Journey of net blotch: From pathotype diversity to useful resistance in barley

Studies on variation, occurrence and distribution of virulence in Pyrenophora teres f. teres are essential to identify effective sources of resistance for net type net blotch. Disease surveys suggested two different stains are prevalent in Western Australia and 13 in all around Australia. Sixty nine barley lines from different breeding groups in Australia and elsewhere were tested against most prevalent pathotypes. Majority of lines have partial to complete resistance while some have elite resistances to net type net blotch. Four lines out of 69 were chosen for further studies. These four lines: WA 4794 (103 IBON 91), Pompadour, CI 9214, and WPG 8412-9-2-1 were highly diverse and resistant to most of the isolates, and were crossed with Stirling-a highly adaptive but susceptible cultivar. Doubled haploids, F2s, and resistant x resistant crosses were studied against five prevalent isolates. Four genes from WA 4794 (all dominant), three (two dominant and one recessive) from Pompadour, five (two dominant and three recessive) from CI 9214, and two (one dominant and one recessive) from WPG 8412-9-2-1 were identified. In total, 11 different genes were operative against P. teres f. teres isolates. Molecular work is initiated to develop markers which would aid screening of the breeding populations for these resistances.

The onset and effectiveness of Adult Plant Resistance (APR) in Tallon barley

Field observations of net blotch epidemics indicated that Tallon barley was quite resistant to infection during later stages of growth despite being susceptible as a seedling. A glasshouse experiment was conducted to determine the effectiveness of this resistance and when it became operative. Three cultivars – Gilbert (very susceptible), Patty (resistant) and Tallon – were inoculated at various stages of growth with conidia of Pyrenophora teres f. teres and the infection response and leaf area diseased, recorded 13 days later. The response of Tallon clearly changed from susceptible to moderately susceptible at growth stage 33. Plants sown two weeks earlier were susceptible and plants sown two weeks later were moderately resistant. The response of the other two cultivars at similar growth stages paralleled their seedling responses. The resistance of Tallon appeared to increase with maturity so that, at its most resistant growth stage, the leaf area diseased was just 10% that of the susceptible, Gilbert. While this resistance appears pathotype specific, this experiment demonstrated very effective APR to net blotch. As most losses to this disease occur during the later stages of plant development, APR offers a valuable source of resistance.

Mapping genes for resistance to net form of net blotch and strip rust in barley (Hordeum vulgare L.)

A dihaploid mapping population comprising 65 lines was developed between barley parent varieties Tallon and Kaputar and used to construct a genetic linkage map. This map, comprising 195 amplified fragment length polymorphism and 38 simple sequence repeat markers, was used to identify markers linked to the net form of net blotch (Pyrenophora teres f.sp. teres) and to stripe rust (Puccinia striiformis f.sp. hordei) in barley. The population was screened with five pathotypes of net blotch at the seedling stage in the glasshouse and subjected to a natural inoculation in Hermitage, Queensland. Stripe rust screening was conducted at the adult plant stage in Toluca, Mexico. Analyses of the markers were performed using Mapmanager and Qgene software. One region on chromosome 6H was highly significantly associated with resistance to the net blotch (R2 = 79%). This association was consistent for all pathotypes studied. One region on chromosome 5H was found to be highly significantly associated with resistance to stripe rust (R2= 65%). There are a number of very closely linked markers showing strong associations in these regions, and these markers present an opportunity for marker assisted selection of these traits in barley breeding programs.