Pumpkin (Cucurbita pepo L., cv. Magic Lantern) and watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai, cvs. Millionaire and Sangrea) plants with wilting leaves and collapse of entire vines were observed during the 2005 and 2006 growing seasons in several fields in southwestern New Mexico (Luna and Hidalgo counties) with an incidence ranging from 7 to 25% and less than 1% in pumpkin and watermelon fields, respectively. Sticky, hyaline strands were visible when vines were cut transversally, indicative of bacterial wilt caused by Erwinia tracheiphila (2). In the pumpkin fields, 12-spotted cucumber beetles, vector insects of E. tracheiphila, were found on plants at the first-true-leaf stage, which were treated with dimethoate. At the 4- to 5-leaf stage, 5 to 10% of the plants were wilted and were removed by hand. Less than 1% of the plants showed symptoms prior to bloom, when a high population of beetles was observed, and the fields were treated with thiamethoxam. To isolate the causal agent of the wilt symptoms, six, 1-cm vine segments and three to five beetles were surface sterilized in 1% NaOCl for 2 min, rinsed and macerated in sterile distilled water, and plated onto potato dextrose agar, nutrient agar, and King's medium B. After incubation at 25°C, bacterial colonies emerged on all media and were grayish white-to-cream, circular, smooth, and glittering. Isolated bacteria were gram negative, did not grow at 39°C, produced hydrogen sulfide gas from hydrolysis of cysteine, and did not hydrolyze litmus milk and starch. With Ready-To-Go PCR beads and 16S rDNA-based primers ET1/ET2 (1), a 700-bp product was obtained from each of two isolates, consistent with previously reported data for E. tracheiphila (1,3). For the pathogenicity tests, 10 seedlings of pumpkin cv. Magic Lantern and watermelon cv. Millionaire were inoculated with each isolate in the greenhouse at the second fully expanded leaf stage using two methods. In the first method, stems were injected with bacterial suspension (106 CFU/ml) using a hypodermic needle. In the second method, a dab of bacterial colonies was taken with a sterile toothpick to stab the cotyledonary axils. Control seedlings were stem injected with distilled water or stabbed with a sterile toothpick. The experiments were conducted four times. Inoculated plants were placed in a humid chamber at 23 to 25°C. Plant wilting was observed within 4 days when stab inoculated with toothpicks and within 7 to 10 days when stem injected with bacterial suspension. Bacterial colonies recovered from inoculated plants were identical to those recovered from field infected plants. To our knowledge, this is the first report of bacterial wilt on pumpkin and watermelon in New Mexico.
References: (1) B. Bruton et al. Phytopathology (Abstr.) 89(suppl.):S10, 1999. (2) R. X. Latin. Page 36 in: Compendium of Curcurbit Diseases. T. A. Zitter et al., eds. The American Phytopathological Society, St. Paul, MN, 1996. (3) N. W. Schaad et al. Laboratory Guide for the Identification of Plant Pathogenic Bacteria. 3rd ed. The American Phytopathological Society, St. Paul, 2001.