Etiology and factors affecting the development of fruit blotch of watermelon (Citrullus lanatus (Thunb.) Matsum and Nakai) in Northeastern Thailand

Thirty isolates of the fruit blotch bacterium were isolated from three locations, namely: Nakhonratchasima (K1 to K10), Roiet (WM1 to WM10), and Khon Kaen (KK1 to KK10). All isolates were gram-negative, non-spore forming rods, about 0.9 x 1.8 um, having single polar flagellum. The colonies were creamy to translucent, about 1 mm in diameter after 2-3 days, convex, circular and entire in NA, NYDA, and NGA. No fluorescent pigment was produced on KB. All isolates were aerobic, oxidase positive, strong lipase producers and reduced nitrates to nitrites. There was no pectolytic activity observed on CVP medium. The isolates were negative for arginine dihydrolase, levan production, and starch hydrolysis. Gelatin liquefaction was negative to very slow. Many isolates were able to grow at 37 deg C and 41 deg C. Hypersensitivity response on tobacco leaves was positive. Sorbitol, mannitol glucose, sucrose, fructose, galactose and trehalose were utilized but not lactose, rhamnose, and cellobiose. Based on morphological, physiological abd biochemical properties, the bacterium appears that it is closely related to Acidovorax avenae subsp. citrulli (formerly (Pseudomonas pseudoalcaligenes subsp. citrilli) Schaad, et al, 1978) which was reported by several authors to cause disease on watermelon seedlings and fruits. Based on limited no. of seeds tested, no seed transmission of the pathogen occurred in seeds collected from infected fruits and stored at room temperature and air conditioned room for one to six months. Seeds from 10 different hybrids treated with bacterial suspension at 3.5 x 10 to the ninth power cfu/ml before planting developed symptom of the disease seven to ten days after planting with 0 to 83 percent disease incidence. Factors that affect disease development were inoculum concentration, age of plant, wounding and varietal resistance. The last inoculum concentration for pathogenicity test was 3.5 x 10 to the ninth power cfu/ml. The seedlings were most susceptible to infection than the young and mature fruits. Insects in watermelon seed farm found were Trigonalid, American serpentine leafminer (Liriomyza trifolu complex), honeybee (Aphis mellifera Linn.), bombay locust (Padanga succinata L.), fire ant (Solenopsis geminata (F.)), squash bug (Anosa tristis DeGreen), cucurbit beetle (Aulacophora similis Oliver), melon fly (Dacus cucurbitae Coq.), leaf-eating caterpillar (Palpita indica)