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

Abstract

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 £ 20 cm, 13 £ 25 cm and 25 £ 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 £ 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.

Effect of host and inoculum patterns on take-all disease of wheat incidence, severity and disease gradient

“…In the naturally infested subplots (all subplots from 2005 and control subplots of 2006), the significant effect of sowing pattern on disease severity and incidences at all disease measurement scales (Table 1) indicates that host aggregation increased epidemic development. This result is in accordance with the observations made on other pathosystems (Willocquet et al 2000) and indicates that the natural inoculum was spread out rather uniformly. Indeed, if the inoculum had been aggregated, one would have expected a lower proportion of diseased plants in the hills than in broadcast sowing, because of the lower hill-to-hill infection rate (Gosme and Lucas 2009).…”

“…when the probability of infection between an infected and a susceptible individual decreases with distance between these individuals). In phytopathology, this effect was experimentally demonstrated in the case of rice sheath blight caused by Rhizoctonia solani (Willocquet et al 2000) and was explained theoretically, either in the case of vector-borne diseases (Caraco et al 2001) or in a more general framework (Bolker 1999). Host aggregation creates spatial discontinuities, thus causing the speed of epidemics to fluctuate as host aggregates are depleted of susceptible individuals and new aggregates are being colonised.…”

Agronomical management influence on the spatiotemporal progress of strawberry dry wilt in Michoacan, Mexico

“…Remarkably, A+G had the smallest aggregates, but the restrictive space could explain the highest displacement of the inoculum within the row. This level of dispersal is consistent with root pathogens, because it depends on the spatial pattern of host population, especially when disease transmission requires contact between healthy and susceptible tissues, that is, root to root (Sujkowski et al, 2000;Willocquet et al, 2000) and the airborne pathogens have a less restrictive dispersion. Since 2006, Fusarium wilt of strawberry has increased in incidence and severity in California, USA.…”

Influence of silicon on sheath blight of rice in Brazil

“…Consequently, cultural and biological control methods are used to minimize the intensity of sheath blight (Mew and Rosales, 1986;Belmar et al, 1987;Jones and Belmar, 1988;Damicone et al, 1993;Roy, 1996;Willocquet et al, 2000). Sheath blight control, however, has mainly focused on fungicides and these pesticides are of environmental concern (Groth et al, 1990;Rush and Lee, 1992).…”