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Etiology and Factors Affecting the Development of Fruit Blotch of Watermelon (Citrullus lanatus (Thunb.) Matsum & Nakai) in Northeastern Thailand
Abstract:
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 (WM 1 to WM 10), and Khon Khaen (KK 1 to KK 10). All isolates were Gramnegative, non-spore forming rods, about 0.9 x 1.8 urn, 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 hydrolys is. Gelatin liquefaction was negative to very slow. Many isolates grew at 37°C and 41 oc. Hypersensitivity response on tobacco leaves was positive. Sorbitol, mannitol, glucose, sucrose, fructose, galactose, and trehalose were used but not lactose, rhamnose, and cellobiose.
Based on morphological, physiological, 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 the limited number 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 109 cfu/ml before planting developed symptoms of the disease 7-10 days after planting, with 0-83 percent disease incidence.
Factors that affected disease development included inoculum concentration, age of plant, wounding, and varietal resistance. The best inoculum concentration for pathogenicity test was 3.5 x 109cfu/ml.Seedlings were the most susceptible to infection.
Insects found in the watermelon seed farm tested were Trigonalid, American suspentine leafminer (Liriomyza trifolu complex), honeybee (Aphis mellifera Linn.), bombay locust (Patanga succinata L.), fire ant (Solenopsis geminata (F.)), squash bug (Anasa tristis DeGreen), cucurbit beetle (Aulacophora simi/is Oliver), melon fly (Dacus cucurbitae Coq.), and leaf-eating caterpillar (Palpita indica).