Seleção de genótipos de cebola resistentes à podridão bacteriana das escamas e reação de isolados de Burkholderia cenocepacia e B. orbicola ao cobre

Abstract

Onion (Allium cepa L.) is subject to several diseases of different etiologies, being sour skin, caused by species from the Burkholderia cepacea complex (BCC), one of the most important diseases for onion cultivation. Among the BCC species, B. cenocepacia, B. orbicola, B. semiarida, and B. sola have recently been associated with disease in northeastern Brazil, with a predominance of of B. cenocepacia and B. orbicola, in this region. Given the above, this study aimed to select onion genotypes resistant to B. cenocepacia and B. orbicola, and to evaluate the reaction isolates of these species to copper. Initially, 9 isolates of B. cenocepacia and 10 isolates of B. orbicola were inoculated into commercial onion bulbs to determine the levels of aggressiveness. Then, bulbs of 44 onion genotypes belonging to the onion genetic improvement programs of Embrapa Hortaliças and Instituto Agronômico de Pernambuco were inoculated with the most aggressive isolate of each species, selecting 28 genotypes resistant to B. cenocepacia and 23 genotypes to B. orbicola. Subsequently, the three most aggressive isolates of each species were inoculated into onion bulbs of the selected genotypes, and it was observed that 15 genotypes remained resistant to B. cenocepacia and 9 genotypes to B. orbicola, while the genotypes Baia Periforme Agro-ceres, Beta Cristal, BRS Rio Verde, Crioula Mercosul, IPA 7, Primeiro and Rainha remained stable for both species, indicating the presence of genes with potential to be explored in breeding programs aimed at obtaining disease-resistant onion varieties. The reaction to copper was evaluated by submitting the three most aggressive isolates of each species to copper sulfate solutions at concentrations of 0, 100, 200, 300 and 400 mg.L-1 of Cu2+. No bacterial growth was observed from the concentration of 300 mg.L-1 of Cu2+. The treatments at concentrations of 300 and 400 mg.L-1 of Cu2+ were plated in NYDA culture medium and inoculated in commercial onion cataphylls, observing bacterial growth in plaque and the characteristic symptoms of the disease in the inoculated cataphylls, demonstrating an action copper bacteriostatics. Subsequently, suspensions of 18 B. cenocepacia isolates and 45 B. orbicola isolates were inoculated in Petri dishes containing minimal medium plus 300 mg.L-1 of Cu2+, with no bacterial growth being observed. It was concluded that 7 onion genotypes showed satisfactory levels of resistance to B. cenocepacia and B. orbicola, and that copper showed bacteriostatic activity on these species from the concentration of 300 mg.L-1 of Cu2+.