Visited a couple of commercial watermelon producers in Washington and Tangipahoa Parishes with Henry Harrison last week. Poor pollination early in the season has been the major problem in the area due to lack of bee activity early in the season. However, we did see quite a bit of gummy stem blight on one farm. The relatively cool, wet spring weather was ideal for the development of this disease. Spores of the causal fungus, Didymella bryoniae, are released during or shortly after periods of rainfall or dew and are wind-borne. Infection of the crown leaves results in the development of greasy-looking lesions in which pycnidia of the anamorph, Phoma cucurbitacearum, are formed. Spores produced in the pycnidia are then rain-splashed and often cause lesions on the runner vines and the crown of the plant. These lesions may eventually girdle the stems causing individual vines or the whole plant to collapse. The pathogen may be seed-borne, and an early season fungicide spray program is essential to manage it in years with consistently wet spring weather.
Donald M. Ferrin, Ph.D., Louisiana State University Agricultural Center, Baton Rouge, LA