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-E-A-B-C-D-E where A, B, C, D, and E 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.

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

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

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.

Cucurbit downy mildew map – click on map to enlarge it. https://cdm.ipmpipe.org/

Downy mildew was found on cucumbers in Haldimand-Norfolk, Ontario on July 1 and Chatham-Kent, Ontario on July 4. Even though it has been hot recently, cooler nighttime temperatures and cloudy skies can allow the spores of the downy mildew pathogen to survive during transport from source plants in Ontario on air currents, and to infect cucumbers and melons. The spores are usually deposited on plants during rainfall events. In our experience, we can find downy mildew in northern Ohio 1-3 weeks after outbreaks in these counties in Ontario, depending on the weather. Normally we only see Clade 2 isolates in northern Ohio during this time. Clade 2 isolates attack only cucumbers and melons e.g. cantaloupe, and circulate primarily in the Great Lakes region. Clade 1 isolates attack watermelon, pumpkins, squash and other cucurbits and work their way up the east coast from Florida and into the eastern parts of the Midwest. We see them mainly in southern and central Ohio in August or later in most years. The clades of the downy mildew pathogen differ somewhat in sensitivity to fungicides, so Ohio growers should use recommendations for fungicides from labs in this region.

Before symptoms appear in northern Ohio, cucumbers and melons should be protected with a chlorothalanil or mancozeb product. Fields should be scouted often; once downy mildew is reported in the northern Ohio counties, growers should add an effective downy mildew fungicide to their protectant (chlorothalanil or mancozeb) spray program. Good options for Ohio are Orondis Opti (FRAC 49+M05; contains chlorothalanil so don’t tank mix with additional chlorothalanil), Previcur Flex (FRAC 28), Elumin (FRAC 22), Ranman 4SC (FRAC 21), Omega (FRAC 29), and Zampro (FRAC 45+40). For more information on these fungicides, see here (OSU data until 2021) and here (Michigan State University 2022).

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 growers thanks to a grant from the Ohio Produce Growers and Marketers Association’s Ohio Vegetable and Small Fruit Research and Development Program.

Early symptoms of cucumber downy mildew. Photo by Raven Schaffter.

With the recent mild, cloudy weather, we could see an early arrival of downy mildew in northern Ohio.  Downy mildew is favored by moderate temperatures, overcast skies and rain. The pathogen, Pseudoperonospora cubensis, does not overwinter in northern regions and is introduced via sporangia (a type of spore) on wind currents from other areas. In Ohio and other Great Lakes states and Canada, the first “wave” of downy mildew likely originates in greenhouse production systems in the region. This wave rarely extends to central or southern Ohio. Over the years the first report of downy mildew on cucumbers in northern Ohio has been about 7-10 days before or after July 4. Melons are also susceptible to this pathogen “type” that occurs in the first wave. Pumpkin, squash, watermelon and other cucurbits are usually not affected. The second wave of this pathogen occurs later in the summer, with a variant of the pathogen arriving on wind currents from the south, affecting all cucurbits. This variant affects mainly southern and central Ohio, although it may spread north.

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

Cucumber and melon growers in northern Ohio should intensify scouting these crops and apply a protectant fungicide such as chlorothalanil. 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.  Once downy mildew is reported in the area, growers should ramp up their spray programs to include fungicides highly effective against downy mildew such as Orondis Opti, Ranman, Omega, Previcur Flex, and Elumin. More information on these fungicides can be found here.

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 C. Wayne Ellett Plant and Pest Diagnostic Clinic (CWEPPDC), now in Wooster. 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.

Verticillium wilt symptoms in high tunnel tomatoes. Photo by Anna Testen.

Tomatoes are prone to damage by many foliar, fruit and root diseases. While producing tomatoes in high tunnels protects them from diseases spread by rainsplash such as bacterial spot and Septoria leaf spot, diseases favored by high humidity, including  Passalora leaf mold and Botrytis grey mold, can be exacerbated. Producing tomatoes year after year in high tunnels often results in a slow decline over the years in crop productivity due to a buildup of soilborne tomato pathogens. In Ohio, corky root rot, black dot root rot, Fusarium wilt, Verticillium wilt and root-knot nematode are quite common. Our fact sheet describing these diseases can be found here. Our research has shown high levels of these diseases in Ohio high tunnel tomatoes.

Corky root rot symptoms in high tunnel tomatoes. Photo by Anna Testen.

If tomatoes have been produced year after year in the same place, even if there are no obvious aboveground symptoms, a few plants in the high tunnel should be dug up as the season comes to a close and their roots washed off and inspected. Healthy roots are abundant and white with an intact taproot and many smaller feeder roots. Plants with corky root rot, for example, may have rotted tap roots and banded dark or “corky” lesions.

What to do if tomato roots appear diseased?

