Bacterial leaf blight caused by Xanthomonas axonopodis is a major problem in the production of eucalyptus seedlings. The pathogen causes leaf lesions that reduce the photosynthetic area and limit plant growth. The objective of this study was to evaluate the antibiotic activity of secondary metabolic compounds of a Pseudomonas sp. against X. axonopodis, following extraction with dichloromethane and fractionation by vacuum liquid chromatography. The antibiotic activity of an ethyl acetate phase (F3 fraction) was determined by agar diffusion tests and bioautography. The minimum inhibitory concentration of F3 fraction and the action of F3 on the cell morphology of X. axonopodis was determined by scanning electron microscopy. Electron microscopy revealed changes in exopolysaccharides and morphology of treated cells. When eucalyptus seedlings were sprayed with the F3 fraction before or after inoculation with X. axonopodis in a greenhouse experiment the number of leaf lesions was respectively 93.9% and 89.7% lower than in untreated control plants. High antibiotic activity of the F3 fraction against X. axonopodis and leaf blight suggests that it has potential to control the disease in eucalyptus seedlings.

A mancha angular do algodoeiro (Gossypium hirsutum) causada por Xanthomonas axonopodis pv. malvacearum (Xam) é uma doença de importância econômica para o Brasil, e sua severidade depende de fatores climáticos e da cultivar. A doença não é controlada por produtos químicos sendo que seu controle depende da utilização de sementes sadias, e resistência varietal. DeltaOPAL, EPAMIG Liça, e Fibermax 986 são importantes fontes de resistência a Xam, pois além de resistência, apresentam boas características agronômicas. O objetivo do presente trabalho foi estudar o mecanismo de resistência à mancha angular envolvendo cruzamentos entre as três cultivares resistentes e a suscetível BRS Ita 90, em casa de vegetação. Populações parentais, F1 e F2 foram avaliadas após inoculação com um isolado agressivo de Xam. As três cultivares apresentaram mecanismos diferentes de resistência. Nas cultivares DeltaOPAL e EPAMIG Liça a resistência é conferida por um gene dominante, enquanto que em Fibermax 986, a resistência é dada por dois genes dominantes, independentes e complementares.

Angular leaf spot of cotton (Gossypium hirsutum) caused by Xanthomonas axonopodis pv. malvacearum (Xam) is an economically important disease in Brazil. The severity of the disease depends on climatic conditions and the type of cultivar. The disease is not satisfactorily controlled by the use of chemicals, but with control depending basically on the use of healthy seed and resistant varieties. Cultivars DeltaOPAL, EPAMIG Liça and Fibermax 986 are the best sources of resistance to Xam, since besides resistance, these cultivars possess desirable agronomic characteristics. The objective of the present investigation was to verify the mechanism of resistance to angular leaf spot in these three resistant cotton cultivars by crossing them with the susceptible cultivar BRS Ita 90, under glasshouse conditions. Parental populations F1 and F2 were evaluated in the glasshouse after inoculation with an aggressive isolate of Xam. All the three of the cultivars revealed different mechanisms of resistance. In cvs. DeltaOPAL and EPAMIG Liça, resistance is governed by one dominant gene, whereas in cv. Fibermax 986, the resistance is governed by two dominant, independent and complementary genes.