Phytophthora blight is caused by several species of Phytophthora. P. infestans causes leaf blight and fruit and tuber rot of potato and tomato, whereas P. capsici may damage all cucurbits, eggplant, pepper, and tomato plant parts at any stage of growth. P. parasitica causes a fruit rot of tomato, pepper, and eggplant that is commonly called buckeye rot. The Irish potato famine (1844 to 1846) bears stark evidence to the destructive potential of this disease. Favorable weather for blight development, and the almost total dependence of the Irish population on the potato for food, resulted in an estimated one million persons dead of starvation and in the emigration of over one and one-half million Irish, mostly to the United States and Canada. At present, Phytophthora blight, cuased by P. capsici, is considered the most important disease of cucurbits and peppers in the United States as well as in the world.    

Symptoms

Leaf symptoms are highly variable, depending on temperature, moisture, light intensity, and the plant variety. Typically, small, pale to dark olive green, angular to irregular, water-soaked spots appear first on the lower leaves of a few plants. Lesions may develop at the leaf tips or edges, causing young, expanding leaves to be misshapen. During cool, moist weather, these lesions rapidly expand into large, dark brown to purplish black dead areas. Under conditions favoring rapid spread, entire vines can be blighted and killed within just a few days. If leaves are inspected early in the morning during cool damp weather, a sparse, white, moldy growth may be seen, mainly on the undersides of leaves.

Early fruit infection symptoms include a grayish green or brown, water-soaked lesion that usually occurs where the fruit touches the soil. Buckeye rot lesions have a smooth surface with a diffusely defined margin, whereas late blight lesions develop a rough surface and sharply defined margin.

Potato tuber symptoms include small to large, irregular, slightly depressed, red to brown or steely purple surface lesions. A tan to dark reddish brown, dry, granular rot extends into the tuber.

In cucurbit fields, infection usually appears first in low areas of the fields where soil remains wet for extended period. Seedling damping-off, crown rot, foliar blight, and fruit rot are common symptoms caused by P. capsici on cucurbits. Crown rot causes the entire plant to collapse and die. Water-soaked lesions develop on vines. The lesions are dark olive in the beginning and become dark brown in a few days. The lesions girdle the stem, resulting in rapid collapse and death of foliage above the lesion site. 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, which expands, and becomes covered with fluffy white mold.  

Life Cycle

The late pathogen overwinters in infected tubers and in diseased tomato transplants from the South. The spores can be blown long distances during moist weather. Phytophthora is considered a water mold, meaning that it does well under high moisture conditions. Night temperatures of 50 to 60 F accompanied by light rains, heavy dew or fog, followed by day temperatures of 60 to 75 F with high relative humidity over a four- to five-day period are ideal for late blight development. Tubers often become infected when rain or irrigation water washes spores off the foliage and down into the soil to the tubers. Tuber rot develops within the first few weeks of storage even though the crop appears healthy going into storage.

P. capsici survives in soil for several years as thick-walled spores (oospores). Spores (sporangia) form when soil is moist and release motile spores (zoospores). Zoospores infect plants, and further disease development occurs rapidly because sporangia are produced abundantly on infected tissues and easily dispersed.  

Management

• Select fields with no history of Phytophthora and that have not been planted to a host crop for the last three years.
• If irrigation is used, irrigate in the early morning hours to reduce the amount of time that leaves are wet.
• Whenever possible, plant varieties that are resistant to Phytophthora blight. However, even the most resistant varieties will require some fungicide applications under persistently wet conditions. It is essential that each fungicide application provide thorough and uniform coverage of all aboveground plant parts.