ANALYSIS OF CROP LOSS IN THE MULTIPLE PATHOSYSTEM GROUNDNUT RUST LATE LEAF-SPOT .1. 6 EXPERIMENTS

A series of six experiments was conducted to study the relationships between production situation, injuries, and damage in the groundnut-rust-late leaf spot pathosystem. The production situation, represented by attainable yields, was varied by replicating the experiments over seasons and incorporating several input factors at different levels. Injuries, represented by log-transformed areas under disease progress curves, were manipulated by means of inoculations and fungicide applications. The resulting database was used to develop damage functions, represented by yield and relative damage response surfaces, using multiple regression analysis. The corresponding equations indicate significant interactions between attainable yield and injuries on actual yield and relative damage. Further analysis indicates that injury-damage relations differ in rust and late leaf spot: whereas damage due to late leaf spot was mainly related to reduction of green leaf area and defoliation, damage due to rust was attributable to different mechanisms in addition to reduction of green leaf area. The negative interaction between the injurious effects of the two pathogens was ascribed to this difference.