Abstract
Moisture levels are a crucial meteorological factor affecting all epidemiological stages of foliar diseases, but their effect on pathogen development has been much less studied that of temperature, for example. There has been no detailed investigation of the tolerance or adaptation of pathogen populations to contrasting or changing moisture conditions. We investigated differential tolerance to moisture stress in the wheat fungal pathogen Zymoseptoria tritici at the individual and population levels. We retained 48 genetically different isolates from populations originating from two Euro-Mediterranean climatic zones – Ireland and Israel – with very different moisture conditions (annual air relative humidity – RH – of 85.1% vs. 69.7%, respectively). We assessed the responses in planta of these isolates to four different moisture regimes imposed on wheat seedlings during the first three days after inoculation (RH of 88.3%, 92.2%, 96.1% and 100%). Visual assessments of lesion development conducted at 14, 17 and 20 dpi were performed to establish individual- and population-level moisture reaction norms expressing the sporulating area of the pathogen on inoculated leaves for the mean RH over the three days following inoculation. Our findings highlight: (i) the critical effects of moisture regime on the development of Z. tritici during the earliest infection stages (pycnidiospore germination, epiphytic hyphal growth and penetration) despite the relatively high RH in the driest conditions tested (88.3%); (ii) the considerable individual variation in the phenotypic plasticity of Z. tritici with respect to sensitivity to RH; (iii) the greater tolerance of the Israeli population to drier conditions at 14 dpi, together with the absence of genotypic differentiation for neutral microsatellite loci between the two populations, providing the first evidence of a moisture adaptation signature in a fungal pathogen of plants. These findings provide insight into drought stress tolerance and the diversity of its responses between pathogen individuals and populations in terms of the likely effects of climate change on plant disease epidemics.
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