| Grain mold of Sorghum bicolor is one of the leading constraints for the production of optimum quality sorghum grain worldwide. Several fungi are able to colonize the grain surface after physiological maturity. Only a few species of fungi actually infect early in grain development at or near anthesis. Two important species of grain molding fungi are Fusarium thapsinum and Curvlaria lunata. Curvularia isolates and microconidial Fusarium isolates were obtained from the College Station and Lubbock grain weathering tests (1996), respectively. Twenty-seven microconidial Fusarium isolates that shared the characteristics of slow growth rate, yellow pigment production, and benomyl insensitivity were identified as F. thapsinum. C. lunata was identified on the basis of morphological characters.;Panicles of three resistant genotypes (Sureno, SC170, and Tx2911) and one susceptible genotype (Tx430) were inoculated at anthesis with conidial suspensions of F. thapsinum and C. lunata to test for effects on mature grain at harvest. Disease severity and incidence parameters included empty spikelet ratio (ESR), average weight per seed (AWS), re-isolation of fungal inoculum from seed (RIS), emergence test (ET), estimation of seedling vigor (ESV), and an assessment of peduncle colonization (PC). Inoculations at anthesis resulted in significant decreases in seed weight, emergence, and vigor especially in the susceptible genotype (Tx430). Increases in empty spikelets, peduncle colonization, and re-isolation levels from mature grain were also observed.;Panicles were also inoculated at anthesis with conidial suspensions of F. thapsinum and C. lunata to test for induction of defense response genes in spikelet tissues at several timepoints after flowering. Four PCR fragment clones, phenylalanine ammonia lyase (PAL1-1), chalcone synthase (CHS2G), beta-1,3-glucanase (GLUC2-1) and chitinase (CHIT25-1) were tested and showed increases in expression after inoculation of spikelet tissue with fungi. Accumulation of red-pigmented compounds and increased mRNA levels for PALM and CHS2G indicated that the phenylpropanoid and flavonoid pathways were induced by inoculation, but greater induction did not associate with resistance indicating that this pathway is not solely responsible for defense against grain mold. Defense genes appeared to be elicited non-specifically. |
