Cucurbit Downy Mildew Outbreaks Increasing

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

  1. Northern and Central Ohio cucumber and melon growers: add effective fungicides shown in green in the table to the spray program now. Tank mix with a protectant fungicide such as chlorothalanil (Bravo, Equus, etc.), 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.
  2. Southern and Central Ohio cucurbit growers (all types): apply a protectant fungicide on a 7-10 day schedule now.

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.

  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.

Click to enlarge.

Wayne County IPM Notes from July 19 – 23

Vegetable Crops

Powdery mildew found on a cucurbit plant in a Wayne County field.

The Vegetable Pathology Lab at OARDC has confirmed several more cases of downy mildew, on both cucumbers and cantaloupe. It is important to take steps to either protect your crop or stop the spread of any ongoing infections. Powdery mildew is also spreading rapidly through the area. Although some heavy rains may have slowed its spread, favorable conditions have led to some fields rapidly becoming infected.

Flea beetles feeding on young green cabbage plants.

Bacterial diseases continue to spread in pepper and tomato plantings. Pay close attention to these crops in particular, and make sure that you are taking the necessary precautions so as to not spread bacterial diseases. Bacteria can be spread from plant to plant via clothing, equipment, or animals. More from APS

Flea beetles are feeding heavily on recently planted cole crops, which left uncontrolled can cause stunted and underperforming plants. Another insect we have seen quite a few of is the squash vine borer. Although these are not typically going to harm large numbers of plants, they can still be a nuisance, especially in smaller plantings.

Small Fruit and Orchards

 This week we found our first incidence of scab in apples. While this was only an isolated find on a few leaves, it is a good reminder to take some time to scout your apple trees and look for any signs of scab. Oriental fruit moth numbers were significantly above threshold again this week. Japanese beetles were also

Severe damage from Japanese beetles feeding on the foliage of apple trees.

still feeding heavily in many of the fruit crops we scout. Spotted wing drosophila are still being found in all of our traps, and for anyone with small fruit in the area, it is recommended that you treat for SWD.

Cucurbit Downy Mildew Continues to Spread in Northern Ohio

Downy mildew map. 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.

Cucumber downy mildew – photo by J. Amrhein.

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. Photo by F. Rotondo.

Basil Downy Mildew Spotted in Wayne County, Ohio

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.

Wayne County IPM Notes from the Week of July 12th – July 16th

Vegetable Crops

Of most importance, the Vegetable Pathology Lab in Wooster confirmed Downy Mildew on a cucumber plot at OARDC. Cucumber growers are highly encouraged to begin taking action to protect their plants, especially as more cases are confirmed around the area.

Squash vine borer on a pumpkin plant. Tommy Becker photo.

Japanese beetles are out in force this year and continue to be one of the most consistent insect pests from week to week on a wide range of crops. Other insect pests of note included Colorado Potato Beetles that have migrated off of harvested potato plantings in into tomato and eggplant plantings. In squash, we have still been finding quite a few squash bug egg masses. Squash Vine Borers have also been spotted in some area pumpkin plantings.

Colorado Potato Beetles feeding on a tomato plant. Tommy Becker photo.

During these heavy rains, we have noticed a significant amount of soil splashed up onto the plants and fruit. This will likely encourage more disease incidence. Accordingly, take extra time and care to scout your crops in the coming weeks.

Sweet corn pests like corn ear worm and European corn borer are not showing much activity in our traps. We occasionally find damaged tassels from ECB feeding, however, we have yet to have any fields go over the 10% damage threshold.

Small Fruit and Orchards

 Between last week and this week, we have seen a sustained flight of oriental fruit moth in area peach blocks as our traps have been well over threshold for the last two weeks. Our codling moth traps still do not show much activity.

Over ripe Lodi apple that had spilt following a heavy rain.

We did find some interesting things while scouting apples this week, including blister spot on some “Delicious” apple varieties and Lodi apples that had burst and fell off the trees due to being over ripe.

As a note for all small fruit growers, all of our traps for SWD in the area are currently catching SWD, therefore, we recommend you treat your small fruit. Pay close attention to the label, especially the REI (re-entry interval) and PHI (pre-harvest interval). Another note for all fruit crops, Japanese beetles are feeding on grapes, apples, peaches, and blueberries. The beetles can do significant defoliation as well as damage to the fruit.

Cucumber Downy Mildew Confirmed in Northern Ohio

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.

Wayne County IPM Notes for the Week of July 5 – July 9

Vegetable Crops

High tunnel tomatoes were one of the areas of focus for disease development this past week. Many high tunnel tomatoes are currently experiencing cases of leaf mold. As a side note on high tunnel tomatoes, we did observe a thrips outbreak in a high tunnel, where heavy feeding by the thrips was causing significant discoloration of the foliage and stunting of the plants. Peppers took the other majority of the focus with disease development, mainly due to favorable conditions for bacterial diseases to manifest. Several pepper samples tested positive for bacterial infection.

Flea beetles seemed to make a comeback this week. Several fields of cole crops were over threshold for flea beetle counts. Japanese beetles are continuing to feed in the majority of the crops planted in the area. Other insects like squash bugs are starting to be found more frequently, specifically in early planted summer squash.

Sticking with cucurbits, we are starting to find spots of powdery mildew in plantings of zucchini and yellow squash. It is important to scout your crops and look for powdery mildew, especially if you have succession plantings of young cucurbit crops nearby. Powdery mildew is easily spread onto younger plants, so it is recommended that you keep up with spraying for powdery mildew and terminate the older infected plantings once you are done harvesting.

Small Fruit and Orchards

 First and foremost, we have found spotted winged drosophila in the Wayne County area. The threshold for SWD in small fruit is 1 fly. Since we are now finding adults, we encourage growers to also do salt tests on the berries to check for larvae.

Wooly apple aphids continue to show up in the several apple orchards in the area. These are a tough pest to get under control due to their protective waxy coating.

OFM counts in some peach blocks were above threshold for the first time in several weeks. OFM counts remained well below threshold.

Addendum: More Powdery Mildew Fungicides for Cucurbits

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.

Managing Mildews in Home Garden Cucurbits

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).

Cucurbit Powdery Mildew Management Recommendations

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.