Cucurbit downy mildew outbreaks in Ohio counties. Red color indicates reports less than 7 days ago. https://cdm.ipmpipe.org
Cucumber downy mildew, Hardin County OH. Photo by Francesca Rotondo.
Just after posting yesterday, July 30 I received a confirmed report of downy mildew in cucumbers in Hardin County. See that post for recommendations.
Cucurbit downy mildew outbreak map. https://cdm.ipmpipe.org.
We have three new reports of downy mildew on cucumbers this week, all from home or community gardens in Coshocton, Muskingum and Geauga Counties. Thanks to OSU Extension educators David Marrison and Clifton Martin for the finds in Coshocton and Muskingum counties. The Geauga County outbreak was reported by one of our Ohio Master Gardener Volunteers participating in a multi-state Citizen Science project to monitor and report cucurbit downy mildew (thanks Annie!). Numerous reports in other states show that downy mildew is on the move (see map), mostly still in cucumbers and melons. In Ohio the cucumber/melon outbreaks have been in the northern and central counties, likely caused by the downy mildew “pathotype” that circulates in the Great Lakes region and infects only cucumbers and melons. However, last week downy mildew was reported on squash and pumpkins in Central Kentucky and on squash in Western New York. The downy mildew “pathotype” that infects squash and other cucurbits moves up into the Midwest from the deep South, so Ohio growers, especially in the southern counties, should step up scouting of all cucurbits and apply protectant fungicides such as chlorothalanil products. When downy mildew is confirmed nearby and conditions are conducive (cool, rainy, humid, overcast), more effective fungicides should be applied.
Spray Program Recommendations for Ohio Cucurbit Growers
If favorable conditions for downy mildew persist and downy mildew is present in your area, add effective fungicides shown in green in Table 3 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.
Click to enlarge.
Cucurbit downy mildew map, July 21, 2021. https://cdm.ipmpipe.org
After last week’s very wet weather, downy mildew has appeared on cucumbers in a research plot on the OSU Muck Crops Research Station (Huron County) and on cucumbers and melons in sentinel plots on the OSU CFAES Wooster campus (Wayne County) and the OSU North Central Agricultural Research Station in Fremont (Sandusky County). In addition, downy mildew continues to spread in commercial fields in Seneca County. Downy mildew is likely to be present in cucumbers and/or melons in other northwest and northcentral Ohio counties and growers should protect these crops with appropriate fungicides. Fungicides must be applied preventatively – once plants are infected they will show symptoms within ~5-7 days even if fungicides are applied after infection. Only the fungicide Curzate has moderate “kick-back” activity.
OSU is part of a multi-state cucurbit downy mildew monitoring project, for which this year we have enlisted the help of Master Gardener Volunteers (MGVs) as Citizen Scientists in counties throughout Ohio. Most (25) are monitoring cucumber and squash plants in their gardens, while six groups of MGVs are managing and monitoring full sentinel plots. We have had no reports of downy mildew on cucurbits outside of northern Ohio and none in squash, pumpkins, gourds, or watermelons in any part of Ohio. We will continue monitoring and reporting downy mildew occurrences as they happen.
If you suspect cucurbit downy mildew on your farm or in your garden, please send pictures (as close-up as you can) of the tops and undersides of affected leaves to me by text (330-466-5249) or email (miller.769@osu.edu). Please include your name and the name of the town or city nearest you.
Downy mildew sporulating in a lesion on the underside of a cucumber leaf.
Downy mildew lesion on the upper surface of a cucumber leaf.
Basil plants with severe downy mildew symptoms in a big box store garden center in Wooster, OH. Photo by F. Rotondo.
This is shaping up to be a summer of downy mildews in Ohio. Rainstorms, high humidity, overcast skies and cool-ish nighttime temperatures are very conducive to downy mildew occurrence and spread. We reported the first “sightings” of cucumber downy mildew last week in Seneca and Wayne Counties in Ohio, and today OSU diagnostician Dr. Francesca Rotondo spotted a very severe outbreak of downy mildew in potted basil in a big box store garden center in Wooster (see photos). The pathogens that cause downy mildew in cucurbits and basil are different and don’t cross-infect among the two different plant groups. However, they do respond similarly to environmental conditions.
