infection are small circular powdery growths on either side of the leaf. These spots enlarge and can eventually cover most of the leaf surface and kill the leaves. Stems and leaf petioles are also susceptible, but the disease is not observed on fruit. In pumpkins, powdery mildew may also attack the “handles”, which can be further damaged by secondary pathogens. It is time to start scouting cucurbits for powdery mildew.

Powdery mildew is managed using disease-resistant varieties and fungicides. Pumpkin and squash varieties vary in resistance to powdery mildew; in general, the more susceptible the variety, the more fungicide needed. The choice of fungicide is important because insensitivity to overused fungicides is common. It is critical that a fungicide resistance management program is followed. Alternate fungicides in different FRAC (Fungicide Resistance Action Committee) groups, indicating different modes of action against the fungus. Fungicide applications should begin when the disease first appears and incidence is low. Fungicides that are labeled for use against cucurbit powdery mildew can be found in the searchable Midwest Vegetable Production Guide for Commercial Growers.

OSU evaluations of efficacy of powdery mildew fungicides in Ohio in 2021 indicated that Aprovia Top, Luna Experience, Inspire Super, Rally, Miravis Prime, Luna Sensation, Microthiol Disperss, Vivando and Procure provided very good control of powdery mildew on pumpkins (see table below).  Velum Prime, Cevya, Prolivo and Gatten provided good control of powdery mildew on upper leaf surfaces but poor control on the lower surfaces.

Quintec provided good control in 2021 but in other years and other states has failed due to resistance. Fontelis, Bravo Weather Stik, Merivon Xemium, Pristine, and Torino have been shown to provide poor or variable control in Ohio or other states and are not recommended.

Always check label for full list of allowed crops and use recommendations and restrictions.

Alternaria leaf spot on cabbage

The OSU Vegetable Pathology Lab carried out an active field research program in 2021, with ten full field trials spread across three research sites in Wooster, Celeryville and Fremont, OH, and three bioassays for downy and powdery mildew management. We tested fungicides, biological control products, and disease-resistant varieties to manage diseases of tomatoes, peppers, cucumbers, pumpkins, cabbage and collards. Our vegetable disease diagnostic lab service for commercial growers, led by Dr. Francesca Rotondo, diagnosed 241 physical samples and 20 digital samples for Ohio growers at no cost. You can click here Plant Pathology Series 2022_Veg Pathology Research Rpts 2021_final to read the research trial and diagnostic lab reports. Most of these trial results will be published in 2022 in Plant Disease Management Reports (subscription required).

Research projects (those with an * were supported financially by the Ohio Small Fruit and Vegetable Research and Development Program):

*Tomato anthracnose – fungicide evaluation

Tomato black mold, Septoria leaf spot and anthracnose – fungicide evaluation

Pepper bacterial canker – evaluation of cultivar resistance

Pepper Pseudomonas leaf spot – evaluation of cultivar resistance

*Pepper anthracnose – fungicide evaluation

Cucumber downy mildew – evaluation of cultivar resistance

Cucumber downy mildew – fungicide resistance screening (bioassay)

*Pumpkin powdery mildew and Plectosporium blight – fungicide evaluation

*Pumpkin powdery mildew – fungicide resistance screening (bioassay)

*Pumpkin powdery mildew – OMRI-listed products evaluation (bioassay)

Cabbage Alternaria leaf spot and soft rot – fungicide and biological product evaluation

Cabbage white mold – fungicide and biological product evaluation

Collards black rot and peppery leaf spot- biological product testing

*Vegetable disease diagnostic report

Phytophthora blight on pumpkins received in October 2021 by the OSU Plant Disease Diagnostic Lab in Wooster. Photo by Francesca Rotondo.

This has been a rainy growing season in much of Ohio and other states east of the Mississippi River. With intense rain events leading to standing water in fields and stretches of hot weather, both favorable for Phytophthora capsici, the cause of Phytophthora blight, the disease has been such a problem that shortages of jack-o-lantern as well as pie pumpkins are expected this Fall.  If Phytophthora blight was detected in a field at any time this season, growers are advised to harvest mature, uninfected fruits as early as possible.  These fruits need to be laid out individually (not touching, so bins are not acceptable) in a shaded area with good ventilation so that they can cure.  A barn floor would be an ideal location since they would not get rained on, but outside under a tree (to prevent sunscald) would be better than nothing.  If putting them outside, do NOT put them on a tarp or plastic that would tend to hold rainwater and spread the disease to the other fruits.  If any of these fruits start to show signs of infection (discolored areas or white, cottony growth – see photo) remove them from the area immediately and discard them in an area away from the fields or curing location.

Dr. Meg McGrath of Cornell University found that hosing pumpkins off first to remove soil (using a garden hose with a trigger spray nozzle) was the second most important step in reducing disease incidence (getting them out of the field being the most important).  Washed fruits need to be dried as quickly as possible. Dipping fruit in 10% Clorox, GreenShield or Kocide was no better than just hosing them off, and these products are not labeled for this use.

Pumpkins (or other cucurbits) with symptoms in the field should be removed and destroyed away from the field and surface water sources. Leaving them in the field will contribute to inoculum buildup; if Phytophthora blight was present in a field, practice rotation of at least four years away from susceptible crops including all cucurbits, peppers, tomatoes, and beans.

Ron Becker contributed to an earlier version of this post in 2007. We thank the Ohio Produce Growers and Marketers Association’s Ohio Vegetable and Small Fruit Research and Development Program for financial support of our disease diagnostics program.

Downy mildew symptoms on untreated pumpkin leaves in Clark County, OH. Photo by Jim Jasinski.

Downy mildew was reported for the first time in pumpkins in Ohio in Clark County at the OSU Western Agricultural Research Station. Jim Jasinski noticed possible symptoms in an untreated control in a pumpkin research trial as many as 10 days ago, but the disease did not progress rapidly in last week’s heat. However, we were able to confirm downy mildew today. This is so far the furthest north that downy mildew has been found and reported on pumpkins or squash in the US. It is likely caused by Clade 1 of the pathogen, which comes from SE US and is more aggressive on pumpkins, squash and watermelons than other cucurbits.  Pumpkin and squash growers throughout Ohio should apply appropriate protectant fungicides now and scout fields thoroughly. Growers in Clark County should begin a fungicide program with downy mildew-effective fungicides now.

In the last week or so Ohio Master Gardener Volunteers in our Citizen Science project to monitor this disease have also reported downy mildew on cucumbers in Belmont, Jefferson and Geauga counties. Rainy or cloudy weather and moderate temperatures are expected this week in much of Ohio – these conditions are ideal for downy mildew.

For all cucurbits: If favorable conditions for downy mildew persist and downy mildew is present in your area, add effective fungicides shown in green in the Table to the spray program. Tank mix with a protectant fungicide, alternate fungicides with different modes of action (FRAC codes) and follow the label restrictions and requirements. Shorten the application interval to 7 days under favorable weather for downy mildew.

1. Forum, Presidio, Quadris, Zampro and Revus are not recommended. Curzate may be moderately effective in some locations.
2. Follow this blog, Twitter @OhioVeggieDoc or the Cucurbit Downy Mildew IPM PIPE for downy mildew reports.

Map of US counties with cucurbit downy mildew reports. https://cdm.ipmpipe.org/