Interactions between winter oilseed rape canopy structure at flowering and Sclerotinia sclerotiorum epidemiology

Abstract: Sclerotinia stem rot is one of the major diseases of winter oilseed rape. It is mainly controlled by fungicide application. Risk occurrence is estimated on petals, through the use of a petal kit. Nevertheless this epidemic potential is often limited by different environmental factors and plant infection capacities are strongly restricted. At the field level canopy structure can modulate conditions more or less able to favour the epidemic. We have looked for canopy structures able to modify disease impact. The initial hypothesis was that a closed canopy could favour the disease development by increasing local relative humidity and temperature. Field trials were carried out during two agronomic seasons with different genotypes and different plant densities. Infection potential, plant branching, petal fall kinetics on different parts of the canopy, wetness and temperature conditions inside the canopy and symptom occurrence was recorded. Results indicated (i) the plasticity of the plant and its ability to compensate at low plant densities, (ii) number of contaminated fallen stuck petals on leaves are not limiting for the epidemic, (iii) microclimatic differences inside and outside the canopy are not so important, (iv) disease incidence was higher for the lowest plant densities for the two locations where we got symptoms. This allows us to arrive at a new hypothesis that has not yet been tested.