Lodging And Grain Yield Of Double-season Rice As Affected By Sheath Blight And The Associated Mechanism

The canopy structure had a deep influence on rice grain yield and lodging-resistance. Meanwhile, canopy structure and its micro-climatic are essential factors for disease development. Canopy structure may affect sheath blight epidemic directly or indirectly. So, in order to settle present problem, we must investigate the relationships of canopy structure with sheath blight epidemic, lodging-resistance and grain yield performance. Field experiments were conducted in both early and late seasons in2008,2009and2010. The effects of nitrogen rate, hill density, fungicide treatment, and inoculation with R. solani on sheath blight severity, lodging-related traits and grian yield performance were investigated. The main results are as follows:(1) Sheath blight index increased with the increase of nitrogen rate and hill density. Tiller number, leaf area index and biomass production was high under dense canopy structure due to high nitrogen and dense density in non-inoculated plants. Dense canopy structure led to higher leaf contact frequency and lesser light transmittance, which was conducive to sheath blight epidemic. In inoculated plants, however, sheath blight severity was not necessarily restricted by autoinfection because enough hypha was available in infected tillers, which is mainly controlled by vertical spread of the pathogen within each hill. Micro-climate within a hill favorable to sheath blight development regardless of nitrogen rate and hill density was not changed. So, canopy structure due to difference in nitrogen rate and hill density has less effect on sheath blight epidemic.(2) An effective inoculation method with short woody toothpicks is a critical component of an accurate disease assay for quantifying levels of sheath blight resistance. Nitrogen rate and hill density have no significant effect on lesion length in infected plants, and the relationships of lesion length with tiller number, leaf area index and biomass production was not consistent and significant. These results suggest that canopy structure deep influences sheath blight epidemic under the common field conditions due to change in leaf contact frequency, light transmittance, leaf area index, tiller number and biomass production, but has little effect on plant resistance to sheath blight in the severe infection condition.(3) Adaptation of 'healthy' canopy structure resulting from appropriate crop management practices such as rational use of fertilizers and optimum planting density can suppress sheath blight. Improving plant traits such as erect leaves based on crop management or breeding selection is an efficient way to control disease infestation.(4) Lodging index increased and breaking resistance decreased with the increases of nitrogen rate. Nitrogen rate have no consistent effect on dry weigh per unit length and stem diameter. Breaking resistance, bending moment, stem diameter and dry weight per length decreased with the increases of hill density, following by an increase in lodging index. So, lodging index increased with the increases of canopy structure.(5) It's feasible to decrease lodging index with increase in breaking resistance. Correlation analysis revealed that breaking resistance was positively related with stem diameter and dry weight per unit length significantly; lodging index was negatively related with stem diameter and dry weight per unit length significantly. So, it is feasible to increase breaking resistance with increase in stem diameter and dry weight per unit length. Improvement light transmittance such as increasing leaf angle, can affect the leaf distributing in canopy under the same canopy structure, and then breaking resistance increased.(6) In early season, the attainable yield was higher under dense planting (13.3×20cm) when N was applied at a rate of120-180kg ha-1. However, the effect of hill density on grain yield was relatively small for late season, while moderate hill density (13.3×26.7cm) and low nitrogen rate (120kg ha-1) were recommended for their advantages in terms of grain yield and lodging resistant. The loding index of late season was higher than those of early season by58%and45%in2009and2010, respectively.(7) Remarkably higher grain yields were achieved in late season than in early season, as late season had advantages over early season in sink size and biomass production. The comparatively less yield under early season rice resulted due to the slower growth during vegetative phase, which can be ascribed to the lower temperature rather than reduced mean daily radiation. In addition, the averaged air temperature during grain filling and the number of days with maximum temperature above34℃around flowering stage were significantly greater in early season than in late season across the three years, which might have led to a decrease in grain-filling percentage.(8) The direct effect of sheath blight infestation on lodging resistance was found by using fungicide treatment and inoculation of R. solani. Sheath blight development does not affect tiller number. But, it has a great effect on leaf area index and biomass production after flowering. Compared with control, fungicide treatment has significantly increased leaf area index and biomass production, especially under dense canopy structure.(9) Compared with control, inoculation with R. solani treatment has decreased grain yield significantly. Generally, the yield loss was great high under high nitrogen and dense planting, following by the lowest attainable yield. The maximum yield loss by sheath blight inoculation in early-and late-season of2009and2010were30.1%,23.8%,23.7%and48.6%. Sheath blight infestation caused yield loss is driven mainly by the decreased biomass production, rather than decreased harvest index. On the other hand, sheath blight epidemic raised yield loss is contributed by the decreased grain filling, rather than spikelets per m2or grain weight.