Basil plants with severe downy mildew in a garden center in Wooster, OH.
The garden center manager in Wooster was notified and will have the diseased basil plants removed and destroyed promptly.
Management options for commercial growers are 1) resistant varieties and 2) fungicides applied preventatively. Dr. Andy Wyenandt and colleagues at Rutgers University recently published an excellent discussion of available management tactics, including those for conventional and organic commercial basil producers. Information is also available on the Basil Ag Pest Monitor website, including photos of symptoms, management options and a map of basil downy mildew reports.
Management options are limited for home gardeners – resistant varieties are the most viable choice. In established garden basil plantings of susceptible varieties, monitor plants closely and remove and destroy leaves and stems with downy mildew symptoms. This may or may not slow disease progress, depending on environmental conditions and the presence of inoculum in nearby plantings. Leaves without symptoms are safe to eat, so if downy mildew appears in one’s garden basil, it may be time to make pesto or a nice Caprese salad before the disease spreads.
Cucurbit downy mildew occurrences in the eastern US. https://cdm.ipmpipe.org.
Downy mildew was reported on pickling cucumbers in Seneca County today. The field was heavily damaged, with about 90% of the plants showing symptoms. We also found one confirmed downy mildew lesion in our sentinel plot in Wooster (Wayne County). These are the first reports for 2021 of cucurbit downy mildew in Ohio, following an outbreak in Chatham-Kent, Ontario reported on June 17. Growers in northern Ohio should protect cucumbers and melons with effective downy mildew fungicides such as Orondis Opti, Ranman, Omega, Elumin, Previcur Flex or Zing!. Remember to alternate products in different FRAC groups and tank mix with a chlorothalanil product or other protectant fungicide – except for Zing!, which contains chlorothalanil in the premix. Check the labels carefully for use instructions and restrictions. 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.
Thanks to Jim Jasinski for providing more info on fungicides for powdery mildew that he has tested on cucurbits. The following table is amended from my post on July 6, 2021 to include three additional effective fungicides: Miravis Prime, Luna Experience and Luna Sensation. Please read the July 6 post for more details on how and when to use these products.
Downy mildew of cucumber.
Downy mildew and powdery mildew are very different diseases of cucurbits but they have in common that they are obligate pathogens that require a living plant for survival and thus don’t overwinter in Ohio. Downy mildew appears from late June to August in northern Ohio (but not observed yet!) and August to September for central and southern Ohio. Powdery mildew appears without fail in the first half of July throughout Ohio. Both mildews are managed by resistant varieties when available and fungicides. For gardeners, fungicide options are limited. For downy mildew the best option is a chlorothalanil product applied preventatively. You can follow me on Twitter @OhioVeggieDoc for alerts on downy mildew appearance in Ohio, or check this blog or the national cucurbit downy mildew forecasting site: https://cdm.ipmpipe.org.
Advanced powdery mildew on pumpkin.
Powdery mildew will show up on pumpkins and other cucurbits soon if not already here. Fungicides should be applied as soon as powdery mildew is observed. Sulfur products are most effective of those listed. Potassium bicarbonate and Serenade Garden also have some efficacy against powdery mildew. Note these are all protectants and only work on the leaf surface. Therefore full coverage is needed throughout the canopy (top and bottom of leaves, petioles).
Powdery mildew colonies on upper leaf surface.
Powdery mildew normally appears on pumpkins and other cucurbits in Ohio in early July. 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
Powdery mildew colonies on lower leaf surface.
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 Midwest Vegetable Production Guide for Commercial Growers; product ratings and FRAC codes are on page 11.
Our evaluations of efficacy of powdery mildew fungicides in Ohio in 2020 indicated that Aprovia Top, Inspire Super, Gatten, Vivando and Procure provided very good control of powdery mildew on pumpkins (see table below). Microthiol Disperss was not tested in 2020 but historically has provided good control. Quintec and Fontelis provided moderate (53-65%) control and Bravo Weather Stik, Merivon Xemium, Pristine, Torino and Rally provided poor control. The bioassay full report can be found here (pp. 15-16).
