Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Excerpted from articles by Dr. Mary Hausbeck, Michigan State University, and the Mid-Atlantic Commercial Vegetable Production Recommendations.
Under wet conditions, another major issue in some vegetable crops is disease caused by the organism Phythophthora capsici. Death of plants in wet or waterlogged soils may be due to this disease organism which can survive in soils for many years. It is most common in fields with a history of production of susceptible vegetables including vine crops (cucurbits), tomatoes, peppers, eggplants, and lima beans. Phytophthora capsici can cause root or crown rots in peppers, summer squash, winter squash, and pumpkins which will be evidenced by wilting, black or brown colored roots, or blackened stems at the crown area at the soil line. This is most common under warm conditions (above 75°F) in wet soils. Tomatoes and cucumbers are not as susceptible to root and crown rots although they can occur. However, they are susceptible to Phytophthora fruit rots (as are watermelons and cantaloupes). Early plantings of cucumbers and summer squash are at the most risk at this time for fruit infections.
The following are some recommendations for control from the Mid-Atlantic Commercial Vegetable Production Recommendations:
Peppers
In peppers, Phytophthora blight typically develops in low-lying areas of fields after rain and can spread quickly throughout the entire field. Planting on a ridge or raised, dome-shaped bed will help provide better soil drainage. Use a minimum 3-year crop rotation with crops other than peppers, cucurbits, lima and snap beans, eggplants, or tomatoes. In fields with low-lying or wet areas, plant only Phytophthora-tolerant cultivars such as ‘Paladin’, ‘Aristotle’, or ‘Revolution’. In heavily-infested fields with a known history of Phytophthora blight, plant only resistant or tolerant cultivars to help reduce plant losses. The pathogen Phythophthora capsici has developed resistance/insensitivity to mefenoxam in some fields. If mefenoxam-insensitivity is known to exist in a field/farm, plant only tolerant cultivars. Do not apply mefenoxam or metalaxyl in fields where insensitivity is known to exist. Preplant fumigants will also suppress disease.
For control of the crown rot phase of Phytophthora blight, apply one of the following: mefenoxam- (Ridomil Gold or Ultra Flourish), metalaxyl (MetaStar) at transplanting and 30 days later. Other labeled fungicides include Presidio and Ranman. Ranman may be applied via transplant water – see label for restrictions. When using polyethylene mulch, apply Ridomil Gold, Ultra Flourish, Ranman, or Presidio at the above rates and timing by injection through the drip irrigation system. Dilute prior to injecting to prevent damage to injector pump. Only apply Ridomil Gold 4SL at transplanting and 30 days later.
Squash
For squash, again multiple practices should be used to minimize the occurrence of this disease. Rotate away from susceptible crops (as above). Preplant fumigants will also suppress disease. Fields should be adequately drained to ensure that water does not accumulate around the base of the plant. Mefenoxam (Ridomil Gold or Ultra Flourish) or metalaxyl (MetaStar) should be applied pre-plant for early season control. Once the canopy closes, subsoil between the rows to allow for faster drainage following rainfall. When conditions favor disease development, apply one of the the following with fixed copper at labeled rates (for suppression only): Revus, Ranman, Presidio, Forum, Gavel, or Tanos. Materials with different modes of action (i.e. FRAC codes) should always be alternated to reduce the chances for fungicide resistance development. Presidio may also be applied through the drip irrigation (see label for details). Soil drench followed by drip application has given good results in some trials on crown rot caused by Phytophthora capsici.
Cucumbers
For cucumbers, again practice good rotation and improve field drainange. Fungicides labeled for crown and fruit rots include Revus, Ranman, Presidio, Forum, Gavel, or Tanos which should be applied with a fixed copper fungicide. Materials with different modes of action (i.e. FRAC codes) should always be alternated to reduce the chances for fungicide resistance development. Presidio may also be applied through the drip irrigation (see label for details). Soil drench followed by drip application has given good results in some trials on crown rot caused by Phytophthora capsici.