Aerated steam treatment against grey mould in strawberry transplants

Grey mould caused by Botrytis spp. is one of the most important fungal pathogens in strawberry and is often present in strawberry transplants in the form of latent infections. In the past, heat treatment has been effective in inhibiting several phytopathogens. However, despite its efficiency, heat treatment is often linked to adverse effects on plants. The ‘Plant Sauna’ is an insulated thermotherapy unit with the ability to provide uniform distribution of aerated steam at desired temperatures. The purpose of this study was to evaluate the efficiency of the ‘Plant Sauna’ in reducing the Botrytis population present in transplants of five strawberry cultivars: Falco, Favori, Murano, Sonsation and Soprano. The heat treatment included pre-treatment at 37°C for 1 h, followed by 1 h of cool-down at ambient temperature and eradicative treatment at 44°C for 2 h or 4 h. Eradicative treatment at 44°C for 4 h led to a 100% reduction in Botrytis populations for transplants of cvs. Soprano and Sonsation. While for transplants of cvs. Murano, Falco and Favori, the incidence of Botrytis spp. was reduced by 72 to 77 % compared to the untreated transplants. On the other hand, the incidence of Botrytis spp. in 2 h heat-treated transplants of cv. Soprano was < 8%. While for the rest of the 2 h heat-treated transplants, the incidence was not remarkably different from the untreated transplants. Furthermore, growth trials were conducted on transplants of cv. Sonsation to determine the effects of ‘Plant Sauna’. The effect of the steam treatment on plant growth and yield was negligible. Aerated steam treatment was further extended for an in-depth study of its effect on Botrytis sclerotia. Eradicative treatment at 44°C lasted for 4 h only which killed 90 to 100% of the sclerotia. Isolates of Botrytis spp. were recovered from strawberry transplants and tested for fungicide resistance. Some of these isolates were resistant to up to five classes of fungicides. Overall, our findings indicate that aerated steam treatment in the ‘Plant Sauna’ is a safe and effective technique for reducing pathogen populations and the quantity of fungicide-resistant strains in strawberry transplants, with no or insignificant negative impacts on transplants.M-P