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Angular Leaf Spot of Phaseolus Beans: Relationships Between Disease, Healthy Leaf Area, and Yield

First, third, and fourth authors: Departamento de Fitopatologia, Escola Superior de Agricultura “Luiz de Queiroz,” Universidade de SÃo Paulo, 13418-900 Piracicaba, SP, Brazil; second author: Instituto Agronômico do Paraná, IAPAR, 86001-970 Londrina, PR, Brazil; fifth author: Plant Pathology Department, University of Florida, Gainesville 32611-0680; sixth author: Institut für Pflanzenkrankheiten und Pflanzenschutz, Universität Hannover, 30419 Hannover, Germany

Five field experiments were conducted to investigate the relationship between the severity of visible disease (X), area under the disease progress curve (AUDPC), healthy leaf area index on any given day (HLAI), radiation intercepted by healthy leaf area on any given day (HRI), healthy leaf area duration (HAD), total healthy leaf area absorption (HAA), and yield of Phaseolus beans, cultivars Rosinha and Carioca, inoculated with Phaeoisariopsis griseola at several doses. In general, yield was not related to disease severity (X) or AUDPC. In contrast, the highest yields were always related to the highest values of HAD and HAA. The relationship between yield and HAD was linear in each of five trials (29.9 < R² < 70.2%, P < 0.001). The relationship between yield and HAA was linear in four of the trials (52.3 < R² < 70.3%, P < 0.001) and exponential in one of them (in which the plant canopy was the largest). Singlepoint models using HRI to estimate yield at various times during the crop season were developed. The slope of the yield-HRI relationship proved to be stable (26.8 ±2.4 g MJ^-1), regardless of cultivar, locale, planting date, and bean growth stage (from R5 to R8). The yield-HLAI relationship proved to be less consistent. HRI is proposed as a key explanatory variable for a transportable system of disease management; it may be useful in producing precise recommendations at the farm level.

Additional keywords: decision making, defoliation, disease assessment, radiation use efficiency, yield loss modeling.