Early symptoms of downy mildew on a cucumber leaf. Image by Francesca Rotondo.

Downy mildew was found on cucumbers in central Wayne County, OH today. This is a bit early for downy mildew in this state, indicating that unless we experience an extended dry spell, downy mildew disease pressure may be significant this growing season. Don’t be fooled by the hot weather – the downy mildew pathogen prefers moderate temperatures, but as long as there is rainfall to drop the spores onto one’s crop and high humidity, the pathogen does just fine. It hides in the crop canopy on the underside of leaves, which protect them from the sun. Moderate nighttime temperatures provide the opportunity for the pathogen to infect the leaves.

Underside of cucumber leaf showing sporulation of the downy mildew pathogen. Image by Francesca Rotondo.

Management recommendations are cited in yesterday’s post.

Oval sporangia (spores) of the downy mildew pathogen, Pseudoperonspora cubensis. Micrograph by Francesca Rotondo.

Downy mildew on cucumber in Medina County, OH, July 2, 2024. Photo by Frank Becker.

Downy mildew was found today in cucumbers in Medina County, OH. Dr. Mary Hausbeck also reports today the first siting of cucumber downy mildew in Michigan (Washtenaw County).  Growers in northern Ohio counties should apply downy mildew-targeted fungicides (Orondis Opti, Ranman, Omega, Previcur Flex, and Elumin) posted here and here in a spray program alternating fungicides with different modes of action/FRAC group.  Follow all label instructions. Cucumber growers in other Ohio counties should scout their fields intensively and protect their cucumber crops with protectant fungicides such as chlorothalanil. Melons are also susceptible to the early strain of the pathogen that has appeared in northern Ohio, so these growers should also protect their melon crops with downy mildew targeted fungicides.

If you see suspicious symptoms in cucumbers or melons, you may send (or drop off) samples to the OSU Plant and Pest Diagnostic Clinic in Wooster. There is no fee for this service for Ohio growers thanks to contributions from the Ohio Produce Growers and Marketers Association’s Ohio Vegetable and Small Fruit Research and Development Program.

I retired December 31, 2023 from OSU after 32.5 years as a professor and State Extension Specialist in Vegetable Disease Management. It

Sally Miller and Andres Sanabria

was a joy and a pleasure to work with OSU colleagues, farmers in Ohio and far and wide, students, and the amazing vegetable pathologists across the country. I am pleased that I will be succeeded by an excellent plant pathologist, Dr. Andres Sanabria Velazquez. Andres is a native of Paraguay, and received his M.Sc. in plant pathology at OSU in the Vegetable Pathology Lab. He then went on to NCSU for his Ph.D and postdoc in plant pathology. He will start his work at OSU in January 2025. In the meantime, Dr. Francesca Rotondo, Director of the OSU Plant and Pest Diagnostic Clinic, will look out for diseases of vegetable crops in Ohio. Please also check out OSU Fruit, Vegetable, and Specialty Crop News for posts from the Veg Team.

Bell pepper fruits with Phytophthora blight, received in late August 2023 by the OSU Plant and Pest Diagnostic Clinic. Photo by Francesca Rotondo.

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

After a hot dry early summer in many parts of Ohio this year, growers may have thought that they escaped the scourge of Phytophthora blight, and for a while this appeared to be true. However, several intensive rainfall events during the past several weeks resulted in flooded soil conditions perfect for spread of Phytophthora blight. Unfortunately, this late in the season, growers have invested a lot of money in these crops and may see significant reductions in yield.

The cause of Phytophthora blight is Phytophthora capsici,  a soilborne oomycete pathogen that thrives in rainy weather. It produces sporangia that release motile spores (zoospores) that are attracted to plants, then form a structure that allows them to infect, and aggressively attack any type of plant tissue. Sporangia and zoospores can be splashed onto leaves, stems and fruits during rain events and overhead irrigation. Phytophthora blight must be  managed preventatively.  This includes the use of resistant varieties (partially resistant varieties are available for pepper but not for cucurbits), cultural practices and fungicides.

We have received several reports of fruit infections of peppers and pumpkins this week. Once fruits have become infected with Phytophthora, nothing can be done to rescue them. Additionally, some lesions on pepper fruits may not be obvious initially but develop during shipping, putting healthy fruits at risk of infection. For pumpkins, if Phytophthora blight was detected in a field at any time during the 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.

Pepper and cucurbit fruits 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.

[Updated from previous posts. Information on fungicides to manage Phytophthora blight in peppers and pumpkins is available in previous posts.]

We thank the Ohio Produce Growers and Marketers Association’s Ohio Vegetable and Small Fruit Research and Development Program for financial support of the OSU PPDC.

