Abstract: The relationships between disease severity of sheath blight and several canopy indices of rice, i.e. stem number (TILPI) and chlorophyll content at panicle initiation (SPADPI), and stem number (TILHD), leaf area index (LAIHD) and chlorophyll content at heading (SPADHD) were investigated in the four-season field experiments conducted in Guangzhou, China during early and late seasons of 2004 and 2005. Yueza 122, a two-line hybrid rice, was arranged in a randomized complete block design with eight N-fertilizer treatments and four replications. There were two categories of factors, i.e. canopy conditions and climatic factors, which affected sheath blight development. Main canopy indices influencing disease severity were TILPI, SPADPI, TILHD, LAIHD, and SPADHD, which explained 44.8% of variation in disease severity among years, seasons and N treatments. Main climatic factors were daily mean temperature (Tav) and relative humidity (RH). The multiple regression equation with TILPI, SPADPI, LAIHD, SPADHD, Tav and RH as independent variables explained 82.7% of variation in disease severity. The disease severity decreased as light transmittance ratio of the canopy at heading (LTR) increased. The LTR, Tav and RH explained 80.2% of variation in disease severity. It is suggested that the reduced maximum tiller number, lower chlorophyll content of leaves, smaller LAI at heading and better ventilation of the rice canopy are the direction to establishing a healthy canopy with high grain yield potential and low sheath blight occurrence.

Key words: rice (Oryza sativa), sheath blight (Rhizoctonia solani), disease severity, nitrogen fertilizer, canopy index, leaf area index, chlorophyll content

摘要： 2004-2005年早季和晚季，以两系法杂交稻粤杂122为材料，设置8个氮肥处理，在广州进行连续4季田间试验，研究了纹枯病病情指数与穗分化始期茎蘖数(TILPI)、叶绿素含量(SPADPI)、抽穗期茎蘖数(TILHD)、叶面积指数(LAIHD)、叶绿素含量(SPADHD)等5项水稻群体指标的关系。结果表明，影响纹枯病病情指数的因素包括气象条件和群体条件两大类。TILPI、SPADPI、TILHD、LAIHD和SPADHD是影响纹枯病病情指数的主要群体指标，它们可以解释不同年份、不同季节、不同施氮处理纹枯病病情指数变异的44.8%。日平均温度和相对湿度是影响纹枯病病情指数的主要气象因素，它们与TILPI、SPADPI、LAIHD、SPADHD一起，可以解释不同年份、不同季节、不同施氮处理纹枯病病情指数变异的82.7%。纹枯病病情指数随着抽穗期群体透光率的提高而下降，由日平均温度、相对湿度和抽穗期群体透光率可以解释纹枯病病情指数变异的80.2%。在保证穗数的前提下尽量降低苗峰，提高成穗率，在保证光合生产能力的前提下，适当降低叶片叶绿素含量和抽穗期叶面积指数，提高群体通透性，是协调高产与防病矛盾、培育健康群体的方向。

关键词: 水稻, 纹枯病, 病情指数, 氮肥, 群体指标, 叶面积指数, 叶绿素含量