How to Manage Pests

UC Pest Management Guidelines


Bacterial Fruit Blotch

Pathogen: Acidovorax avenae subsp. citrulli

(Reviewed 11/05, updated 11/05)

In this Guideline:


Although all cucurbits are susceptible, bacterial fruit blotch is only a problem on watermelon. Two- to three-week old fruit are most susceptible to infection. Fruit symptoms begin as small, water-soaked spots. These lesions expand rapidly and may cover the upper surface of the fruit. Later the lesions may turn red brown and develop cracks and a general fruit rot may follow. A white bacterial ooze may form in the lesion during wet weather..


The occurrence of blotch in California is apparently rare. Although the bacterium is capable of causing a seedling collapse and foliage symptoms, it is the fruit symptoms that are most obvious and distinctive. Disease is favored by high humidity and high temperature. It is spread through the field by mechanical means or by rain or sprinklers. All watermelon varieties are susceptible, but differences in rind color affect disease severity: dark-colored fruit is least susceptible, light green most susceptible, and striped fruit moderately susceptible.


Use clean seed and disease-free transplants. Disease control in transplant greenhouses involves minimizing handling, keeping temperatures and humidity low, and practicing general greenhouse sanitation. Control in the field involves rotation out of cucurbits and control of volunteer watermelon plants. To prevent spread through the field, avoid sprinkler irrigation and do not work in fields with wet foliage.


[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Cucurbits
UC ANR Publication 3445


R. M. Davis, Plant Pathology, UC Davis
T. A. Turini, UC Cooperative Extension, Imperial County
B. J. Aegerter, UC Cooperative Extension, San Joaquin County
J. J. Stapleton, UC IPM Program, Kearney Agricultural Center, Parlier

Acknowledgment for contributions to Diseases:
W. D. Gubler, Plant Pathology, UC Davis

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