Basil plants with severe downy mildew symptoms in a big box store garden center in Wooster, OH, 2021. Photo by F. Rotondo.

We received a report on August 8 from an Ohio Master Gardener of sweet basil downy mildew in a garden near the  OSU main campus in Columbus, Franklin County. We often see this disease in Ohio for the first time in late July or early August, although our reporting, aside from sentinel plots in northwest and central Ohio, depends on the growing and gardening communities giving us a heads up when observed. So, downy mildew may have been present in Ohio earlier than this first report. The pathogen, Peronospora belbahrii, does not overwinter in Ohio and arrives most years on air currents from the south. The pathogen is also seedborne and has been introduced earlier on basil seedlings and transplants in nurseries and retail stores as we observed in 2017 and 2021. For future plantings, there are sweet basil varieties now available with good resistance to downy mildew. These include Prospera Compact DMR, Prospera DMR, Prospera Red DMR, Rutgers Devotion DMR, Rutgers Obsession DMR, Rutgers Passion DMR, and Rutgers Thunderstruck DMR, available from a number of seed companies as organic or non-organic seeds. Resistance in the varieties may break down under severe disease pressure from favorable weather conditions (cool, overcast, high humidity, rainy) and high inoculum levels, so crop protectants may also need to be applied. A detailed listing of fungicides and biologicals registered for downy mildew management was published recently by Dr. Andy Wyenandt, Rutgers University.

Downy mildew was also reported on cucumbers in Huron County, Ohio this week.

Partial map of 2023 cucurbit downy mildew reports as of August 7, 2023. Counties in red indicate new reports (<7 days), while counties in green have older reports. cdm.ipmpipe.org.

Bill Holdsworth of Rupp Seeds confirmed a severe outbreak of downy mildew on pumpkins in his field trials in Fulton County, Ohio today. Downy mildew was extensive in this field, indicating that it had begun some time ago, perhaps a week or two. Last year, Bill reported pumpkin downy mildew in the same county on August 10. It was likely caused by Clade 1 of the downy mildew pathogen Pseudoperonospora cubensis, which has a broad host range among cucurbits, preferring pumpkins, squash and watermelons, but also infecting cucumbers and melons. Clade 2 isolates infect and cause damage to cucumbers and melons, and are seen in northern Ohio first. As in 2022, the Fulton County outbreak on pumpkins was unusual for the Great Lakes region. There are no reports of downy mildew on cucurbits other than cucumber in the Midwest, Northeast or Canada at this time – although not being reported doesn’t mean it is not there. We have not seen downy mildew on pumpkins, squash, melons or watermelons in sentinel plots in Sandusky and Franklin Counties, and Jim Jasinski, OSU Extension, has not found downy mildew in scouted pumpkin fields on OSU’s Western Agricultural Research Station in South Charleston.

Downy mildew can kill cucurbit foliage, including that of pumpkins, which will stop the fruit from maturing unless it is controlled preventatively. Pumpkin, squash, and watermelon growers should be applying at least a protectant fungicide such as chlorothalanil (Bravo, Eqqus, etc.) and amp up scouting efforts. Growers in Fulton and surrounding counties should apply downy mildew-targeted fungicides (Orondis Opti, Ranman, Omega, Previcur Flex, and Elumin) posted here and here in a spray program alternating fungicides with different modes of action/FRAC group.  Follow all label instructions.

Bacterial spot on pumpkin – can be mistaken for downy mildew.

Downy mildew symptoms on a pumpkin leaf (top)

If you suspect cucurbit downy mildew you can 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. It is harder to confirm downy mildew in pumpkins and squash from photos because lesions are less distinctive and sporulation is less than in cucumbers.  So you may need to send a sample to the OSU C. Wayne Ellett Plant and Pest Diagnostic Clinic (CWEPPDC) in Wooster for confirmation. Instructions for sample submission are here. Digital images may also be sent to the CWEPPDÇ.

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.

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.

[Updated from 2022 post] Powdery mildew usually appears on pumpkins and other cucurbits in Ohio beginning in early July, but this year it appears to have come in a little later. The pathogen, Psudoperonospora cubensis, 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. Scouts observed a small number of powdery mildew colonies about 2 weeks ago on squash in our downy mildew sentinel plot at OSU’s North Central Agricultural Research Station in Fremont.

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.

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 (rule of thumb: at least one leaf of 50 scouted). 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 in color 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.

Jim Jasinski, OSU Extension, has been running field trials in central Ohio for many years to assess fungicide efficacy against powdery mildew on pumpkins. Some effective fungicide combinations, based on 2021 and 2022 data, are shown in the second table. Some of these fungicides were not effective in our bioassays (table in color), but when paired a broad spectrum protectant like Manzate may prove more effective than when applied alone. Most experts suggest adding a broad spectrum protectant fungicide like Manzate to more powdery mildew-targeted fungicides to reduce the risk of fungicide development, boost fungicide efficacy and protect the crop from other diseases.

