Cucurbit host range of Myrothecium roridum isolates from watermelon

Authors: Fish, Wayne; Bruton, Benny; POPHAM, THOMAS - Retired ARS Employee

Submitted to: American Journal of Plant Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/29/2012
Publication Date: 3/6/2012
Citation: Fish, W.W., Bruton, B.D., Popham, T.W. 2012. Cucurbit host range of Myrothecium roridum isolates from watermelon. American Journal of Plant Sciences. 3:353-359.

Interpretive Summary: In 2010, watermelon production in Oklahoma was estimated at about 25,000 metric tons and valued at $5.5 million. Diseases are generally the greatest yield-limiting factor for watermelon across the US. A new foliar and stem-lesion disease, caused by Myrothecium roridum, was observed for the first time in Oklahoma causing moderate to severe defoliation. Studies were set up to establish the host range within the cucurbit family and determine what environmental conditions may have led to the disease outbreak. It is likely that a combination of three factors facilitated this outbreak of M. roridum on watermelon under field conditions. First, the field soil was likely heavily infested with M. roridum as a result of continuous cropping of cantaloupe; a susceptible host. Second, weather conditions were warm and wet during the four weeks leading up to the disease outbreak. Third, the frequency of rain during this four week period prevented the regular weekly application of fungicides targeted at foliar diseases on the watermelon, allowing M. roridum-induced leaf spot and crown/stem canker to gain a substantial foothold in the watermelon crop. Once the weather turned drier and foliar fungicide weekly were performed, disease spread was slower. 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. Natural infection of watermelon fruit has never been reported and none was observed in the field. This information will assist farmers in the selection of resistant cultivars.

Technical Abstract: In 2010, a foliar and stem-lesion disease caused by Myrothecium roridum was observed for the first time in Oklahoma causing moderate to severe defoliation. The objective of this study was to compare the susceptibility of the vegetation and fruit of a broad range of commercially important cucurbits to three isolates obtained from these foliar lesions on watermelon. In greenhouse foliar 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 earlier 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, although natural infection of watermelon fruit has never been reported.