Abstract
Differences in grain mould disease levels among different sorghum varieties grown in the same environment imply that host genes play a role in controlling disease severity. The fungi most often recovered from naturally infected sorghum grain, Fusarium thapsinum and Curvularia lunata, were used to inoculate a set of resistant and susceptible cultivars at anthesis in both field and glasshouse trials. In the field, 12 cultivars were inoculated with a mixture of F. thapsinum and C. lunata and, in the glasshouse, individual panicles from four selected cultivars were inoculated with spore suspensions of C. lunata, F. thapsinum, a mixture of the two or water to serve as a control. Based on grain mould severity ratings and germination tests on the seed from the field trial, Tx2911, Sureno, SC719-11E and SC650-11E displayed a high level of resistance to grain mould. To determine whether resistant and susceptible lines differed in response to the pathogens, PR-10 mRNA levels were measured using real-time reverse transcriptase-polymerase chain reaction. PR-10 is a protein with antifungal properties that has been associated with defence responses in sorghum and other plant species. In field tests, most, but not all, cultivars showed significant induction of normalized relative quantities of PR-10 after dual inoculation with spores of both C. lunata and F. thapsinum. Under glasshouse-controlled conditions, glumes of inoculated plants showed the clear induction of PR-10 mRNA, and the response was greater in resistant (Tx2911 and Sureno) than in susceptible (RTx430 and SC170-6-17) cultivars. Inoculation with spores from a single mould-inducing pathogen generally induced greater responses than when spores were combined. For RTx430, SC170-6-17 and Sureno, the response to C. lunata was greater, whereas Tx2911 showed a stronger response to F. thapsinum. The results indicate that the induction of PR-10 in sorghum glumes may be a factor useful in breeding programmes designed to combine multiple factors for resistance.