RESUMO
Puccinia sorghi Schwein., the causal agent of maize common rust, is an endemic disease in the Argentine Corn Belt region. Virulence surveys of the pathogen population within the region have not been performed; thus, the understanding of the pathogen population is low and it is difficult to deploy resistance genes that could be effective at controlling the disease. In total, 58 single-uredinial isolates derived from infected maize leaves collected in different locations throughout the Argentine Corn Belt region during 2010 to 2012 were tested on a set of 25 maize lines carrying different Rp genes. Maize lines Rp3-A and PIO19802 showed the lowest virulence frequencies (3.4 and 1.7%, respectively) for all tested isolates. Moreover, the combination in a single genotype of the resistance genes carried by lines Rp3-A and PIO19802 or either of these lines combined with the resistance genes from PIO12345 would confer resistance to all isolates tested. Virulent isolates on maize lines Rp-G, Rp1-K, and Rp-GI were most frequent in 2012. Twenty-four virulence phenotypes were identified, with phenotypes TCCG (17.2%), TTBB (15.5%), and TCFG (10.3%) being the most common throughout the region. Adult plant resistance associated with hypersensitive response was identified at vegetative stage 6 in maize lines PIO68752, PIO28427, and PIO36420.
RESUMO
In the cross of the durable leaf rust resistant wheat Sinvalocho MA and the susceptible line Gama6, four specific genes were identified: the seedling resistance gene Lr3, the adult plant resistance (APR) genes LrSV1 and LrSV2 coming from Sinvalocho MA, and the seedling resistance gene LrG6 coming from Gama6. Lr3 was previously mapped on 6BL in the same cross. LrSV1 was mapped on chromosome 2DS where resistance genes Lr22a and Lr22b have been reported. Results from rust reaction have shown that LrSV1 from Sinvalocho is not the same allele as Lr22b and an allelism test with Lr22a showed that they could be alleles or closely linked genes. LrSV1 was mapped in an 8.5-cM interval delimited by markers gwm296 distal and gwm261 proximal. Adult gene LrSV2 was mapped on chromosome 3BS, cosegregating with gwm533 in a 7.2-cM interval encompassed by markers gwm389 and gwm493, where other disease resistance genes are located, such as seedling gene Lr27 for leaf rust, Sr2 for stem rust, QTL Qfhs.ndsu-3BS for resistance to Fusarium gramineum and wheat powdery mildew resistance. The gene LrG6 was mapped on chromosome 2BL, with the closest marker gwm382 at 0.6 cM. Lines carrying LrSV1, LrSV2 and LrG6 tested under field natural infection conditions, showed low disease infection type and severity, suggesting that this kind of resistance can be explained by additive effects of APR and seedling resistance genes. The identification of new sources of resistance from South American land races and old varieties, supported by modern DNA technology, contributes to sustainability of agriculture through plant breeding.