RESUMO
Brown rust (causal agent Puccinia melanocephala) is an important sugarcane disease that is responsible for large losses in yield worldwide. Despite its importance, little is known regarding the genetic diversity of this pathogen in the main Brazilian sugarcane cultivation areas. In this study, we characterized the genetic diversity of 34 P. melanocephala isolates from 4 Brazilian states using loci identified from an enriched simple sequence repeat (SSR) library. The aggressiveness of 3 isolates from major sugarcane cultivation areas was evaluated by inoculating an intermediately resistant and a susceptible cultivar. From the enriched library, 16 SSR-specific primers were developed, which produced scorable alleles. Of these, 4 loci were polymorphic and 12 were monomorphic for all isolates evaluated. The molecular characterization of the 34 isolates of P. melanocephala conducted using 16 SSR loci revealed the existence of low genetic variability among the isolates. The average estimated genetic distance was 0.12. Phenetic analysis based on Nei's genetic distance clustered the isolates into 2 major groups. Groups I and II included 18 and 14 isolates, respectively, and both groups contained isolates from all 4 geographic regions studied. Two isolates did not cluster with these groups. It was not possible to obtain clusters according to location or state of origin. Analysis of disease severity data revealed that the isolates did not show significant differences in aggressiveness between regions.
Assuntos
Basidiomycota/genética , Variação Genética , Repetições de Microssatélites , Sequência de Bases , Basidiomycota/classificação , Brasil , Primers do DNA , Marcadores GenéticosRESUMO
In December 2009, on a farm located near the city of Rincão, state of São Paulo, Brazil, uredinial leaf lesions typical of a rust disease were observed to be affecting sugarcane (a complex Saccharum L. hybrid). Lesions were observed in a nursery plot of a precommercial cultivar (cv. Centauro) known to be resistant to brown rust caused by Puccinia melanocephala Syd. & P. Syd. Leaf samples, inspected under a light microscope, revealed spores identical morphologically to those described for the fungus P. kuehnii E.J. Butler, causal agent of sugarcane orange rust (1,3). Telia and teliospores were not observed. The Ministry of Agriculture was notified on 7 December 2009 in accordance with current Brazilian quarantine legislation. On 9 December, a specialized team from the Federal Phytosanitary Defense Department (DSV/MAPA) started a survey at the farm, collected leaf samples for additional analyses, and confirmed the presence of P. kuehnii E.J. Butler in Brazilian territory. During the following weeks, the disease was observed in several sugarcane-growing areas of the southern states of São Paulo and Paraná. It affected a small proportion of clones under selection in several breeding programs and the commercial cultivars SP89-1115, RB72454, and SP84-2025, which are now considered to be highly susceptible to the disease and currently cover less than 10% of the country's commercial sugarcane area. To our knowledge, this is the first report of P. kuehnii infecting sugarcane in Brazil, the largest world producer with more than 8 million ha of this crop. Orange rust is expected to expand to other important Brazilian sugarcane-producing areas, including the northeastern states and a new sugarcane expansion area in the central part of Brazil. The disease is widespread in the Asia-Pacific Region, caused a major epidemic in Australia in 2000, and was reported in the Western Hemisphere in 2007-2008 in several countries in North (1,2) and Central America (2,3) prior to this detection in South America. References: (1) J. C. Comstock et al. Plant Dis. 92:175, 2008. (2) R. C. Flores et al. Plant Dis. 93:1347, 2009. (3) W. Ovalle et al. Plant Dis. 92:973, 2008.