Caracterização morfológica, fisiológica, isoenzimática e controle de isolados de Corynespora cassiicola (Berky & Curt) Wei, agente causal da mancha alvo

Abstract

Corynespora cassiicola is the causal agent of target spot of several crops of economical importance such as soybean, cotton, tomato, cucumber etc. Sixteen isolates of this fungus from several hosts and different regions in Brazil were obtained and conducted morphologic, physiologic, isozymic characterization as well as control, in laboratory and in greenhouse of the Center of Research in Phytopathology of Escola de Agronomia e Engenharia de Alimentos, da Universidade Federal de Goiás , located in Goiânia, State of Goiás. The main objectives of this investigation were to characterize: a) morphology: by determination of dimensions and number of pseudosepta in one hundred spores of each isolate; b) physiology: by evaluating mycelial growth under different temperatures (15oC, 20oC, 25oC, 30oC and 35oC) and photoperiods (0, 12 and 24 hours); sporulation in different photoperiods; hours of continuous humidity to improve spores germination; the pathogenicity of several isolates of C. cassiicola by inoculation on several hosts (cucumber, soybean, cotton, coffee, Barbados cherry, tomato and melon) and the preservation of 10 isolates using three methods: Castellani, mineral oil and periodic transfer; c) isozyme analysis: with polyacrylamide gel eletrophoresis of esterase of 14 isolates; and d) control: by reactions of nine cucumber hybrids to C. cassiicola; and the sensibility of six isolates of C. cassiicola to eight fungicides in vitro (captan, chlorotalonil, mancozeb, azoxystrobin, difeconazole, carbendazim, tebuconazole and thiophanatemethyl) using concentrations of 0, 1, 10, 100 e 1000 μg mL-1; fungicide treatment of cucumber plants inoculated with C. cassiicola. The following results were obtained: a) the dimensions (length and width) of spores were in accord with the key for the identification of pathogen; b) the best temperature for mycelial growth in potato-dextrose-agar medium (PDA) was 25oC; eight isolates exhibited response to the photoperiods used whereas six were sensitive to photoperiod; the majority of isolates sporulated more under 24 hours of light; eight hours of continuous humidity was enough to germinate the majority of spores of the isolates; the pathogenicity of isolates was observed and some hosts were more specifics to their isolate of origin, such as tomato plant, and others such as melon and cotton plants were less specifics; and the mycelial growth of the great part of isolates were not affected by the preservation method; sporulation showed great variation among the methods and all preservation methods conserved the pathogenicity of the isolates; c) the eletrophoresis analysis of sterase isozyme showed variation in number of the bands and positions in the gel which indicated differences among isolates, these were divided in two different groups of similarity, probably indicating the existence of races among them; d) the less susceptible hybrids were Taisho, Nikkey, Yoshinari and Safira; the difenoconazole fungicide resulted in better porcentage of pathogen inhibition in vitro and carbendazim and thiophanate-methyl presented the worst results, and this last one can not be able to inhibite the mycelial fungi growth and before seven and fourteen days of artificial inoculation of pathogen and application of fungicides, only azoxystrobin was efficient to control the disease.