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Scientific Research

Phytophthora Fruit Rot – A Menace to Watermelon Production
Bill Jester, Horticultural Science Department and Gerald Holmes, Department of Plant Pathology, North Carolina State University

An ugly profit-robbing disease reared its head in 2003 on the east coast of the United States.  The Eastern North Carolina watermelon production region experienced unusually high rainfall in the months of June, July and August. The rains were frequent and the ground had very little time to dry out.  Over 2,000 acres of watermelon were affected by Phytophthora fruit rot.  Watermelon growers in Eastern NC who raised pepper and squash knew the ravages of the disease, caused by Phytophthora capsici, and were surprised to find another crop on its list of victims.

Diseases caused by Phytophthora capsici are not new. This fungus was first described on pepper in New Mexico in 1922. At least 49 economic species can be infected. Among the major hosts of this organism are cucurbits (watermelon, cantaloupe, honeydew melon, cucumber, squash, gourd, etc.) and solanaceous vegetables (pepper, tomato and eggplant).

Watermelon fruit rot caused by P. capsici is not a new problem to North Carolina either. We have seen this disease repeatedly with increasing severity over the last twenty years. In watermelon the disease only affects the fruit and has not been observed on roots, crowns, stems or leaves.  The disease often starts on the lower part of the melon where fruit makes contact with soil. The fruit rot stage can occur anytime from fruit set through harvest.  Splashing water, either from rain or overhead irrigation, disperses the pathogen and can result in numerous infections on the upper fruit surface. Infected fruit can rot after harvest, during transit or in storage.

Watermelon fruit rot begins as one or numerous small water-soaked depressed lesions, often on the bottom of the fruit (Fig 1.).  The lesions spread and coalesce; eventually the fruit is covered with a white, yeast-like growth that contains great masses of spores (Fig. 2).  Lesions often show a concentric ring pattern at the margin.  The infected fruit soon collapse and disintegrate, and in the process produce fungal spores for another round of infections..

The disease can develop between a wide range of temperatures (52-95F). Generally the disease is more destructive during the higher temperatures of July and August.
P. capsici loves water.  Soil moisture conditions are the most important factor for disease development. Spores form when the soil is at field capacity for 24 hours. Another spore type (a swimming spore) is released when soil is saturated for only six hours. The disease is usually associated with heavy rainfall, excessive-irrigation, or poorly drained soil. Over irrigation increases the incidence of the disease.

Control is difficult and usually involves using a number of practices. One practice is to select fields without a history of Phytophthora blight. Rotation for three years or more with nonhost crops may be beneficial. Perhaps the most important control strategy is water management (i.e., preventing extended periods of high soil moisture). Growers’ selection of well-drained fields is imperative.  Low-lying areas prone to flooding should not be planted.  Avoid spreading the pathogen to uninfested fields by cleaning farm equipment of soil before moves.   It is important to avoid irrigation water that is contaminated with water from infested fields. Scout fields for fruit with symptoms of Phytophthora blight and destroy these areas to prevent the spread of the disease.

Work is underway at North Carolina State University with the support of the National Watermelon Growers Association to assess the effectiveness of control using various fungicide treatments in different cultural regimes.  Fungicide effectiveness against Phytophthora blight of watermelon has not been tested before.  Because the disease primarily attacks fruit, controlling it may be simpler in watermelon than in pepper or squash where the entire plant is susceptible.

Fig. 1. Early symptom of Phytophthora blight in watermelon: water soaked lesion where fruit contacts soil.
Fig. 2. Large Phytophthora blight lesion covered with spores.  Note the concentric rings at the margin.

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