Phytophthora Blight Phytophthora blight, caused by Phytophthora capsici, is one of the most serious diseases of cucurbits. This pathogen also infects eggplants, peppers, tomatoes, and more than 40 species of other crops and weeds. Symptoms. The infection usually appears first in low areas of the fields where soil remains wet longer. P. capsici causes seedling damping-off (Fig. 1), root and crown rot, leaf spots, vine lesions, foliar blight, and fruit rot. Crown rot (Fig. 2-A) causes the entire plant to collapse and die. Water-soaked lesions develop on vines. The lesions are dark olive in the beginning (Fig. 2-B) and become dark brown in a few days (Fig. 2-C). The lesions girdle the stem, resulting in rapid collapse and death of foliage above the lesion site (Fig. 2-C, D). Fruit rot can occur from the time of fruit set until harvest. Fruit rot also can develop after harvest, during transit or in storage. Fruit rot typically begins as a water-soaked lesion, expands, and becomes covered with fluffy white mold (Fig. 3). The pathogen produces numerous sporangia on infected fruit. Fruit infection progresses rapidly, resulting in complete collapse of the fruit. Phytophthora foliar blight and fruit rot may result in total loss of the crop. Disease Cycle. P. capsici survives as oospores (Fig. 4-A, B) in soil for several years. Sporangia (Fig. 4-C) form when soil is moist and release zoospores (Fig. 4-D). Zoospores infect plant, and further disease development occurs rapidly because sporangia are produced abundantly on infected tissues and dispersed. Disease Management. No cucurbit varieties with measurable resistance to Phytophthora blight are available. An effective rotation period has not been identified. Management practices currently include selecting fields with no history of Phytophthora blight, seed treatment, management of soil moisture, sanitation, and fungicide application. Seed treatment with mefenoxam prevents seedling infection. Well-drained fields should be selected and soil moisture should be kept as low as possible. Soil movement between fields should be avoided. Infected plants should be removed from the field and destroyed. Spray application of fungicides (e.g., dimethomorph) plus a copper fungicide controls foliar and fruit infection.

Fig. 1. Pumpkin seedling damping-off. Fig. 2. A, crown infection; B, a newly infected vine; C, a fully-developed lesion; D, a girdling lesion affecting a part of a vine. Fig. 3. Fruit rot: A, cucumber; B, pumpkin; C, acorn squash; D, watermelon. Fig. 4. A, oospores; B, an oospore; C, a sporangium releasing zoospores; D, sporangia and zoospores.