Donald L. Hopkins

Occurrence of a bacterial watermelon fruit blotch in Florida.

Mature watermelon fruit with large, firm, dark green, water-soaked lesions were observed in spring 1989. Losses of up to 50% in marketable fruit occurred in some fields in Florida. A gram-negative, aerobic, rod-shaped, nonfluorescent, oxidase-positive, arginine dihydrolase-negative bacterium was isolated from diseased tissues. In pathogenicity tests, the bacterium produced symptoms in foliage and fruit. Morphological, physiological, and biochemical properties of the bacterium related it to a previously described pathogen of watermelon, Pseudomonas pseudoalcaligenes subsp. citrulli

Wet seed treatment with peroxyacetic acid for the control of bacterial fruit blotch and other seedborne diseases of watermelon

Prevention of seed transmission of Acidovorax avenae subsp. citrulli into the transplant house or field is the most effective control of bacterial fruit blotch of watermelon currently available. Peroxyacetic acid was evaluated as a disinfectant that might efficaciously eradicate A. avenae subsp. citrulli from contaminated seed and also be efficacious against other seed-transmitted diseases of watermelon. Peroxyacetic acid at low concentrations eliminated A. avenae subsp. citrulli, Fusarium oxysporum, and Didymella bryoniae from microbial suspensions. Treatments of seed contaminated with A. avenae subsp. citrulli and D. bryoniae with peroxyacetic acid at 1,600 μg/ml and higher for 30 min were effective in preventing seed transmission of bacterial fruit blotch and gummy stem blight. Hydrochloric acid treatments at 10,000 μg/ml, while effective in eliminating seed transmission to watermelon seedlings, can adversely affect seed germination, especially with triploid seed. Efficacious dosages of peroxyacetic acid can be applied safely to freshly harvested triploid watermelon seed without concerns for reduction in seed quality. A most effective wet seed treatment protocol involved a 30-min treatment with peroxyacetic acid at 1,600 μg/ml followed by seed drying at low humidity in a 40°C drying oven for 48 h.

Ingress of the watermelon fruit blotch bacterium into fruit.

Bacteria were observed with the scanning electron microscope to be located randomly on water-melon (Citrullus lanatus) fruit surfaces 2 hr after inoculation with drops of suspensions of the fruit blotch bacterium. For the next 4 days, bacteria were observed around and in stomata. Nine days after inoculation, masses of rod-shaped bacteria were observed in stomatal chambers. The incidence of disease decreased with age of the fruit at the time of inoculation