Experimental investigation on disease transmission of rice blast from leaves to necks

Rice blast can affect different plant organs, including leaves and necks. Leaf blast epidemics mainly occur in the early, vegetative stage of crop growth, whereas neck blast appears after heading, when disease severity on leaves in general is declining and low. This typical sequence therefore leads to questioning the extent to which airborne inoculum originating from diseased leaves contributes to neck infection. Two microfield experiments were conducted to address this question. Leaf blast epidemics with differing onset times were artificially generated on two susceptible varieties (Co39 and IR50). In each microfield, the upper parts of 22 tillers (boot and flag leaf) were bagged at booting stage for three weeks, resulting in a physical protection of the neck nodes from airborne inoculum. Neck blast was assessed immediately after the bag removal. Levels of leaf and neck blast were higher in the first than in the second experiment. In both experiments, leaf blast dynamics displayed an increase, a plateau, then a decrease in disease severity; both leaf and neck blast levels were higher in Co39 than in IR50, and were highest when epidemic onset on leaves was early (at seedling); and neck blast was observed in protected panicles, but its average incidence (28%) was about half of that on un-protected panicles. These results suggest the concomitant occurrence of neck infection from airborne propagules, as well as from within-tiller, non-airborne disease transmission. These results and their potential implications for disease management are discussed in the context of epidemiological hierarchies from plant to landscape.