A list of products for powdery mildew management in organic cucurbits prepared by Dr. Meg McGrath of Cornell University can be found here.
Numerous intense rainstorms that have moved across Ohio during the 2021 growing season so far have created conditions for the establishment and spread of bacterial diseases. Unseasonably cool weather in early June plus rain favored leaf spots and blights caused by Pseudomonas spp. such as bacterial leaf spot of parsley and peppery spot of crucifers. In late June we are seeing bacterial diseases favored by rain and warmer weather, caused by Xanthomonas spp., such as
Cabbage black rot.
black rot in crucifers and leaf spot of radish. These bacteria enter vegetable crops through infested seeds and seedlings, overwintered crop debris or weeds. Epidemics tend to be more severe when started at the seed or seedling stage. Other bacterial diseases to watch for this summer are bacterial canker of tomato and pepper (Clavibacter), bacterial wilt of cucurbits (Erwinia), yellow vine decline of cucurbits (Serratia), and various soft rots (Pectobacterium).
Microscopic image of bacteria streaming from a leaf lesion.
Bacterial disease symptoms can be similar to those caused by certain fungi or oomycetes, so an accurate diagnosis should be obtained before applying fungicides or bactericides. When in doubt, growers may send samples to us for diagnosis. A simple microscopic evaluation for bacterial streaming from diseased tissue allows us to separate bacterial diseases from others (see image, right). 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 commercial vegetable growers.
Unfortunately, bacterial diseases are very difficult to manage in susceptible varieties at the field stage if rainy conditions persist. The go-to treatment for bacterial diseases in the field has been a fixed copper product with or without mancozeb. However, copper application has never been highly effective due to its inability to reach the inside of plants, relatively low residuals and sensitivity to pH. It is not uncommon for bacterial populations in a leaf with lesions to exceed 100 million bacterial cells per gram of leaf tissue. Copper compounds in currently available products are too large to enter lesions and thus high levels of inoculum are protected. In addition, our research and studies conducted in other states have shown high levels of resistance to copper in Xanthomonas spp. causing bacterial spot of tomato and pepper. Other bacterial species also develop copper resistance.
So what can be done if a bacterial disease is found in an open-field vegetable crop? If it is early in the epidemic and few plants with symptoms are observed during scouting, various non-copper-based products can be applied preventatively. These products are listed in my July 18, 2020 post. Many of these products, falling in three categories of 1) plant resistance inducers, 2) antimicrobials and 3) antibacterial viruses have been shown to be partially effective under low- to moderate-disease pressure. If disease pressure is high, and rainy, humid conditions persist, these products are not likely to control a bacterial disease epidemic.
Now that we know cucumber downy mildew has appeared in the Great Lakes Region (Ontario report – see my post on 6/17/2021), cucumber and melon growers should prepare for fungicide applications. There are a number of products labeled for downy mildew management in cucurbits, but not all are consistently effective due to development of fungicide insensitivity in downy mildew pathogen populations.
The OSU Vegetable Pathology Lab conducts bioassays each summer to assess the efficacy of fungicides commonly used to manage cucurbit downy mildew. We treat potted young cucumber plants with fungicides individually at the labelled rate or with water alone, and the next morning place the plants in a cucumber field with active downy mildew. Depending on the year, tests were conducted in Wayne, Sandusky and Huron counties, all in northern Ohio. After 48 hrs of exposure the plants are returned to the greenhouse and rated for symptom severity 7 days later. The results for 2018, 2019 and 2020 are shown in the table.
Click to enlarge.
The fungicides Previcur Flex, Ranman, Omega and Orondis were highly effective against cucumber downy mildew for all three years. Zing! and Elumin were only tested in 2018 and 2019/2020, respectively, and both were highly effective. One or more of these fungicides should be part of a downy mildew management program.
Curzate, Forum, Presidio, Quadris and Zampro were inconsistent across years and locations and were ineffective in a least one location in one of three years. Revus was consistently ineffective.
Spray Program Recommendations for Northern Ohio Cucumber and Melon Growers