BROWN APICAL NECROSIS (BAN): A FUNGAL DISEASE CAUSING FRUIT DROP OF ENGLISH WALNUT

Abstract

Apical necroses occurring at the stigmatic end of walnut fruit are quite common on Juglans regia. The typical and specific pathogens of this culture such as Xanthomonas arboricola pv. juglandis and Gnomonia leptostyla are both able to cause black spots at the fruit apex as well as all over the husk. Amidst the apical necroses, brown apical necrosis (BAN) is caused by ubiquitous and polyphagous fungi which produce brown to dark-brown spots exclusively at the blossom end of walnut fruit. The economic importance of this disease is related to fruit drop that can cause over 20% yield loss. The presence and the severity of BAN depend upon the occurrence of conducive cultural and environmental conditions promoting the dissemination and colonization of the fungal pathogens. Previous studies highlighted several fungi associated with BAN among which Alternaria and Fusarium are the main genera. As a result of ten-years of investigation, based on isolations from flowers, fruit, catkins and artificial inoculations, it is possible to identify the fungal genera/species involved in the disease and give a picture of BAN as a complex disease. Artificial inoculations performed in planta directly on flowers and fruit, confirmed that the fungi enter trough the style, which represents the key point for BAN occurrence. Though several Fusarium species are involved in causing the necrosis of the style, endocarp and kernel, depending on site and time, F. semitectum and F. graminearum are the most aggressive with a pioneer role. Among the small-spored catenulate taxa related to Alternaria alternata, which are the agents of inner rot, A. arborescens is the most represented and aggressive species. The efficacy of tebuconazole and/or mancozeb treatments against BAN, which were also registered against anthracnose, is a further confirmation that BAN is strictly caused by fungi.