The spatial and temporal dynamics of rice blast disease in Arkansas

Field experiments were conducted over a four year period from 1995 to 1998 to determine the spatial and temporal dynamics of rice blast disease in Arkansas. The three objectives addressed in this dissertation are as follows: (1) to determine if seed infested with P. grisea can cause rice blast epidemics in small plots, (2) to determine if plant maturity affected the rate and distance of rice blast development from foci, and (3) to determine if conidia from asymptomatic and symptomatic flag leaf collars caused neck blast infections. A unique aspect of these epidemiological studies was the use of a sulfate nonutilizing (sul) mutant of Pyricularia grisea as a marked strain. The presence of the sul mutant in leaf, collar and neck blast lesions, typical of parental strains, illustrated the point that the sul mutant initiated and perpetuated rice blast epidemics in these studies.; These studies illustrate six important points regarding the spatial and temporal dynamics of rice blast disease in Arkansas. First, the non-systemic, aerial dissemination of conidia from infested seed was responsible for the initial onset of rice blast and the development of disease in these field plots throughout the season. Secondly, the number of infested seed on the soil surface at plant emergence significantly affected the early development of leaf blast and the maximum disease incidence observed at the panicle primordia stage of plant growth. Third, the development of leaf blast on a susceptible cultivar, M204, was more severe, increased more rapidly and spread farther within populations of rice plants that were in their vegetative growth stages than within populations of rice plants that were in their reproductive growth stages. Fourth, the incidence of flag leaf collar rot was positively correlated to the incidence of neck blast; however, low incidences of neck blast could occur in the absence of flag leaf collar rot. Fifth, P. grisea conidia were observed on symptomatic and asymptomatic flag leaf collars prior to neck node emergence, indicating that the absence of collar infections does not mean that inoculurn that could cause subsequent neck infections is not present. Sixth, inoculation of flag leaf collars with a sul mutant demonstrated that inoculum originating from both symptomatic and asymptornatic flag leaf collars caused neck blast infections at the end of the season. These studies have strengthened our understanding of the temporal and spatial dynamics of rice blast disease throughout the season and may assist in the development of suitable and effective management strategies for controlling rice blast disease in Arkansas.