EFFECT OF PATTERN AND SEVERITY OF MOISTURE-DEFICIT STRESS ON STALK-ROT INCIDENCE IN SORGHUM .2. EFFECT OF SOURCE SINK RELATIONSHIPS

Stalk-rot of grain sorghum [Sorghum bicolor (L.) Moench] is most commonly associated with weakly parasitic root and stalk-rot fungi when the host plants are subjected to environmental stresses. The incidence of rots is generally more in high-grain-yield environments associated with high plant density or fertilizer application. The effects of time of occurrence and degree of moisture deficit stress (moisture stress) on grain and biomass yields, and the natural incidence of stalk-rots were studied. Stress during grain-filling had a greater effect on incidence of rots than stress at earlier stages. Grain-yield and disease incidence were differently affected by timing and severity of moisture stress. Stalk-rot incidence was most commonly and strongly associated with moisture stress at the terminal stage of growth. However, stress during panicle development induced changes in the sink size (grain number) or root-growth pattern, which in turn influenced both the timing and extent of stalk-rot incidence. The amount of biomass produced during the later part of the grain-filling period was positively correlated with lower disease susceptibility. The distribution index (i.e., the ratio of grain-yield to biomass produced after flowering) could be generally used to predict disease susceptibility. There were no simple correlations between biomass, grain-yield or yield components, and stalk-rot. Implications of these findings for sorghum production and for stalk-rot resistance screening are discussed.