Relationships of disease and leaf area variables with yield in the Alternaria helianthi–sunflower pathosystem

Abstract

Three field experiments were carried out in 1997/1998, 1998/1999 and 1999/2000 to investigate the relationship between severity (S) of alternaria leaf spot, caused by Alternaria helianthi, area under disease progress curve (AUDPC), healthy leaf area index at any given day (HLAI), healthy leaf area duration (HAD), radiation intercepted by healthy leaf area at any given day (HRI), total healthy leaf area absorption (HAA), and yield components of sunflower sown on four dates. AUDPC showed a relationship with yield in two years, following the negative exponential model coefficient of determination (R2 = 0·79 and 0·60). The HAD–yield relationship was linear for each of three trials (0·73 < R2 < 0·83). Among the integral variables analysed, the HAA–yield relationship was the best, fitted by the exponential model (0·77 < R2 < 0·89). Single‐point models using disease severity, HLAI and HRI to estimate yield at various times during the crop season were tested. The slope of the severity–yield relationship was stable from the R1 (inflorescence visible) growth stage. The HLAI–yield relationship was stable between R1 and R6 (anthesis completed) and increased from the R7 (first phase of achene development) stage. The HRI–yield relationship was variable and not stable. The relationship between severity and yield in the R3 (second phase of inflorescence elongation) growth stage proved that plants with disease severity higher than 10% had yield lower than 500 kg ha−1, regardless of the sowing date. This value can be used as a damage threshold for the disease. Disease severity can be used as an independent variable in a sunflower–alternaria leaf spot management system and can be used to produce recommendations at the research level, such as genetic breeding for disease resistance or studies on sowing date.