Stand competition and slope increase the probability of occurrence of Gremmeniella abietina in forest stands in Spain

Aim of study: This study aimed to determine the distribution of the fungal pathogen Gremmeniella abietina in forest stands close to the first detection zone in Spain. Additionally, we aimed to identify stand characteristics associated with the presence of the pathogen and evaluate the use of remote sensing methods, such as the Normalized Difference Vegetation Index (NDVI), for early disease detection. Area of study: Province of Palencia, northern Spain. Material and methods: We surveyed 36 forest stands to assess the distribution of G. abietina. Stand inventories, NDVI values derived from Deimos-1 satellite imagery and DNA amplification techniques were used to characterize each stand and determine the presence or absence of the pathogen. Finally, a combination of a random forest algorithm and a permutational multivariate analysis of variance (PERMANOVA) was applied to identify the most significant predictors of the pathogen presence. Main results: The presence of G. abietina was confirmed in 13 plots and was found to be correlated with lower NDVI values during the summer months and with greater competition (i.e., high basal area) on steeper slopes. The algorithm had limited predictive power but it was sufficiently reliable for descriptive purposes. Infected stands also showed higher levels of defoliation, distortion of terminal twigs and dry needles. Research highlights: The results indicate that although the pathogen has spread beyond the initial detection zone, it currently causes only moderate damage. The absence of epidemic outbreaks in the region may be due to environmental conditions that are not conducive to disease development. Nevertheless, the presence of G. abietina could negatively affect forest productivity, particularly in stands located on steep slopes and characterized by a high level of competition. Use of the NDVI may be a useful tool for detecting trees affected by the pathogen at an early stage, but confirmation through field diagnosis surveys and molecular diagnostics remains essential.