1Plant Pathology and Microbiology Division and 2Field Crop Research Institute, Department of Agriculture, Jatujak, Bangkok, 10900, Thailand

Background and objectives
Sugarcane yellow spot, caused by Mycovelosiella koepkei, spreads widely throughout sugarcane growing areas in Thailand. Several commercial varieties have shown high susceptibility to the disease.[1]. Reduction of cane yield occurred when the percentage leaf area infected by yellow spot at the sugar-accumulation stage was in excess of 15% [2]. For a better understanding of yellow spot epidemiology, a study of the progress of the pathogen population and its relation to disease development was carried out.

Materials and methods
Susceptible cane variety F134 was planted over 1600 m2 at Rayong Field Crop Research Centre. Airborne spores were trapped over the field at 2-day intervals using glass slide spore traps for monitoring the progress of the pathogen population. To monitor yellow spot development, 200 stools in an experimental field were selected. Visual estimates of per cent leaf area infected on the top six visible collar leaves of all stalks in each selected stool were made at monthly interval. Meteorological data were also recorded. All data were collected throughout two growing seasons and were analysed using correlation and regression analysis.

Results and conclusions
Yellow spot spores were found in the field throughout the growing season but the number of spores trapped was greater in the rainy months from May to October. By applying correlation analysis, the amount of rainfall, relative humidity and maximum temperature showed significant effects on spore catch number, with correlation coefficients of 0.82, 0.60 and -0.68, respectively. Weather conditions defining high spore counts during epidemics were more than 1 mm of rainfall, more than 70% r.h. and less than 32C (maximum temperature).

Increased yellow spot infection occurred during June to November. Per cent leaf area infected was significant related to the average number of spores trapped for 15- and 30-day periods preceding the date of disease assessment, and to the average number of spores trapped for 15-day periods 15 days preceding the date of disease assessment, with correlation coefficients of 0.68, 0.76 and 0.79, respectively.

The regression equation describing the relationship between the average percentage of leaf area infected (y) and the average number of spores trapped for 15-day periods 15 days preceding the date of disease assessment (x) was y=6.7+0.2x. The equation will be applied for the estimation of yellow spot development.

1. Ouvanich W, Srisink S, 1994. Abstracts of the 20th Thai Science and Technology Conference, pp. 464-465.
2. Ouvanich W, Srisink S, 1995. International Sugar Journal 97(1161), A29.