Myrothecium roridum Leaf Spot and Stem Canker on Watermelon in the Southern Great Plains: Possible Factors for Its Outbreak

In 2010, a foliar and stem-lesion disease was observed for the first time in Oklahoma causing moderate to severe defoliation. Using microscopic examination, the physical features of the fungus were consistent with Myrothecium roridum. There was a wide range of susceptibility among watermelon cultivars in the field indicating a moderately high level of resistance to Myrothecium leaf spot in some cultivars. In greenhouse inoculation experiments, cantaloupe, honeydew, cucumber, squash, and watermelon were all susceptible to the fungus with cantaloupe and honeydew being the most susceptible and watermelon the most resistant. Furthermore, greenhouse inoculations supported the field observations as differential resistance was exhibited among the watermelon cultivars as well as the cucurbit types. Although cantaloupe was most susceptible to the foliar phase of this disease, watermelon was most susceptible to the fruit-rot phase. Natural infection of watermelon fruit has never been reported.

Accepted for publication 15 December 2011. Published 30 January 2012.