Hymenoscyphus fraxineus is an invasive fungus in Europe and causes a severe decline affecting ashes since the late nineties. One of the symptoms associated with the disease is lesions in the outer bark of the collar area. However, the etiology of these basal lesions and in particular the relative role of H. fraxineus and Armillaria species is still controversial; moreover, little is known on the influence of environmental factor on the disease epidemiology. We therefore surveyed 42 plots located in Northeastern France, in an area affected by ash decline since 2008 in order to determine which environmental factors condition the severity of lesions associated with H. fraxineus on ash collar. The spatial pattern that is the consequence of the invasive spread of the disease was taken into account in the analysis using a spatial hierarchical Bayesian model fitted by Integrated Nested Laplace Approximation (INLA). Results show that while basal lesions are tightly associated with H. fraxineus, their severity is not linked to the Armillaria species present in the plot. Sites with vegetation indicating moist condition or more humid topographical position were associated with more developed basal lesion.