The expression of resistance in subterranean clover (Trifolium subterraneum) to races 1 and 2 of Kabatiella caulivora is affected by inoculum pressure but not by combinations of the two races

The performance of subterranean clover (Trifolium subterraneum) varieties in the field in the presence of the clover scorch pathogen, Kabatiella caulivora, can be highly variable in terms of the disease levels that develop. Although the influence of environmental conditions on development of this disease is documented, the influence of inoculum pressure or race combinations has not previously been investigated. Hence, separate controlled environment studies were undertaken to identify the effects of inoculum concentration and the effects of race combinations upon the expression of resistance to clover scorch disease in seedlings of seven subterranean clover varieties. In relation to the percentage of petioles with lesions following inoculation with different conidial concentrations of either race 1 or race 2 of K. caulivora, there was a significant effect of races (P<0.001), varieties (P<0.001) and of inoculum concentration (P<0.001) and also a significant interaction between races and varieties (P<0.001), and of inoculum concentration with varieties (P<0.05). Although levels of disease observed on the varieties tested were generally comparable to those observed in the field, the findings in relation to inoculum concentration may explain why the performance of some subterranean clover varieties in the field in the presence of the clover scorch pathogen can be highly variable, as evidenced by certain incidences of severe disease and/or collapses of field swards of Meteora and Karridale varieties in Western Australia. When inoculated with varying proportions of race 1 and race 2, there was a significant effect of race combination treatments (P<0.001), but there was no indication of any additive or interference effects between the two races when present together as a combination on the same plant in relation to host expression of resistance to each individual race. Together, these studies showed, for the first time, that expression of resistance to K. caulivora in subterranean clover is dependent on pathogen inoculum level but independent of the races present as mixtures. The implications of these studies are 3-fold. First, that variable expression of resistance in the field is related to inoculum pressure. Second, that seedling resistance to individual races should be able to be identified even where both races occur together in combination, eliminating the current expensive requirement of utilising a separate field screening site for each race. Third, these results indicate that while effective deployment of host resistance is particularly rewarding where cultural and/or chemical strategies are in place to keep the inoculum of the pathogen at manageable levels, it can also be rewarding even under high inoculum pressure.