The Effect of Plant Age on the Spatial and Temporal Dynamics of Rice Blast in Arkansas

Abstract

Field experiments were conducted in 1997 to determine how plant maturity affected the rate and distance of rice blast development from foci. A sulfate non-utilizing (sul) mutant of P.grisea was used as a marked strain that could be distinguished from wild-type (background) inoculum. Plants, 28 or 42 days old, of the susceptible cultivar M204’ were inoculated with P. grisea in the greenhouse. Inoculated plants were then transplanted as foci in the centers of 12.5-m² plots of M204 which also were either 28 or 45 days old (660 and 1090 degree day thermal units, respectively). Lesion formation was detected three days after transplanting on greenhouse inoculated plants and the focal strength was determined to be 45 lesions / focus in the younger plants and 90 lesions / focus in the older plants. The number of lesions per plant were recorded weekly at distances of 0.3, 1.6, 3.1, 4.7, and 6.3-m from the focal center (0.09-m²). Disease developed rapidly at the focus within 10days in both the younger and older plants (6 and 5 lesions / plant respectively). However, disease spread from foci differed considerably between the different aged plants whereby disease occurred earlier and was more severe in the younger plants. For example, a mean of 15 lesions / plant were observed on the younger plants I.6-m from the foci after 18 days, while only 1 lesion / plant was observed at the same distance in the older plants. After 30days, the mean number of lesions observed on the younger plants at all distances (45, 32, 24, 14 and 12 lesions / plant, respectively) was significantly higher than on the older plants (12, 12, 4, 2, and 0.5 lesions / plant, respectively). The sul mutant was recovered from over 90% of the lesions collected 10 and 40 days after transplanting, indicating inoculum introduced at foci at the beginning ofthe season was responsible for initiating and perpetuating the epidemic.