Phytophthora Fruit Rot of Watermelon

This disease is more likely to develop during periods of heavy rains in relatively poorly drained soils. June started out dry for many areas of Indiana, however recent rains increase the likelihood of Phytophthora diseases.

Phytophthora fruit rot of watermelon causes large, soft areas to develop on mature watermelon fruit. These lesions can be several inches across and are often covered with a white mold. The lesions usually form first on the bottom of the fruit, close to where the fruit comes into contact with the soil. Further development of the disease often results in lesions on the top of the fruit as well.

The first application of a systemic fungicide for this disease should occur when watermelon are about softball stage. Since Phytophthora does not usually affect the foliage of watermelon, there is no need to apply fungicides for this disease until fruit are present. Applications to small fruit may include Forum® or one of the products with phosphorous acid as an active ingredient (e.g., Agri-Phos®, Phostrol®, Kphite®). If conditions are conducive to disease, subsequent applications may include Presidio® which can be alternated with Revus®.  A newer product, Zampro®, has also proven effective. An earlier article describes the product Orondis® that should be effective during conducive conditions.

Phytophthora diseases can affect several vegetable crops. This article is primarily aimed at early applications of systemic fungicides to watermelon. Later articles will cover other facets of this disease. More information about Phytophthora fruit rot of watermelon can be found here https://vegcropshotline.org/article/phytophthora-fruit-rot-of-watermelon/. More information about the fungicide Oronids can be found here https://vegcropshotline.org/article/new-fungicide-for-vegetables-3/.

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