Staygreen Mitigates the Impacts of Fusarium Stalk Rot and Charcoal Rot in Grain Sorghum.

Stalk rots are known to be the most prevalent and destructive sorghum disease worldwide. Despite the absence of reported experimental evidences, the staygreen trait is generally accepted as a physiological means of stalk rot resistance. Staygreen has been shown to be highly correlated with chlorophyll content (measured by Soil and Plant Analytical Development meter: SPAD). The objectives of this study were to test the effects of Fusarium stalk rot and charcoal rot on SPAD readings at three maturity stages and to investigate the relationships between SPAD and stalk rot resistance and tolerance when plants were inoculated with causal organisms. Staygreen and non-staygreen lines and hybrids established in the greenhouse and field were inoculated with three Fusarium spp. (F. thapsinum, F. proliferatum, F. andiyazi) and Macrophomina phaseolina at 14 d after flowering. SPAD readings were obtained at soft-dough, hard-dough, and physiological maturity. Panicles harvested at physiological maturity were measured for total seed weight (TSW) and 100-seed weight (SW). Disease severity was measured by splitting stems longitudinally and measuring lesion length (LL). ANOVA with combined greenhouse and field data revealed significant pathogen, genotype and growth stage main effects on SPAD. All pathogens significantly reduced SPAD readings of tested genotypes in comparison to the control. LL was significantly and negatively correlated to SPAD at physiological maturity, revealing the potential impact of staygreen trait in reducing the extent of pathogen infection. The difference between control and pathogen-treated TSW was significantly and positively correlated to the difference between control and pathogen-treated SPAD at all maturity stages, demonstrating the ability of staygreen trait to enhance stalk rot tolerance under disease pressure.