Thesis Abstract of AGS Students

Ephidemiology of major soybean diseases in relation to environment and farming practices in rainfed area

Vorapun Charumas (1988)

Studies on the environmental factors and farming practice in relation to major soybean diseases were conducted at the end of rainy season, 1985. Two locations in Chiang Mai were selected from rainfed areas at Baan Huay Nam Kaow, Tambol Yang Kram, Amphur Chomthong and Baan Huay Bong, Tambol Nam Prae, Amphur Prao, which are major soybean seed production areas.

Cropping patterns, disease incidence and severity were recorded and analysed. Yield loss and seed quality (% disease and germination) were also measured. Results showed that cropping patterns were more diversified at Prao. Surveys of soybean diseases in 25 farms at Chomthong and in 23 farms at Prao showed that rust and downy mildew occurred in both locations, but bacterial pustule was only found at Chomthong. Anthracnose was a minor disease of soybean at Prao but was absent at Chomthong. Climatic data indicated that total rainfall and average temperature duringthe growing season were almost the same but the distribution was different in two locations. Average soybean yield at Chomthong (185 kg/rai) was higher than Prao (128 kg/rai). The effect of climatic variables on disease incidence was statistically analysed by using simple correlation. The disease severity was described by transforming the data by using Gompertz model: and its relationship with the climatic variables was explained by regression model: Y = f(temp, RH, rain). Results clearly showed that low tem perature and high humidity were the important factors determining the disease incidence and severity. Rust was found during flowering stage (R1), mostly in the lower and the middle parts of the infected plant and the severity was increased with age. Downy mildew and bacterial pustule were found during vegetative stage and pod forming (R4). Symptoms appeared mostly in the middle and top parts of the plant. Anthracnose was found at seed filling stage (R5) and severity was subsequently decreased. Rust was found more severe in the first and second planting date than the third. At Chomthong yield loss due to rust was 15 and 12 percents in the first and second planting date, respectively; which was higher than the third planting date (2 percents), while at Prao yield loss at both planting dates averaged 14 percents. All seeds tested showed as good quality and high percentage of germination.

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