Author : Payorn Srichumpa Pinyapong

Summary : The study was conducted to: 1) isolate the bacterium causing fruit blotch of watermelon; 2) conduct pathogenecity tests on watermelon fruits and seedling leaves; 3) characterize and identify the bacterium causing watermelon fruit blotch; and 4) determine some environmental factors that affect disease development. Thirty isolates of the fruit blotch bacterium were isolated from their locations, namely: Nakhonratchasima (K1 to K10), Roiet (WM1 to WM10), and Khon Khaen (KK1 to KK10). All isolates were Gram-negative, non-spore forming rods, about 0.9 x 1.8 um, and had singular polar flagellum. The colonies were creamy to translucent, about 1 mm in diameter after 2-3 days, convex, circular and entire in nutrient agar (NA), nutrient yeast-dextrose agar (NYDA), and nutrient glucose agar (NGA). No flourescent pigment was produced on King's medium B agar (KB). All isolates were aerobic, oxidase positive, strong lipase producers and reduced nitrates to nitrites. There was no pectolytic activity observed on Crystal Violet Pectate (CVP) medium. The isolates were negative for arginine dihydrolase, levan production, and starch hydrolysis. Gelatin liquefaction was negative to very slow. Many isolates grew at 37 degree C and 41 degree C. Hypersensitivity response on tobaco leaves was positive. Sorbitol, mannitol, glucose, sucrose, fructose, galactose and trehalose were utilized but not lactose, rhamnose, and cellobiose. Based on morphological, phsiological, and biochemical properties, the bacterium appeared to be closely related to Acidovorax avenae subsp. Citrulli (formerly, Pseudomonas pseudoalcaligenes subsp. citrulli) which was reported by several studies to cause disease on watermelon seedlings and fruits. Based on limited number of seeds tested, no seed transmission of the pathogen occured 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 (superscript 9) cfu/ml.