Phytophthora symptoms include stunting, crown rot and fruit rot depending on the cucurbit species and the time of infection. The crown rot phase is the most destructive. Dark-green, “water soaked” lesions form on the crown or vines, girdling it and causing the entire plant to turn yellow or brown and die. (Figures 1 and 2).
Fruit infections begin as a large water-soaked lesion. Under humid conditions, a thin layer of white spores develop on the surface of the fruit. These spores resemble a fine dusting of powdered sugar (Figure 3). In pumpkins, the lesions frequently are circular in shape, while in cucumbers the spores often cover the entire fruit.
Phytophthora is a water mould. It thrives under wet soil conditions caused by excessive rainfall, poor drainage or even compaction. Fields with short rotations are at high risk of developing phytophthora. The spores survive in the soil for long periods of time. Other host crops include the solanaceous crops (peppers, eggplant, tomatoes and potatoes.)
Products registered for phytophthora in cucurbits include Zampro (ametoctradin/dimethomorph) and Presidio (fluopicolide). Apply fungicides before disease symptoms appear, when the risk of infection is high based on local soil and weather conditions.
In Michigan, phytophthora spores have been found in irrigation ponds and streams. See: The Potential for Spread of Phytophthora Blight of Cucurbits and Peppers in Irrigation Water
The most effective way to manage this disease is through early identification and crop rotation. While a 3-yar rotation is normally sufficient for most cucurbit diseases, fields infested with phytophthora may require a longer (4-5 year) rotation. If you suspect phytopthora in a cucurbit crop field, contact OMAFRA or your local agri-business.
 Suppression only. Refer to the product label for rotational crop restrictions.