Former Buckeye Dr. Andy Wyenandt (Rutgers Univ.) has suggested that spray programs for cucurbit powdery mildew should contain multiple fungicides with different modes of action/FRAC numbers. He suggest a schedule of A-B-C-D-A-B-C-D where A, B, C, and D represent fungicides from different FRAC groups applied at weekly intervals. A protectant fungicide like Manzate should be tank mixed with the powdery mildew-targeted fungicide. This program is primarily to reduce the risk of resistance to any single fungicide, but it will also provide some cover if the pathogen has come in already resistant to one of the fungicides in the program.

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

Effective Fungicide Treatments to Control PM in Pumpkin in Ohio in 2021 and 2022.

*sequential applications for research only, must be rotated per label for grower use.

Map of 2023 cucurbit downy mildew reports. Red = reported <1 week ago; Green = reported > 1 week ago. cdm.ipmpipe.org

Downy mildew is now confirmed in cucumbers in five Ohio counties: Sandusky, Fulton, Medina, Wayne and Knox. The latest report, from Sandusky County, was from our sentinel plot on OSU’s North Central Agricultural Research Station (NCARS) in Fremont. Our interns Raven and Audrey scouted the week before and saw no symptoms or signs of downy mildew. One week later, downy mildew symptoms and signs were present on leaves of every cucumber plant in the sentinel plot (see photos).  This illustrates the explosive potential of this disease on a highly susceptible host like cucumbers and the necessity

Downy mildew on cucumbers in the OSU Sentinel plots at NCARS, Fremont, OH, July 24, 2023. Photo by R. Schaffter.

Underside of cucumber leaf showing downy mildew lesions and signs of the pathogen, Pseudoperonospora cubensis. NCARS, July 24, 2023. Photo by Raven Schafter.

of managing it preventatively.  This is mainly done through applications of appropriate bound spectrum fungicides such as chlorothalanil and mancozeb plus specific fungicides (e.g. Orondis Opti, Elumin, Ranman) targeted against the downy mildew pathogen and other oomycete pathogens. See previous recent posts on management tactics for cucurbit downy mildew.

Map of reported cucurbit downy mildew outbreaks. New for this year in Ohio: Medina, Wayne and Knox counties. cdm.ipmpipe.org.

Severe cucumber downy mildew in Medina County, OH, July 24, 2023. Photo by Frank Becker.

Cucumber downy mildew was confirmed yesterday in three Ohio counties (Wayne, Medina and Knox), following a report last week from Michigan and several weeks ago from Ontario. While these reports are later in July than average for Ohio, it is likely that infections occurred at least a week earlier. Thanks to OSU’s Plant Pest and Disease Clinic Director Dr. Francesca Rotondo and Wayne County Extension Educator Frank Becker for these finds. Knox County is in central Ohio, so I’ll amend my July 19 post to include cucumber and melon growers statewide, who should add fungicides very effective against downy mildew to their spray program now if they have not already done so. Waiting until symptoms appear may be too late to avoid yield losses; effective fungicides should be applied preventatively. The best ones, according to research in Ohio, Michigan  and other Great Lakes states and provinces are Orondis Opti (FRAC 49+M05), Ranman (FRAC 21), Omega (FRAC 29), Previcur Flex (FRAC 28), and Elumin (FRAC 22). These should be tank mixed with chlorothalanil (Bravo, Equus, etc.) or mancozeb (Dithane, Manzate, etc.). Orondis Opti is a premix already containing chlorothalanil, but at a reduced rate.  Fungicides have restrictions on how much product can be applied and how often, so follow the label. The more effective fungicides should be rotated to avoid resistance development in the pathogen.  More information can be found here and here.

See my June 24, 2023 post for pictures of symptoms and instructions for submitting live or digital samples to OSU for diagnosis. Diagnosis is free for Ohio vegetable growers thanks to a grant from the Ohio Produce Growers and Marketers Association’s Ohio Vegetable and Small Fruit Research and Development Program.

Downy mildew of cucumber.

Downy mildew was confirmed today in Saginaw County, MI. Dr. Mary Hausbeck’s team at Michigan State University also detected spores of the downy mildew pathogen, Pseudoperonospora cubensis, in air samples in several MI counties.  Although we haven’t seen or heard of downy mildew on cucumbers yet this year in Ohio, it is likely already here or soon will be. Cucumber and melon growers in northern Ohio should start adding fungicides very effective against downy mildew to their spray programs now. These are Orondis Opti, Ranman, Omega, Previcur Flex, and Elumin. More information on these fungicides can be found here.