Crop canopy indicators for the early prediction of Phomopsis stem canker (Diaporthe helianthi) in sunflower

摘要

Phomopsis stem canker is a worldwide fungal disease which causes drastic reductions in yield and oil content as a result of premature plant wilting and lodging. Although it is agreed that the sunflower canopy plays a major role in the success of infection, no quantitative variable has been proposed until now for basing spraying decisions on the agronomic risk at field level. The objective of this study is to evaluate the suitability of two early indicators of a sunflower crop canopy for predicting crop injury for a wide range of agronomic, climatic and cultivar conditions in south-western France. Linear regressions were established between the proportion of stems with girdling lesions (SGL, %) and each of the two canopy indicators: plant height (H) and fraction of photosynthetically active radiation intercepted (fPARi). Critical values of H=52cm and fPARi=48% were determined for SGL=15%, which is adopted as the damage threshold. A decision rule for chemical spraying based on these indicators was evaluated using a large database (17 trials, 431 plots, five cultivar classes of susceptibility). The value of spraying or not according to the observation of early indicators was correct in 83% of the situations using fPARi and in 77% of the situations using plant height. This simple rule is proposed to refine the characterization of the agronomic risk in the decision grid used in France.