MJ BARBETTI, P SI and PGH NICHOLS

Agriculture Western Australia, 3 Baron-Hay Court, South Perth, Western Australia 6151, Australia; Centre For Legumes In Mediterranean Agriculture, University Of Western Australia, Mounts Bay Road, Nediands, Western Australia 6009, Australia

Background and objectives
Clover scorch, caused by the fungus Kabatielia caulivora, has been a major disease of subterranean clover (Trifolium subterraneum) in Australia since the early 1970s. Cultivars with resistance to the disease have been released and widely grown. However, in 1990 a new race of the fungus, able to overcome the previous resistance of many of these cultivars, was discovered in Western Australia [1]. This new race has become widespread, causing severe losses in south coastal areas of Western Australia. In order to breed resistant cultivars, an understanding of the genetic control of resistance to both races of Kabatielia in subterranean clover is required. A half diallel study, using 7 cultivars with differing reactions to the two races crossed in all possible combinations, was conducted. For both races, 120 single plants of F2 progeny from each cross and the 7 parents were tested.

Results and conclusions
Disease scores between the parents varied widely for both races. Denmark and Goulburn were highly resistant to both races. Meteora, the only member of ssp. yanninnicum in the study, also had resistance to both races. Daliak was the most resistant to the old race, but was susceptible to the new race. Karridale and Mt Barker both had an intermediate reaction to the old race but were susceptible to the new race. Woogenellup was susceptible to both races. For each cross, F2 population means were generally within the parental range. Progenies from Denmark crosses had low mean disease scores to both races, while progenies from Daliak and Meteora crosses generally had low mean disease scores to only the old race. However, for both races, all progenies from Goulburn crosses had higher mean disease scores than the Goulburn parent.

Diallel analysis showed that the genetic control for resistance to both races was governed by simple additive and dominant gene effects. Regression analysis indicated an absence of non-allelic gene interactions and the presence of partial dominance.

For the old race, Daliak, Denmark and Meteora showed dominant gene effects for resistance. This is consistent with the results of Beale and Thurling [2] for ssp. yanninnicum. Goulburn, however, showed recessive gene effects for resistance. The most susceptible parent, Woogeneflup, had recessive genes for susceptibility, while Karridale and Mt Barker showed intermediate gene effects. For the new race, resistance was controlled by dominant genes in Denmark, with Goulburn showing recessive gene effects. Daliak and Mt Barker showed recessive gene effects for susceptibility. However, Meteora, Karridale and Woogenellup showed intermediate effects.

The number of genes controlling resistance to the old race was determined in three crosses between a resistant cultivar (Daliak, Meteora, or Denmark) and the susceptible Woogenellup. F2 plants were examined for their segregation ratios. Daliak and Meteora both had one completely dominant gene governing resistance. However, the resistance in Denmark was controlled by two genes with complete dominance, which may explain its resistance to both races in the field.

Incorporating multiple gene resistance to Kabatielia into a range of subterranean clover cultivars is now a major subterranean clover breeding objective, in order to provide more durable resistance.

References
1. Barbetti MJ, 1995. Australian Journal of Experimental Agriculture 43, 645-53.
2. Beale PE, Thurling N, 1980. Australian Journal of Agricultural Research 31, 927-33.