Pathogenicity and progression of Botryosphaeriaceae associated with dieback in walnut orchards in Australia

Dieback of the fruiting spurs of walnut (Juglans regia) results in yield losses in walnut orchards in Australia. Botryosphaeriaceae spp. have been implicated as the causative agents, with Diplodia seriata and Neofusicoccum parvum reported as the most prevalent species. Pathogenicity assays demonstrated inter- and intra-species variation with N. parvum more virulent than D. seriata. Both species were re-isolated beyond the dieback lesion. At 12 months post-inoculation on one-year-old trunks, N. parvum and D. seriata were re-isolated at 31.5 and 9 mm respectively beyond lesion edges. The lesion length as well as pathogen progression beyond the lesion increased over time. The pathogen progressed faster acropetally than basipetally. Following wound healing at the site of inoculation, and in the absence of any external lesion, both pathogens continued to move through the xylem, causing internal discolouration of the vascular tissues. Both mycelia and conidia were pathogenic, with mycelia showing higher infection rates. There was a strong linear relationship between the dieback length caused by the two types of inocula. As little as two conidia were able to infect wounded tissues. Although wounds were susceptible to infection for more than 4 months post-wounding for N. parvum and 1 month for D. seriata, the highest disease incidence occurred in the first week following wounding. The implications for the walnut industry for appropriate control strategies are discussed.