Phytophthora blight of pepper is caused by the fungus Phytophthora capsici. Other names applied to this disease of peppers are damping off and Phytophthora root rot, crown rot, and stem and fruit rot. All of these names can apply since all parts of the pepper plant are affected. The disease has been responsible for serious losses. Other crops infected have included eggplant, tomato, summer and winter squash, and pumpkin. Other reported hosts include cucumber, watermelon, and honeydew melon. The pathogen involved in these latter crops may be Phytophthora parasitica or P. capsici. Phytophthora blight of peppers can attack the roots, stems, leaves, and fruit, depending upon which stage plants are infected. A grower not knowing what to expect might first encounter the disease at mid-season when sudden wilting and death occur as plants reach the fruiting stage. Early infected plants are quickly killed, while later-infected plants show irreversible wilt. Often a number of plants in a row or in a roughly circular pattern will show these symptoms at the same time. Fungus-infected seedlings will damp off at the soil line, but relatively few plants die when temperatures are cool. Far more commonly, the disease will strike older plants which then exhibit early wilting. Stem lesions can occur at the soil line and at any level on the stem. Stems discolor internally, collapse, and may become woody in time. Lesions may girdle the stem, leading to wilt above the lesion, or plants may wilt and die because the fungus has invaded the top branches before the stem lesions are severe enough to cause collapse.
P. capsici reproduces most rapidly in warm and wet or humid weather by producing millions of short-lived, lemon-shaped spores on the surface of infected plants. These spores can be splashed from the soil to plants, or between plants, and they can also be carried by moving water in a field. Each one can also release 20-40 mobile spores that can swim short distances through standing water or saturated soil towards plant roots. Both of these spores can be produced very rapidly, and only require the presence of a single isolate of P. capsici. A second type of spore with much thicker walls is produced inside infected plant tissue, and requires the presence of at least two isolates of P. capsici. All P. capsici isolates can be classified as either A1 or A2 isolates, and these thick-walled spores are only produced when A1 and A2 isolates grow close to each other. Although these spores are produced more slowly, they are very important to the life cycle of Phytophthora blight because they can survive for years in the soil until a susceptible crop is planted. For this reason, once these thick-walled spores are in the soil Phytophthora blight is there to stay. In FieldClimate.com we calculate the formation and infection by Oospores (sexual) and the formation and infection by Sporangia (asexual). Oospores need the presence of two isolates of P. capsici, are thick walled inside the plant tissue and are able to survive a long time periode, while sporangia are the asexual form, which are dispersed quickly.
Leaves first show small dark green spots that enlarge and become bleached, as though scalded. If the plant stems are infected, an irreversible wilt of the foliage occurs. Infected fruits initially develop dark, water-soaked patches that become coated with white mold and spores of the fungus. Fruits wither but remain attached to the plant. Seeds will be shriveled and infested by the fungus. Because of the wide host range and the various phases at which plants can be infected, refer to the table for clarification of the crops affected and the Phytophthora species involved.
The symptoms of buckeye rot of tomato consist of leathery tan or brown spots, often appearing as concentric rings or bands on green fruit. Lesions can appear on the shoulder or, more commonly, on the blossom end where the tomato has contact with wet soil. On butternut squash (and on several of the other crops listed with fruit symptoms) tan or brown lesions may give a banding affect or appear as large circular spots. Under humid conditions, white cottony mycelium and spores occur on the surface, and fruit are likely to rot quickly from secondary organisms.