Relative ratio of mature pustules: a simple method to assess partial resistance of barley to Puccinia hordei

In plant breeding and germplasm evaluation, large-scale assessment of quantitative resistance is desirable. but feasible only if a simple and accurate measure is available. In several plant-pathogen systems. latent period (LP) is a parameter that is well correlated with the level of partial resistance observed in field trials. However, measuring LP or relative LP (RLP), i.e., relative to the reference accessions, is laborious. We investigated the value of relative ratio of mature pustules (RRMP. relative to the susceptible control) as a simple and rapid alternative to replace LP estimation in barley to barley leaf rust (Puccinia hordei). A set of 103 F-9 recombinant inbred lines (RILs) derived from a cross L94 x Vada was sown in a greenhouse compartment, and was inoculated at the seedling stage with isolate Uppsala or 24, and at the adult plant stage with isolate 24. In demarcated sections of leaves, the number of mature pustules was counted several times after inoculation. The ratio of mature pustules (RMP) and LP50 were calculated to assess RRMP and RLP. respectively, and to identify the quantitative trait loci (QTLs) contributing to the genetic variation. The contrasts in RRMP among accessions were highest when the susceptible reference line had developed 70 to 90% mature orange pustules, the immature infection sites being visible as pale flecks. At this optimal time of observation, the correlation between RRMP and RLP in both the seedling stage and the adult plant stage was highly significant (r = -0.82 similar to -0.98). Compared with RLP, RRMP was much easier and simpler to measure and still showed good correspondence with RLP in the identification of QTLs for partial resistance. In another experiment at the seedling stage with 25 barley cultivars and lines inoculated with isolate 1.2.1, the coefficient of correlation between RRMP and RLP was -0.98. Therefore, we conclude that RRMP should have great application potential in breeding programs and germplasm screening and could be used in fundamental studies as well.