New Watermelon Disease Reported (June 11, 2014)

Dr. Mike Stanghellini at the University of California, Riverside and colleagues have reported the identification of a new disease on watermelon plants in the Coachella Valley of California. Symptoms on foliage include yellowing, wilting, and death of leaves, beginning at the crown and rapidly making progress towards the tips of vines. The large main roots look healthy but the small feeder roots display a brownish discoloration. This disease resulted in collapse of mature watermelon plants at first harvest and was found in several drip-irrigated fields. A fungus was recovered from the discolored feeder root tissue and experimentally shown to cause symptoms observed in the field. The fungal pathogen, called Olpidium bornovanus, has not until now been known to cause disease on watermelon plants anywhere in the world. This fungal pathogen produces motile zoospores as part of its life cycle. Zoospore production, movement, and disease development are enhanced by wet soil conditions. Soil in affected fields apparently was nearly saturated due to daily application of irrigation water through drip-tape. Does this disease occur in Arizona? At this time, we do not know. If watermelon plants with the foliar and feeder root symptoms described above are found in Arizona watermelon plantings, especially in drip-irrigated fields with wet soil, please bring the root system of affected plants, including the small discolored feeder roots, to the University of Arizona Yuma Agricultural Center for analysis.

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To contact Mike Matheron go to: matheron@ag.arizona.edu.