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.

White, diamond-shaped lesions of Plectosporium blight on pumpkin vine (photo by Francesca Rotondo)

We’ve had several reports this week of Plectosporium blight appearing in pumpkins in Ohio. Sometimes also called white speck, this is a disease of pumpkins and squash caused by the fungus Plectosporium tabacinum. The symptoms on vines, leaf petioles and veins on the back of the leaves are small, white and diamond-shaped lesions; on fruits the lesions are also small and white, generally

Advanced symptoms of Plectosporium blight on pumpkin vine (photo by Brad Bergefurd)

round. Under favorable environmental conditions (rainy, moderate temperatures) the lesions can coalesce, and the affected tissues appear white and become brittle. Plectosporium produces spores in the lesions that are dispersed long distances by the wind. The fungus survives in soil associated with plant debris for several years.

Plectosporium blight management requires an integrated approach that includes crop rotation, cultural practices and fungicide applications. Although pumpkin and squash varieties vary somewhat in susceptibility to this disease, none are resistant. In a study we conducted in 2018 at the OSU South Centers in Piketon, OH, the varieties ‘Hulk’, ‘Cronus’, ‘Warty Gnome’ and ‘Bayhorse Gold’ had less Plectosporium blight than other varieties in the trial.

Plectosporium blight lesions on pumpkin vine and handle (photo by Brad Bergefurd)

Plectosporium blight management:

1. Rotate out of cucurbits for 4-5 years.
2. Choose a site with good air circulation to allow plants to dry quickly.
3. Use drip irrigation instead of overhead irrigation.
4. Scout fields, looking for diamond-shaped lesions on leaf veins, vines and petioles within the canopy; if present, apply fungicides on a 7-day schedule.
   • Chlorothalanil (e.g. Bravo), the strobilurin fungicides such as Quadris, Quadris Top and Pristine, and Topsin M are the most effective fungicides against Plectosporium blight, but do not fully control the disease.  It is important to get good coverage of the fungicides well into the canopy.  Use high water volumes – at least 40 gal/acre. Strobilurin fungicides have translaminar activity and move through the leaves, improving coverage.

5. After harvest plow down the crop residue to encourage rapid decomposition.