Epidemiology and sustainable control of Podosphaera aphanis (strawberry powdery mildew)

Abstract

Until recently strawberries grown in the United Kingdom were grown in open fields, the plants and fruit were exposed to the British weather. This resulted in a short 6 week harvest period where the fruit was often damaged by rain and infected by Botrytis cinerea. Strawberry growers started to use polythene tunnels to extend the cropping season, protect the fruit from rain damage and reduce the incidence of infection by B. cinerea. However the conditions produced by the polythene tunnels were ideal for the growth and development of Podosphaera aphanis (strawberry powdery mildew). Growers are now under pressure from the retailers to reduce the amount of fungicides that they use to control P. aphanis. The symptoms related to P. aphanis infection have been identified (leaf cupping, visible mycelium and red blotches) and a progression has been established: From the symptom progression two new s~ring methods for the identification of P. aphanis infections were developed wmch have greater relevance to current cultivation methods than the previous method. The source of initial inoculum for newly planted and established sites was identified. The inoculum was planted into new sites on the plants coming from the propagators and overwintering on plants within established sites. This was contrary to what the growers believed. They were basing their early season tunnel management on keeping the perceived air borne infection out of their tunnels. A rule based prediction system has been developed that has the potential to reduce the number of fungicide applications applied by the growers. The prediction system ensures that fungicide applications are not applied too close together. Potassium Bicarbonate has been shown to provide comparable control of P. aphanis to that achieved with Systhane (Myclobutanil). Significantly better control of P. aphanis was achieved using a new (at the time) product, Fortress (Quinoxyfen). There were significant differences in the resistance °to infection by P. aphanis displayed by different cultivars of strawberry. Elsanta, the cultivar favoured by the retailers was not one of the most resistant. Control of inoculum . already present on plants as they are being planted could be achieved by dipping the plant in Systhane. Growers are under considerable pressure from the retailers to reduce the amount of fungicides used to control P. aphanis. Growers could achieve this by implementing the recommendations made in this report.