A simple model to interpret the effects of sunflower crop management on the occurrence and severity of a major fungal disease: Phomopsis stem canker

Abstract

Phomopsis stem canker ( Diaporthe helianthi Munt.-Cvet. et al.) causes drastic reductions in yield and oil content in the sunflower ( Helianthus annuus L.) crop. The use of disease-tolerant genotypes and fungicides is the basis of current disease control. Both for economic and environmental reasons, control by means of cultural practices should be promoted more. From 1992 to 2002, INRA and Cetiom collaborated to study the effects of sunflower crop management on the frequency and severity of Phomopsis attacks. As plant density and nitrogen rate significantly affect the development of sunflower leaf area, use of the fraction of photosynthetically active radiation intercepted by the canopy (fPARi) was suggested as a synthetic indicator of canopy management. In 1994, an exponential relation was observed between the proportion of sunflower stems bearing a Phomopsis lesion in mid-August and the fPARi by the canopy at anthesis in early July. This relation was successfully tested for a range of unprotected situations (1994–2000) corresponding to moderate infection pressure and various combinations of plant density and N fertilization. Such a reference curve was used to analyse the response of sunflower to stem canker infection in a range of cultural conditions: (1) high inoculum; (2) drought during head development or soon after leaf infection; (3) application of a fungicide at the early stages of flower bud development; (4) late sowing (May); (5) use of very susceptible or resistant genotypes. This simple model could help to interpret the origin of genotypic susceptibility to stem canker (crop architecture and/or physiology) and also to predict the potential risk of disease development for a given management scenario.