Assessing the seasonal patterns of spore deposition of Gnomoniopsis castaneae in some chestnut orchards in northern Italy.

Gnomoniopsis castaneae Tamietti is an emerging fungal pathogen of sweet chestnut causing symptoms ranging from nut rot to leaves necrosis, up to bark cankers. To date, little is known about the epidemiology of this pathogen. The aims of this work were i) to detect and quantify the airborne inoculum of G. castaneae, ii) to assess the effect of seasonality on spore deposition, and iii) to study the correlation between spore deposition and climatic conditions. Samplings were performed between 2013 and 2015 in three orchards in the northwest of Italy using a novel and optimized spore trapping method combined with a real-time PCR approach. In each orchard, one permanent transect was established by installing 14 spore traps at approximately 15 m intervals. The airborne inoculum was sampled continuously every two weeks. The disease incidence was also assessed by performing fungal isolations from 40 ripe nuts collected from chestnut crowns in each orchard. Climatic data were obtained by installing thermo-pluviometric stations. Airborne inoculum of G. castaneae was detected in all sampling sites. The spore deposition showed different patterns depending on site and climatic conditions; however, similar patterns were recorded in the two years of samplings. On average, disease incidence ranged from 10% to 42.5% in 2013, from 5% to 57.5% in 2014, and from 0% to 40% in 2015. This study provides key information that may be used for the development of epidemiological models.