First, find out which diseases/pathogens are present. The Ohio State University Vegetable Pathology Lab and the USDA-ARS Application Technology Research Unit in Wooster are offering free soil testing this Fall for soilborne tomato pathogens.  See the flyer here: Soil_testing_flier_1Sep2022

Secondly, consider management tactics for next season. Anaerobic soil disinfestation (ASD) is a non-chemical soil treatment that reduces the populations of soilborne plant pathogens. It is very suitable for conventional and organic high tunnels but should be done while the soil is fairly warm; in Ohio this means ASD treatment should begin by late September. More information on ASD can be found here (factsheet) and here (step by step video).

Once the pathogens have been identified, growers can also choose disease-resistant rootstocks and produce or purchase grafted seedlings. Information on rootstocks and propagators can be found here (see Resources).

CDM.ipmpipe map of pumpkin downy mildew reports, August 10, 2022

We have had a confirmed report today from Bill Holdsworth of Rupp Seeds of downy mildew on pumpkins in research plots near Wauseon, Ohio, in Fulton County. This is the first report of downy mildew on pumpkins, not only in Ohio but in the Great Lakes region and in fact a big swath of the Midwest and Northeast

CDM.ipmpipe map of cucumber downy mildew reports, August 10, 2022

(map on left). Compare this to the cucumber downy mildew map on the right, with widespread distribution. As a reminder, these maps are constructed from voluntary reports of downy mildew. The disease is likely more widespread than the maps show. Bill has also seen downy mildew on gourds in the area.

The downy mildew pathogen, Pseudoperonospora cubensis, exists as two clades. Clade 2 is quite specific to cucumbers and melons and tends to circulate early in the Great Lakes region, having possibly overwintered on cucumbers in greenhouses. It also makes its way up the Eastern US from southeastern states. Clade 1 has a broader host range, attacking all cucurbits, and also originates in the Southeast, moving up the through the eastern states. Due to typical wind currents from west to east, we don’t usually see clade 1 outbreaks in the Midwest until mid-August or September, when spores can be carried in storms that are remnants of hurricanes originating in the Southeast. We don’t know with certainty yet if the Fulton County outbreak was caused by clade 1 strains, but this is likely. We hope to conduct molecular tests to confirm the clade in the near future.

Downy mildew symptoms on a pumpkin leaf (top)

Downy mildew symptoms on the underside of a pumpkin leaf.

Downy mildew can be a bit harder to diagnose in pumpkins, squash and other Cucurbita species since the lesions may be smaller and not always crisply angular as in cucumbers. Downy mildew can also cause a lot of damage on pumpkins, squash, gourds and other Cucurbita spp.

Growers should be scouting all cucurbits for symptoms of downy mildew. The best time to do this is in the morning before 9 or 10 am before the lesions dry out and the sporangia disperse. If you find suspicious symptoms you may text me photos of the underside and top of symptomatic leaves (330-466-5249) and/or send us samples for confirmation.

Growers of any cucurbits throughout Ohio should apply appropriate protectant fungicides such as chlorothalanil (e.g. Bravo). Growers in Fulton County should begin a fungicide program with downy mildew-effective fungicides now. See my July 11 post for a list of recommended fungicides.

Downy mildew on basil

Downy mildew was spotted in a garden in Wooster, Wayne County, Ohio and confirmed by Dr. Francesca Rotondo, interim director of the OSU Plant and Pest Diagnostic Clinic. This is the first report in Ohio this summer but it has likely been here for a few weeks. The pathogen, Peronospora belbahrii, is related to but different from the cucurbit downy mildew (CDM) pathogen, Pseudoperonospora cubensis. These pathogens don’t cross-infect hosts: P. belbahrii does not infect cucurbits and P. cubensis does not infect basil. However, their biology is similar; they are both obligate parasites that require living plant tissues to survive. They disappear from the outdoor environment in northern areas during the winter and are introduced the following spring or summer from infected plant material or via spores carried on wind currents and rain. Cucurbits or basil grown over the winter in greenhouses can be a source of inoculum. Last year we found basil plants with severe downy mildew symptoms in a big box store in Wooster. We tend to begin

Light microscope image of sporangia of the downy mildew pathogen Peronospora belbahrii. Image by Francesca Rotondo.

seeing basil downy mildew in Ohio in July or August. Rainy, cloudy weather favors spore (sporangia) transport and infection; the sporangia are sensitive to UV light and tend to be killed by sunshine.

Management of basil downy mildew, like CDM, is entirely preventative. There are resistant varieties and a number of fungicides and biologicals are available for conventional and organic production systems.  Fungicides and biologicals are only effective if applied before infection. Home gardeners should keep an eye on their basil plants and harvest non-diseased leaves as soon as downy mildew is observed. More specifics can be found in last year’s July 17 post.