Diversidade de populações de Xanthomonas phaseoli pv. manihotis do estado do Pará e reação de cultivares de mandioca à bacteriose

Abstract

Cassava (Manihot esculenta Crantz) is a crop of food and industrial importance. However, one of the main diseases that may represent limitations for this crop is bacteriosis caused by the bacteria Xanthomonas phaseoli pv. manihotis (Xpm), which is responsible for yield losses ranging from 30 to 80%. Therefore, selecting resistant cultivars is considered the best control strategy against this disease. However, the intensity of the disease can vary depending on the aggressiveness of the strain and there is currently no information available on the genetic diversity of Xpm isolates from the northern region of Brazil. Furthermore, the study of pathogen variability is an important tool to assist genetic improvement programs and epidemiological studies of the disease. Therefore, the objective of this work is to evaluate the genetic and pathogenic variability of Xpm isolates from the State of Pará using MLSA, MLVA, and rep-PCR techniques, as well as to assess the aggressiveness of the strains and select cassava cultivars resistant to bacteriosis in a greenhouse setting, utilizing selected cassava materials through genetic improvement programs. The tests were conducted in the Phytopathology Laboratory and the greenhouse facilities at Embrapa Amazônia Oriental. The results obtained from investigating Xanthomonas phaseoli pv. manihotis using the MLSA technique demonstrated that it was efficient in identifying and demonstrating that there is genetic variability through the opening of several clades among isolates of the same species. Conversely, REP-PCR effectively separated different species but provided limited information on the variability among Xpm isolates. The MLVA technique was considered the most effective method for demonstrating variability among isolates and the proximity relationships of strains from the same origin. The similarity of each VNTR dendrogram varied between 80 and 100%. Therefore, the present work makes it clear that isolates of the species Xanthomonas phaseoli pv. manihotis from the State of Pará present genetic diversity, but the polymorphism of the pathogens cannot be determined based on their places of origin, as different haplotypes are present in a dispersed form in several cassava-producing municipalities. All isolates tested in the BRS Poti cultivar induced the typical symptoms of the disease, demonstrating the existence of variability in the aggressiveness of the pathogen. The cultivar reaction results to Xpm demonstrated that all cassava materials exhibited symptoms with varying levels of incidence. Therefore, isolates of Xanthomonas phaseoli pv. manihotis present a significant genetic diversity, which suggests the occurrence of dissemination of these pathogens to several cassava-producing regions in the State of Pará and the exchange of genetic material between them resulting from mutation processes generating a variety of haplotypes for the species under study. Therefore, the MLVA technique was efficient in demonstrating the genetic distance between the isolates and quantifying the haplotypes present. However, the severity of the disease was not demonstrated by genetic variability or geographic origins, except for isolates Xam 17 and Xam 18, which obtained high genetic similarity in all molecular tests, in terms of disease severity and also originate from same municipality. Even in the face of highly aggressive isolates, 23 particularly resistant cultivars were selected for the State of Pará.