Methods of assessing the incidence of Armillaria root rot across viticultural areas and the pathogen's genetic diversity and spatial–temporal pattern in northern Italy

Armillaria mellea (Vahl:Fr.) P. Kumm. is the causal agent of root rots of numerous perennial woody plants, including grapevine. In grapevine, A. mellea infections cause reduced plant vigour, poor grape quality and, in the last stage of the disease, plant death. No post-infection control agents are available for use against this disease. Disease management programmes rely solely on prophylactic measures. Therefore, knowledge of the distribution pattern of the disease over time, disease incidence and actual losses due to the disease across geographical areas is crucial. We evaluated the use of aerial photographs, surveys of growers’ experiences and field assessments to estimate the incidence of Armillaria root rot in a viticultural area (140 ha) situated in the Piana Rotaliana region (Italy). Results of the 4-year assessment were that 25% of the vineyards (36 ha) were infected by Armillaria, 2% of the cultivated vines exhibited disease symptoms and 0.25% of the vines died each autumn. The causal agent was almost exclusively A. mellea but, in two cases, Armillaria gallica was also isolated. The genetic diversity of the Italian population, according to IGS sequencing and somatic incompatibility testing, was very high, suggesting multiple pathogen introduction sites and the slow, vegetative underground spread of the fungus. Fungal clones were found within individual vineyards. Even if, within a particular area, the disease is spreading very slowly and crop losses are limited (one vine out of 400 dies each year), an effective agronomic plan for the prevention of infections should be implemented.