Feature Story October 5 through October 31, 2000

Phytophthora Fruit Rot

Contributed by
M. T. McGrath
Long Island Horticultural
Research and Extension Center
Cornell University
Riverhead, NY 11901-1098

Phytophthora capsici Leonian and other Phytophthora spp. cause a fruit rot that often occurs together with a crown and root rot, especially in summer squash. Environmental conditions determine the relative importance of the two phases: crown and root rot has a greater impact on summer squash yield under the dry conditions of California, whereas fruit rot is more important in the humid eastern United States. Fruit rot is common in watermelon, although crown rot has not been observed. All cucurbits are susceptible; pumpkin and squash seem to be the most commonly affected.

Phytophthora fruit rot was first reported in the United States in Colorado and California in the late 1930s. It apparently occurred sporadically in most of the United States except California until the 1980s, when the incidence of both fruit rot and crown rot increased notably in Florida, Georgia, Michigan, and the northeastern states. The increase followed a hurricane in some areas.

In addition to fruit rot and crown rot, these fungi cause seedling damping-off, root rot, stem lesions, foliar blight and leaf spots (see Phytophthora Crown and Root Rot in the Compendium of Cucurbit Diseases).

Symptoms

Fig. 1. First indication of sporulation on the earlier water-soaked lesion of Phytophthora blight. Courtesy of M. T. McGrath. (Click image for larger view)

The initial symptoms of Phytophthora fruit rot are a water-soaked or depressed spot (Fig. 1). The underside of the fruit, where it is in contact with the ground, is often affected first. Symptoms also develop on the upper surface of the fruit, especially following rain or irrigation which provides splashing water for fungal dispersal. The fungus produces a white, yeastlike growth that contains many sporangia, especially under moist conditions (Fig. 2). This growth does not become very thick. Affected fruit can be completely covered by sporangia. Infections can progress rapidly until the fruit collapses (Fig. 3). Symptoms also develop rapidly after harvest.

Fig. 2. Phytophthora fruit rot of pumpkin, caused by Phytophthora capsici and other Phytophthora spp. Courtesy of M.T. McGrath. (Click image for larger view)		Fig. 3. Total collapse of pumpkin crop in low areas of field infected with Phytophthora. Courtesy of M.T. McGrath. (Click image for larger view)

Causal Organism

Descriptions of P. capsici and other Phytophthora spp. are given in the section on Phytophthora crown and root rot in the Compendium of Cucurbit Diseases.

Disease Cycle and Epidemiology

The fungus survives in soil between crops for at least 2 years, according to observations of disease occurrence in northern states. Soil moisture conditions are important for disease initiation. Sporangia form when the soil is at field capacity (within 24 hr, under controlled conditions), and they release zoospores when the soil is saturated (5–6 hr, under controlled conditions). Subsequently the disease can develop rapidly, because sporangia on affected fruit are produced and dispersed under a wider range of conditions.

Control

Rotation with nonhost crops is recommended. Other hosts are pepper, tomato, eggplant, cocoa, and macadamia. It is not known how long Phytophthora can survive in soil as oospores, which are believed to be the primary overwintering propagule, or as chlamydospores. Both mating types have been found in the United States.

An important control strategy is to manage soil moisture by selecting well-drained fields, avoiding low-lying areas, subsoiling, preparing dome-shaped raised beds for nonvining crops, and not overirrigating. Lengthening the time between furrow irrigations from 7 to 14 or 21 days is effective in California. Destroying disease foci in low areas at the start of disease development has been effective in commercial fields.

Fumigation and a preventive fungicide program initiated at the start of fruit formation may provide effective disease suppression when combined with these cultural practices.

Movement in soil on equipment is probably an important means by which Phytophthora has been spread between fields and may account for disease occurrence in fields with no history of susceptible crops.

Selected References

Bernhardt, E. A., and Grogan, R. G. 1982. Effect of soil matric potential on the formation and indirect germination of sporangia of Phytophthora parasitica, P. capsici, and P. cryptogea. Phytopathology 72:507-511.

Café-Filho, A. C., Duniway, J. M., and Davis, R. M. 1995. Effects of the frequency of furrow irrigation on root and fruit rots of squash caused by Phytophthora capsici. Plant Dis. 79:44-48.

Kreutzer, W. A., Bodine, E. W., and Durrell, L. W. 1940. Cucurbit diseases and rot of tomato fruit caused by Phytophthora capsici. Phytopathology 30:972-976.

McGovern, R. J., Jones, J. P., Mitchell, D. J., Pluim, R. A., and Gilreath, P. R. 1993. Severe outbreak of Phytophthora blight and fruit rot of cucurbits in Florida. (Abstr.) Phytopathology 83:1388.

McGrath, M. T. 1994. Fungicides provided insufficient suppression of Phytophthora fruit rot of cucurbits when disease pressure was high. (Abstr.) Phytopathology 84:1373.