Phytophthora Blight on Pumpkin Fruits

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.

 

 

New Reports of Downy Mildew on Pumpkins and Cucumbers in Ohio

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/

Cucurbit Downy Mildew – One Day Update

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 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.

Cucurbit Downy Mildew Continues to Spread in Northern Ohio

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 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.

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.

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.