South Australian Research and Development Institute, Urrbrae, South Australia 5064

Background and objectives
The spot form of net blotch caused by Pyrenophora teres f. maculata has become a common disease of barley in south-eastern Australia since it was first identified in 1990. The disease is associated with close rotations, particularly where barley is grown in alternate years. The increasing use of stubble retention in farming systems is also responsible for an increase in the disease. The most effective method for control of the disease is the use of resistant varieties. Sources of resistance for use in Australian breeding programs are being sought using field and seedling tests. The genetics of resistance are being evaluated through the use of genetic marker technologies and useful molecular markers are being developed for use in breeding programs.

Materials and methods
Field screening using naturally occurring inoculum was used to identify resistance in a range of varieties and breeding lines. Two effective sources of resistance in adapted backgrounds have been identified in the cultivar Galleon and the breeding line WI 2976, both of which have Ethiopian ancestry. Ninety-eight doubled haploid-derived lines from the cross Galleon x Haruna Nijo, a very susceptible Japanese variety, were screened as seedlings using a mixture of four fungal isolates. The tests involved growing three clumps of 5 seedlings per 4-inch pot. Each line was represented by four replicated clumps in different pots. The seedlings were inoculated at the two leaf stage, incubated for 24 h at 19C, followed by a 24/14C 12 h day/night regime for 14 days, and then scored using a 0-9 rating scale very similar to that devised by Tekauz [1]. To determine the location of loci contributing to resistance the programs Q-gene and Mapmanager were used to compare the trait scores to previously mapped RFLP and PCR-based marker loci in this cross.

Results and conclusions
Segregation data for spot form of net blotch in the doubled-haploid derived lines from the cross Galleon x Haruna Nijo indicated that resistance is conferred by a single locus. Results from the QTL analysis indicated that this locus was on chromosome 7H in the cultivar Galleon and accounted for 80% of the total variation observed in the seedling screening tests. The designation Ptm is proposed for this locus. A number of markers closely linked to Ptm were identified, the nearest two of which flanked the resistance locus at distances of approximately 8 and 11 cM. One of these markers is polymorphic between Galleon and a range of other varieties including Blenheim, Clipper, Franklin, Halcyon, Natasha, Prisma and Triumph. This marker will be a useful tool for marker-assisted selection of spot form of net blotch resistance in progeny from crosses between these varieties. Further work is in progress with the other linked markers to reveal more polymorphism between breeders' lines and to determine if the resistance in WI2976 is allelic to Ptm.

1. Tekauz A, 1985. Canadian Journal of Plant Pathology 7:181-183.