Indução da supressividade à rizoctoniose do feijão-caupi pela rotação de culturas e adubação verde

Abstract

Rhizoctonia canker, caused by Rhizoctonia solani, is an important cowpea disease in Northeastern region of Brazil. This thesis aimed: a) to evaluate the potential of crop rotation and green manures in inducing suppressiveness to Rhizoctonia canker in cowpea; b) to identify the biotic factors related to suppression; and c) to analysis the stability of the disease suppression by selected crop rotation in relation to different isolates and inoculum density of the pathogen, and soils from different locations. The experiments were conducted in microplots in the field. In relation to crop rotations, Initially, 14 treatments were compared in four growing seasons, including different combinations of cowpea (CA) in rotation with cotton (AL), sunflower (AL), castor bean (MA), corn (MI) and sorghum (SO), and in combination with fallow (PO) and cowpea monoculture. The rotations CA-MI-SO-CA, CA-GI-SO-CA and CA-MI-PO-CA provided reductions of 77.6%, 74.0% and 73.9% in the disease severity, respectively. When the stability of disease control by CA-MI-SO-CA rotation was evaluated in relation to different pathogen isolates, inoculum densities and soils, in all situations the disease severity was significantly (P0,0001) lower under crop rotation as compared to cowpea monoculture, indicating the potential of the crop rotation for controlling the Rhizoctonia canker. In relation to green manures, 13 treatments were compared in five growing seasons, considering different combinations of cowpea and green manures (sunn hemp, breviflora crotalaria, showy crotalaria, jack bean, velvet bean, dwarf mucuna, lablab bean, dwarf pigeon pea, forage pigeon pea, corn, millet and sorghum). For the two evaluations after green manures cultivation, the incorporation of sorghum into the soil led to smallest levels of severity of cowpea Rhizoctonia canker, reducing by 81.3% and 83.3% the disease severity. There was no significant correlation from levels of disease severity with microbiological variables.