Effects of cultivar susceptibility, fruit maturity, and natural wounds on the infection of English walnut (Juglans regia L.) fruits by Botryosphaeriaceae and Diaporthe fungi

Shoot blight and branch dieback of English walnut has been associated with a broad diversity of Botryosphaeriaceae and Diaporthe fungi worldwide. These pathogens affect both wood and fruit tissues, infecting the tree through mechanical or natural wounds. Fruit infections can play an important role in the life cycle of the disease. Thus, the effects of cultivar, and fruit maturity on English walnut fruit infection by Botryosphaeriaceae (Botryosphaeria dothidea, Neofusicoccum mediterraneum and N. parvum) and Diaporthe (Diaporthe neotheicola and Dia. rhusicola) fungi were evaluated. The infection and disease progress from inoculated attached fruits or by inoculating fruit abscission wounds was evaluated both under laboratory and field conditions. An initial experiment evaluating two inoculation methods was conducted, but there were not significant differences in disease severity between inoculations with mycelial plugs or conidial suspensions. A total of eight cultivars were selected to evaluate their susceptibility to fruit infection, with ‘Chandler’ being the most susceptible for all the pathogens tested compared to the other cultivars. Botryosphaeriaceae showed higher aggressiveness on fruit collected at beginning- or middle summer, while Diaporthe showed similar aggressiveness regardless of fruit maturity stage. Botryosphaeriaceae fungi were able to colonize the entire surface of the inoculated fruit, reaching the peduncle and infecting the attached shoot; while Diaporthaceae fungi were not able to colonize the surface of the inoculated fruit quickly enough to infect the attached shoot before the peduncle was naturally separated from the shoot. Finally, we demonstrated that both Botryosphaeriaceae and Diaporthe fungi can infect shoots by inoculating the natural fruit abscission wounds in the field. This study generates new insights into the influence of fruit infection leading to shoot blight and branch dieback of English walnut caused by Botryosphaeriaceae and Diaporthe fungi.