Epidemiology of Diplodia Shoot Blight Outbreaks in Pine Forests After Hailstorm Events

ABSTRACT

Diplodia shoot blight caused by Diplodia sapinea is an emerging disease in European pine forests. Outbreaks can occur after hailstorms. However, key epidemiological aspects associated with these events are not fully understood. We assessed the risk of outbreaks in native Pinus nigra stands in the Pyrenees (north‐east Spain), combining radar maps, meteorological data and ground assessments. A total of 886 storms that occurred between 2022 and 2024 were screened to identify those associated with outbreaks. Disease development was studied using Sentinel‐2 satellite imagery to analyse changes in the normalised difference vegetation index (NDVI). Diplodia shoot blight outbreaks were positively associated with low records of accumulated precipitation in the 10 days preceding hailstorm events. Disease development showed monomolecular growth, with NDVI losses reaching 80% approximately 40 days after a hailstorm. NDVI assessments correlated well with field disease assessments ( R ²  = 0.70), whereas hailstorm estimation map accuracy was low (0.51). Our findings suggest that water stress is a prerequisite for Diplodia shoot blight outbreaks following a hailstorm. Monomolecular disease development seems consistent with a predominant expansion of the endophytic inoculum over the role of the new infections occurring in hail wounds. Under the studied conditions, symptoms should appear 2–3 weeks after a hailstorm or not at all. Although NDVI measurements reliably assessed Diplodia shoot blight outbreaks, the gross scale of hailstorm maps (1 km ² pixels) makes them unreliable for detecting outbreaks. Our findings have implications for pest management, particularly in the context of climate change, with increasing temperatures and hailstorm risk.