The relationship between sheath blight occurrence and dry matter production and yield formation of rice (Oryza sativa L.).

Three field experiments were conducted during 2005-2007 late seasons in Guangdong.Hybrid rice varieties,Shanyou 63 and Boyou 998,were grown under six nitrogen fertilization levels ranged from N 0～185 kg/hm2.Plant samples were taken at key growth stages,i.e.,mid-tillering(MT),panicle initiation(PI),booting(BT),and heading(HD).Stem number,stem thickness,dry weight of leaves(WLV),dry weight of stems,sheath and panicles(if any)(WST),and total aboveground dry weight(TDW) were measured.Yield and yield components were measured at harvest.Disease severity of sheath blight(ShBI) caused by Rhizoctonia solani was investigated 15 days after heading.There were significant differences in ShBI among N treatments,with greater ShBI for treatments with higher N input.Significant differences were also observed between two tested varieties.Shanyou 63 always had greater ShBI than Boyou 998 at all N levels except for zero-N level.The ShBI was positively and significantly(P0.05) correlated with stem number,WLV,WST,and TDW,and negatively and significantly(P0.05) correlated with stem thickness and stem-to-leaf ratio at PI,BT and HD,respectively.The correlation coefficient between ShBI and WLV was always greater than that between ShBI and WST at different growth stages in this study.Significant and positive correlations between ShBI and grain yield were observed for Boyou 998 in all three years.Among the four yield components,ShBI had highly significant positive correlation with number of panicles per m2 and number of spikelets per panicle,but negative or non-significant correlations with filled grain percentage and 1000-grain weight.These results suggest that varieties with fewer and thicker stems and higher stem-to-leaf ratio should be bred to achieve both high yield and low sheath blight occurrence.At a certain yield level,plant populations with fewer panicles per m2,fewer spikelets per panicle and better grain filling may have lower sheath blight occurrence.