Effect of various crop establishment methods practised by Asian farmers on epidemics of rice sheath blight caused by Rhizoctonia solani

Establishment methods for rice crops in tropical Asia are very diverse, leading to variation in the structure of rice canopies. Differences in canopy structure can in turn affect the spread of the rice sheath blight pathogen, Rhizoctonia solani. Rice sheath blight epidemics were compared during two seasons in crops established by different methods: direct broadcasting of pregerminated rice seeds, and transplanting of rice seedlings at spacings of 20 x 20 cm, 13 x 25 cm and 25 x 25 cm between hills (i.e. along and between rows, respectively). In both years, the apparent infection rate based on incidence data and the terminal severity of sheath blight were lower in the direct-seeded crops than in any of the transplanted ones, regardless of spacing. The frequency of leaf-to-leaf contacts (CF) between hills (or plants) was highest in direct-seeded rice, and lowest in rice transplanted at a spacing of 25 x 25 cm. Larger CF is known to favour rice sheath blight epidemics. The apparent contradiction between higher incidence and lower CF in the transplanted stands than in the direct-seeded stands is interpreted in terms of accessibility of healthy host tissues to the spread of the pathogen in the canopy, and accounts for within-host (rice hill or plant) and between-host (hill or plant) disease spread. The analysis of incidence-severity relationships indicated a less aggregated distribution of the disease in direct-seeded rice, which was related to the spatial distribution of the tillers. These findings have direct implications for the management of the disease.