Can oak powdery mildew severity be explained by indirect effects of climate on the composition of the Erysiphe pathogenic complex? B Marçais, D Piou, D Dezette, ML Desprez-Loustau Phytopathology

Abstract

Coinfection by several pathogens is increasingly recognized as an important feature in the epidemiology and evolution of plant fungal pathogens. Oak mildew is induced by 2 closely related Erysiphe invasive species E. alphitoides and E. quercicola, which differ in their mode of overwintering. We investigated how climate influences the co-occurrence of the 2 species in oak young stands and whether this is important for the disease epidemiology We studied the frequency of flag-shoots (i.e. shoots developing from infected buds, usually associated with E. quercicola) in 95 oak regenerations over a 6 year period.. Additionally, in 2012 and 2013, the oak mildew severity and the 2 Erysiphe species relative frequencies were determined both in spring and in autumn in 51 regenerations and 43 one-year-old plantations of oaks. Both the frequency of flag-shoots and the proportion of Erysiphe lesions with E. quercicola presence were related to climate. We showed that survival of E. quercicola was improved after mild winters, with increase of both the flag-shoot frequency and the proportion of Erysiphe lesions with E. quercicola presence in spring. However, disease severity was not related to any complementarity effect between the two Erysiphe species causing oak powdery mildew. By contrast, increased E. alphitoides prevalence in spring was associated with higher oak mildew severity in autumn. Our results point out the critical role of between season transmission and primary inoculum to explain disease dynamics which could be significant in a climate warming context.