Phomopsis castanea infection levels in chestnut from the North and South Islands of New Zealand

The nut rot attributed to the fungus 'Phomopsis castanea' (Sacc.) causes serious losses to the New Zealand chestnut industry. In anticipation of the likely increased importance of the chestnut industry in New Zealand, a study was undertaken as part of the ultimate goal of eliminating or minimising this storage rot problem. This study focused on determining the location, timing and intensity of 'P castanea' infection of chestnuts; possible methods of control; pollen source as a potential vector of infection and resistance; and better quantification of the levels of rot in storage. The experiments were conducted over two growing seasons in both North and South Island sites. 'P castanea' colonised leaves, twigs, and structural parts of the inflorescence and branch tissue. Phomopsis was identified on the stigmatic surface and associated with pollen. Phomopsis infection levels increased with increased rainfall, and when temperatures were high, reaching 90% in the wettest and warmest locations and at most 45% in the driest cool areas. Warm-wet conditions favoured disease development. In the South Island there were varietal differences in both levels of infection and rate of decline of infection levels, in the North Island such varietal differences were not observed, suggesting genotype by environment interactions. Other studies have shown a significant link between pollen source and incidence of 'P castanea' and subsequent nut-rot, this study has confirmed this link in the South Island but with low incidence in the range between 5% and 19% nut rots. The different pollinators and environmental conditions in each trial may explain such results. Trunk injection of a fungicide as a means of control has been found ineffective and this is in agreement with previous attempts to use chemical control. Late pollination was found to increase the incidence of external fungal infection. This study has also found that the spread of the disease from nut to nut after harvest and during storage was not significant. The conclusion of this study is that the control of Phomopsis rot will be achieved by a range of measures such as, choice of well adapted cultivars associated with good pollinators, good orchard management and hygiene, post harvest treatment and screening, improved storage conditions, and intense breeding for Phomopsis tolerance in both cultivars and pollinators.