Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2011
Publication Date: 5/1/2012
Citation: Sikora, E.J., Bruton, B.D., Wayandande, A.C., Fletcher, J. 2012. First report of the cucurbit yellow vine disease caused by Serratia marcescens in watermelon and yellow squash in Alabama. Plant Disease. 96(5):761. Interpretive Summary: Cucurbits, primarily watermelon, cantaloupe, squash and pumpkin are important horticultural crops in the southeastern U.S. that are affected by a variety of insect and disease complexes. Cucurbit yellow vine disease (CYVD) is caused by the phloem-colonizing bacterium, Serratia marcescens Bizio. During the summer of 2010, watermelon and squash exhibiting symptoms of CYVD were observed in Russell and Tallapoosa Counties, Alabama. CYVD affected plants usually exhibit stunting, yellowing, and gradual decline beginning about 10 to 14 days prior to harvest. Occasionally a rapid wilt occurs at flowering and/or fruit-set, and plants collapse within a single day without plant yellowing. Losses can range from less than 5% to 100% in affected fields of watermelon, squash, pumpkin, and cantaloupe. Early-planted watermelons targeted for the 4th of July market generally are impacted more severely than later plantings. The symptom most diagnostic and consistent with the disease in all affected cucurbit species is a honey-brown discoloration of the phloem. Phloem discoloration is not as intense in squash and pumpkin compared to watermelon and cantaloupe. Consequently, field diagnosis may be more difficult in these crops. At present, the squash bug is the only known vector of the CYVD bacterium. Early-season scouting and squash bug control are the only effective disease management strategies for CYVD. The presence of CYVD in Alabama extends the geographic range of this disease into the deep south and a major watermelon production area.
Technical Abstract: Symptoms typical of cucurbit yellow vine disease (CYVD) were first observed in a 2 ha watermelon field in Crawford, Russell County, Alabama on 8 June 2010. Watermelon plants, cv. 'Jubilee,' exhibited a yellow or chlorotic appearance and some plants were completely wilted. On 24 June plant samples were collected from a 1 ha watermelon (cv. 'Jubilee') field near Dadeville, Tallapoosa County, Alabama. At the time of collection, incidence of affected plants was estimated at 25%. Approximately 30% of plants in the field exhibited yellowing and wilting symptomatic of CYVD. On the same date, samples were also collected from a planting of approximately 50 yellow crooked-neck squash plants at another location in Tallapoosa County. Approximately 20% of the squash plants expressed symptoms typical of CYVD. Cross-sections of below-ground stem and primary root revealed a honey-brown phloem discoloration and a healthy appearing xylem, symptoms consistent with CYVD, caused by the phloem-colonizing bacterium, Serratia marcescens Bizio. Isolations were made from the lower crown on four symptomatic watermelon and two symptomatic squash plants. Approximately 2.5 mm**3 tissue pieces from the phloem were excised; surface sterilized for one minute in 10% sodium hypochlorite, and ground in 1 mL PBS buffer. A 10 µL aliquot of slurry was plated onto nutrient agar (NA) (Difco) and the plates stored at room temperature for 3 to 4 days. Slow growing individual colonies (approximately 0.05 mm diameter), having a consistency of peanut butter, were selected and purified by serial dilution plating. Bacteria obtained from watermelon (isolates 22-10B551, 22-10B552, and 22-10B553) were grown on NA plates and suspended in sterile water at 10**8 cells/mL for immediate use in mechanical transmission experiments using an inoculation fork. Sterile water served as a negative control. After 28 da in the greenhouse, all plants were cross-sectioned at the juncture of the root and stem and sections observed for phloem discoloration using a light microscope. Of the 56, 58, and 62 'Lemon Drop' squash plants inoculated in three replicate studies, 78.6, 56.9, and 62.9% developed typical CYVD symptoms, respectively, while no control plant was positive. When cultured bacteria from six of the symptomatic, greenhouse-inoculated plants were subjected to multiplex end-point PCR using primer sets YV1/YV4, specific for the species S. marcescens, and a43F/a43R, which amplifies only the CYVD strains of S. marcescens, all six, and the positive control, were positive, while the negative control was negative. Although rhizosphere-inhabiting and plant growth promoting endophytic strains of S. marcescens have been reported from Alabama, this is the first known report of CYVD and of phytopathogenic S. marcescens in Alabama cucurbits.