Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


This research was conducted to examine the nature of the difference in resistance to blast under different water-management systems. Two cultivars; M-201, susceptible, and Brazos, partially resistant to blast, grown in flooded and upland (unflooded) field plots were inoculated with race IH-1 of Pyricularia oryzae in 1984 and 1985. Final disease ratings on M-201 and Brazos averaged 77.6% and 39.8% under upland conditions and 1.1% and 0.1% under flooded conditions, respectively. When blasted plants in upland plots were flooded, the rate of blast development decreased on Brazos but changed little on M-201. No increase in disease was observed when flooded plots were drained. Air temperature, leaf-wetness period, and relative humidity in upland and flooded plots were similar or favored greater disease development under flooded conditions. Total N and S content in leaf samples taken at intervals were greater under upland conditions than under flooded conditions. Silica, Mn, and Na content were higher in leaf samples from flooded plots. Phosphorus, K, Ca, Mg, Al, B, Cu, Fe and Zn content in leaf tissues did not differ significantly between the two conditions. The site of appressorium formation and site of penetration by P. oryzae on the leaf epidermis did not differ between plants grown under flooded, saturated, and moist soil conditions. The number of penetrated cells and the degree of intracellular hyphal growth in the penetrated cells on inner surfaces of leaf sheaths were reduced by flooding. A computer program written to predict blast occurrence based on microclimate was tested with on-site microcomputers. The microcomputers collected data on air temperature, leaf wetness and relative humidity from upland and flooded rice plots, interpreted this information in relation to blast development and displayed daily blast units of severity (BUS) values. When cumulative BUS values, rather than time in days, were used to predict the logit of disease proportions, the accuracy of prediction was significantly improved. The system did not work where rice was flooded as flooded rice was resistant to leaf blast.