The impact of environmental variables on ash dieback mediated tree mortality and defoliation

Populations of European ash (Fraxinus excelsior L.) are threatened by an invasive fungus, Hymenoscyphus fraxineus (T.Kowalski) Baral, Queloz & Hosoya, causing substantial landscape changes and risks to public safety. Understanding the interplay between seasonal climate and disease severity can provide valuable information for targeted pest management of ash woodlands across Europe. We compiled 80 peer-reviewed papers on the outbreak of European ash dieback, obtaining 681 tree mortality records and 1078 crown damage measurements with geo-referenced climatic variables. While none of the environmental variables exhibited a significant correlation with tree mortality, we observed peak mortality rates at an average spring temperature (March–May) of 13 °C and a maximum summer temperature (June–August) of 25 °C. Higher spring and summer temperatures were significantly associated with lower defoliation. Summer precipitation significantly increased the defoliation levels, while spring precipitation does not have a significant effect on tree deflorations. These results will facilitate tree disease forecasting, providing evidence that will enable more effective disease mitigation strategies.