European Journal of Plant Pathology 109: 625–632, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. Relationships between angular leaf spot, healthy leaf area, effective leaf area and yield of Phaseolus vulgaris W.C. Jesus Junior 1,4 , F.X.R. Vale 1 , R.R. Coelho 1 , P.A. Paul 2 , B. Hau 3 , A. Bergamin Filho 4 , L. Zambolim 1 and R.D. Berger 5 1 Departamento de Fitopatologia, Universidade Federal de Vic ¸osa, Vic ¸osa-MG 36571-000, Brazil (Phone: +55 19 3429 4124; Fax: +55 19 3434 4839; E-mail: wcjesus@esalq.usp.br); 2 Plant Pathology Department, Iowa State University, Ames, Iowa 50011-1020, USA; 3 Institut fur Pflanzenkrankheiten und Pflanzenschutz, Universitat Hannover, Hannover 30419, Germany; 4 Departamento de Entomologia, Fitopatologia e Zoologia Agr´ ıcola, Escola Superior de Agricultura ‘Luiz de Queiroz’, Universidade de S˜ ao Paulo, Piracicaba-SP 13418-900, Brazil; 5 Plant Pathology Department, University of Florida, Gainesville, FL 32611-0680, USA Accepted 25 March 2003 Key words: common bean, disease assessment, Phaeoisariopsis griseola, yield loss Abstract Three field experiments were carried out with the bean cultivar Carioca Comum to investigate the relationships among visual and virtual severity of angular leaf spot (caused by Phaeoisariopsis griseola), area under visual and virtual disease progress curves (AUDPC), healthy leaf area index on any given day (HLAI), healthy leaf area duration (HAD), healthy leaf area absorption (HAA), effective leaf area duration (ELAD), effective leaf area absorption (ELAA) and yield of Phaseolus vulgaris. To obtain a wide range of disease severities, the plots were sprayed with fungicide at different stages of plant growth (before, during and after flowering). Visual and virtual severity and AUDPC showed no significant correlation with yield. However, HAD, HAA, ELAD and ELAA were significantly correlated with yield. Variables that considered the effective leaf area (ELAD and ELAA) provided similar or better coefficients of determination (R 2 ) than those that considered the remaining green leaf area only (HAD and HAA). Single-point models with HLAI, effective leaf area index (ELAI), intercepted radiation by healthy leaf area (HRI) and intercepted radiation by effective leaf area (EHRI) to estimate yield at various times during the crop season were developed. The slope of the relationship between yield and HLAI, ELAI, HRI and EHRI proved to be stable, regardless of planting date and bean growth stage (from R6 to R8). Introduction Angular leaf spot, caused by Phaeoisariopsis griseola, is one of the most destructive foliar diseases of common beans (Phaseolus vulgaris) in Brazil. The pathogen causes lesions on leaves, pods, branches and petioles and may cause severe defoliation. Without adequate disease control, yield reductions of up to 70% have been reported in Brazil (Brenes et al., 1983) and 80% in Colombia (Schwartz et al., 1981). Information about crop loss is the basis for any valid economic analysis of disease management measures. To determine crop losses, plant pathologists have examined the relationship between crop yield and disease severity. The relationship is often disappoint- ing. Waggoner and Berger (1987) discuss some of the reasons why this may be so. In contrast, crop production is often closely related to the amount of solar radia- tion utilized by the plant (Monteith and Elston, 1983). Healthy leaf area duration (HAD) and healthy leaf area absorption (HAA) have been shown to be better pre- dictors of yield than disease severity (Waggoner and Berger, 1987). HAD is the integral of the healthy leaf area index over the growing season and HAA is the