After a hot dry early summer in many parts of Ohio this year, growers may have thought that they escaped the scourge of Phytophthora blight, and for a while this appeared to be true. However, several intensive rainfall events during the past several weeks, some bringing as much as 10 inches of rain over a weekend, resulted in flooded soil conditions perfect for spread of Phytophthora blight. Unfortunately, this late in the season, growers have invested a lot of money in these crops and may see significant reductions in yield.
The cause of Phytophthora blight is Phytophthora capsici, a soilborne oomycete pathogen that thrives in rainy weather. It produces sporangia that release motile spores (zoospores) that are attracted to plants, then form a structure that allows them to infect, and aggressively attack any type of plant tissue. Sporangia and zoospores can be splashed onto leaves, stems and fruits during rain events and overhead irrigation. Phytophthora blight must be managed preventatively. This includes the use of resistant varieties (partially resistant varieties are available for pepper but not for cucurbits), cultural practices and fungicides.
We have received several reports of fruit infections of peppers and pumpkins this week. Once fruits have become infected with Phytophthora, nothing can be done to rescue them. Additionally, some lesions on pepper fruits may not be obvious initially but develop during shipping, putting healthy fruits at risk of infection. For pumpkins, if Phytophthora blight was detected in a field at any time during the 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.
Pepper and cucurbit fruits 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.
We thank the Ohio Produce Growers and Marketers Association’s Ohio Vegetable and Small Fruit Research and Development Program for financial support of the OSU PPDC.