INFECTION AND ROT EXPRESSION BY CRYPTOSPORIOPSIS ACTINIDIAE AND PHOMOPSIS SP. IN KIWIFRUIT, ACTINIDIA CHINENSIS 'HORT16A'

The fungi Crytosporiopsis actinidiae and Phomopsis sp. can cause significant loss of yellow-fleshed kiwifruit Actinidia chinensis 'Hort16A' in long-term storage. To identify appropriate control interventions a greater understanding of the biology of the pathogens is required. The aim of this study was to investigate the time of infection of the fruit by these fungi and to examine the effect of storage conditions on rot expression. Flowers and fruit were inoculated at different times during the 2002–2003 growing season, and the trial was repeated in 2003–2004. For inoculation, Nitrate non-utilising (Nit) mutants of both species were used to enable differentiation between induced and background (wild) infections. In the first year, harvested fruit were stored in normal atmosphere at 1.5°C to simulate standard storage conditions and the incidence of rots was recorded over a 24-week storage period. In the second year, half the fruit were stored in normal atmosphere and half in controlled atmosphere (CA) at 1.5°C to investigate the effect of CA on disease expression. After 18 weeks, fruit were removed from storage, held at approximately 21°C for 14 days to simulate 'shelf' conditions, and then examined for rots. The results suggest that infection by C. actinidiae occurs during flowering and the fungus survives endophytically during fruit development. Phomopsis sp. appears to infect throughout the season, also surviving endophytically in the skin of the fruit. Treatment effects in the first year were confounded by a high level of chilling injury in fruit, precipitating development of rots mediated by either of the inoculated species. In the second year, the expression of rots by both species was significantly greater in CA storage. These findings suggest that the transition from latent survival to active pathogenesis in both species is triggered by changes in the physiology of the fruit.