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Title: Genetic diversity of Puccinia kuehnii, the causal agent of orange rust of sugarcane, from Brazil

Author
item URASHIMA, ALFREDO - Universidade Federal De Sao Carlos
item MISTURA, TATIANE - Universidade Federal De Sao Carlos
item RODRIGUES PORTO, LIS - Universidade Federal De Sao Carlos
item Austin, Paige
item Arias De Ares, Renee

Submitted to: Journal of Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/14/2020
Publication Date: 8/8/2020
Citation: Urashima, A.S., Mistura, T.F., Sakuno, C., Austin, P.D., Arias De Ares, R.S. 2020. Genetic diversity of Puccinia kuehnii, the causal agent of orange rust of sugarcane, from Brazil. Journal of Phytopathology. https://doi.org/10.1111/jph.12937.
DOI: https://doi.org/10.1111/jph.12937

Interpretive Summary: The rust of sugar cane caused by the fungus Puccinia kuehnii results in significant economic loses. In some countries e.g. Australia, the disease has been effectively managed by the use of resistant varieties of sugar cane; but in Brazil this method has shown ineffective as varieties considered resistant were now infected. Given the relative proximity of Brazil to United States, and the fact the rust spores are known to travel long distances by air; it was in our interest to determine the reason for supposedly resistant varieties to get infected. We determined that there are at least five groups of genotypes of the sugar cane pathogen Puccinia kuehnii, and we also identified statistically significant pathogen/sugar cane interactions, where certain genotypes are more virulent than others to each particular variety of sugar cane.

Technical Abstract: The use of resistant genotypes is the preferred method to control orange rust of sugarcane (Saccharum spp) caused by Puccinia kuehnii. This approach has been adopted in Brazil but outbreaks of the disease on previously resistant varieties showed that the efficacy of this method is limited and requires a better understanding of pathogen diversity. Nevertheless, adequate molecular markers for examining pathogen diversity at population level are not available, which limits the success of orange rust control by genetic resistance. Therefore, two independent investigations were conducted to examine genetic diversity of P. kuehnii from São Paulo state, the most important sugarcane growing state of Brazil. First, simple-sequence repeat (SSR) markers were developed in the present work and genotypic diversity of orange rust isolates from different locations investigated. Second, phenotypic diversity was examined by the single-pustule inoculation technique on P. kuehnii isolates retrieved from three susceptible commercial sugarcane cultivars. A total of 96 SSR markers were generated and tested for this species. Subsequently, 29 isolates of P. kuehnii were fingerprinted with nine SSR markers to estimate the genotypic diversity by neighbour-joining and 3D principal coordinates. The 29 isolates of the pathogen clustered into four main groups, which were identified by three SSR markers (NPRL_PK_108a, NPRL_PK_162_spka and NPRL_PK_221_spka). Phenotypic data at 21 days after the single-pustule inoculation showed that P. kuehnii from highly susceptible commercial cultivars harboured a small proportion of variants capable of causing disease on resistant cultivars. A differential reaction was demonstrated for the most virulent variant in a repeated experiment confirming the existence of races within P. kuehnii in Brazil.