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Phytophthora capsici infects more than 50 plant species in more than 15 families. Among the affected plants, cucurbits and peppers are the most susceptible hosts. The following disease lesson describes Phytophthora blight on cucurbits. Development of Phytophthora blight on other hosts is similar to that of cucurbits.

Symptoms and Signs

Phytophthora blight, caused by the oomycete plant pathogen Phytophthora capsici, can develop on cucurbit plants at any stage of development (Figure 1). The pathogen can infect seedlings, vines, leaves, and fruit. The infection usually appears first in low areas of the fields where soil remains wet longer (Figure 2).

Figure 1.

Figure 2.

Damping-off. Phytophthora capsici causes pre- and post-emergence damping-off in cucurbits under wet and warm [20-30°C (68-86°F)] soil conditions. In seedlings, a watery rot develops on the hypocotyl at or near the soil line, resulting in plant death (Figure 3). Mature plants show symptoms of crown rot (Figure 4). Post-emergence plant death is preceded by plant wilting: a sudden, permanent wilt of the plant without a change in color of the foliage (Figure 5). Leaf wilting progresses from the base to the extremities of the vines. Plants often die within a few days of the first symptoms expression or after soil is saturated by excessive rain or irrigation. The stems of infected plants turn light to dark brown near the soil line and become soft and water-soaked. Infected stems collapse and die. The taproot and lateral roots of infected processing pumpkin plants usually do not exhibit symptoms. Following death of the foliage, roots may give rise to new vines if environmental conditions become less conducive for disease development. Phytophthora damping-off may result in partial to total loss of the crop.

Figure 3.	Figure 4.
Figure 5.

Vine blight. Vines can be affected at any time during the growing season. Water-soaked lesions develop on vines (Figure 6, see arrows). The lesions are dark olive (Figure 6-A) and then become dark brown in a few days (Figure 6-B). Lesions girdle the stem, resulting in rapid collapse and death of foliage above the lesion (Figure 6-C, D; Figure 7).

Figure 6.

Figure 7.

Leaf symptoms. Phytophthora capsici can infect both the petioles and the leaf blades of plants. Dark brown, water-soaked lesions develop on petioles (similar to lesions on vines), resulting in rapid collapse of the petiole and leaf death. Infected leaf blades develop spots ranging from 5 mm (0.2 in.) to more than 5 cm (2 in.) in diameter (Figure 8). Infected areas are chlorotic at first, but within a few days they become necrotic with chlorotic to olive-green borders. Under wet and warm conditions, leaf spots expand rapidly, coalesce, and may cover the entire leaf. Under dry conditions, leaf spots cease to expand.

Figure 8.

Fruit rot. Fruit rot can occur at any time from fruit set until harvest. Fruit rot generally starts on the site of the fruit that is in contact with the ground (Figure 9-B and Figure 10). However, occasionally infections will begin in other locations on the fruit where infected leaves or vines come into contact with a fruit (Figure 9-C). Also, symptoms on the upper surface of the fruit develop following rain or overhead irrigation, which can splash water containing the pathogen onto neighboring plants. Fruit rot also can develop after harvest, during transit or in storage. Fruit rot typically begins as a water-soaked lesion (Figure 9-A). Lesions expand, and become covered with white mold (Figures 9-11). The pathogen produces numerous sporangia on most infected fruit (Figures 9-11). Fruit infection progresses rapidly, resulting in complete collapse of the fruit (Figure 9-D). Phytophthora foliar blight and fruit rot may result in total loss of the crop.

Figure 9.	Figure 10.
Figure 11.

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