Hot-Water Dipping of Apples to Control Penicillium expansum, Neonectria galligena and Botrytis cinerea: Effects of Temperature on Spore Germination and Fruit Rots

The efficacy of hot-water dipping against apple storage rots caused by Neonectria galligena, Botrytis cinerea and Penicillium expansum was examined. Pure spore suspensions as well as artificially inoculated 'Elstar' apples were incubated for 3 min in a water bath heated to specific temperatures in the range of 32 degrees C to 70 degrees C, followed by incubation at 2 degrees C (fruit) or 20 degrees C (spores). Whereas there were striking interspecific differences in the dipping temperatures survived by spores, storage rots caused by all three species were significantly reduced by dipping temperatures around 50 degrees C. Temperatures above 52 degrees C caused serious heat scald on the fruit surface and gave rise to increasing levels of fruit rot in the case of N. galligena and P. expansum. Very similar temperature-response curves of blue mould development were observed in apples inoculated with P. expansum before or after hot-water dipping, except for the highest temperature tested (70 degrees C). It is concluded that the major effect of hot-water dipping against these fruit rots is mediated by heat-induced acquired resistance of fruit rather than heat-mediated spore mortality. These results suggest possible applications for hot-water dipping of apples at harvest, after short-term cold storage or after the opening of controlled-atmosphere storage rooms in order to improve fruit quality during subsequent storage periods.