The web blight caused by the fungus Rhizoctonia solani (teleomorph Thanatephorus cucumeris) is an important disease of cowpea [Vigna unguiculata (L.) Walp.] in Roraima state, Brazil. The aim of this work was to characterize 28 Rhizoctonia spp. isolates from cowpea plants with web blight, collected in two ecosystems (savannah and forest) in Roraima state. The characteristics evaluated were nuclear number, radial growth rate, the presence and morphology of microsclerotia, anastomosis group (AG) and pathogenicity test. One savannah isolate was binucleate and the others, savannah and forest isolates, were multinucleate. The radial growth rate on potato-dextrose-aga, at 25 ºC in the dark, was 2.1 to 5.3 cm.day-1 for forest isolates and 2.7 to 5.8 cm.day-1 for savannah isolates. In these conditions, after three to four days, microsclerotia formation was observed. Two types of microsclerotia were differentiated: one type of 10 to 50 microsclerotia.Petri dish-1, formed as flat sclerotia mass, which was white when young and pale brown at maturity, 1 to 2 mm in diameter (most forest isolates) and another type of 100 microsclerotia.Petri dish-1, brown color, 68 to 541 µm in diameter (most savannah isolates). Among 28 Rhizoctonia isolates collected in Roraima state, 24 anastomosed with AG1-1A of Rhizoctonia solani.
Rhizoctonia solani; Thanatephorus cucumeris; Vigna unguiculata
