Sources of crown rot ( Phytophthora cactorum ) infection in strawberry and the effect of cold storage on susceptibility to the disease

Summary Cold‐stored plants of strawberry cultivars Tamella, Cambridge Favourite and Redgauntlet were more susceptible to pathogenic isolates of Phytophthora cactorum than similar plants which had not been cold‐stored. Indigenous nonpathogenic isolates of P. cactorum did not cause crown rot in cold‐stored plants, although a small number of symptomless latent infections occurred. The majority of P. cactorum isolates causing crown rot symptoms were taken from infected strawberry crowns, although two isolates from gooseberry plants, but of uncertain origin, were also pathogenic. Outbreaks of crown rot in areas with no previous history of the disease therefore probably result from the importation of non‐indigenous inoculum with planting material. Assessments of the timing of infection in relation to cold storage revealed that a high incidence of death in the cold store and chronic wilt symptoms on planting from the store resulted from initiating symptomless infections prior to cold storage. However, infection during the period immediately after cold storage resulted in rapid wilt symptoms of Phytophthora crown rot. When plated in sterile distilled water for 24 h, pieces of tissue from infected plants which had died during cold storage produced large numbers of sporangia and zoospores. This indicates that such plant material could provide a potent source of inoculum for infections in the post storage thawing environment. It is proposed that a combination of heightened host susceptibility resulting from cold storage and the presence of scatted latent infections or infected debris among the plants could result in a sudden, large scale appearance of crown rot, as sometimes is seen with cold‐stored plantings of strawberries.