S.E.M. Studies on Penicillium italicum-'Star Ruby' grapefruit Interaction as affected by Hot Water Dipping

Postharvest infection of 'Star Ruby' grapefruit (Citrus paradisi Macf.) by wound-inoculation with the citrus blue mould agent (Penicillium italicum), was clearly influced by the hot water treatments. Dips (2 min at 25, 50, 52, 54, 56 and 58 °C) one hr following inoculation resulted in a signficant reduction of the infection. As dipping temperture was increased from 25 to 50 and 52°C the infection percentage was reduced from 87 to less than 3%. At 54, 56or 58 °C no infection occured for up to 7 days after the treatment when fruit was stored at 17 °C and 90% RH. Scanning Electron Microscopy (S.E.M.) observations of the inoculation sites at 3 and 7 d post-treatment showed that exposure to 50 and 52 °C delayedthe sporegermination for 48 hr, but did not affected spore viability. Higher temperatures caused a gradul reduction of viability. The mycelium that developed from conidia subjected to a 2 min dip at 50 °C was significantly thinner and was not able to colonise the wound court. Heat treatment of spore suspentions caused some delay of spore germination in vitro,while mycelium from conidia treated at 50 °C did not evidenced differences in thickness compared to 25 °C treated ones. Pathogen growth was also arrested when fruit were dipped in hot water (50°C, 2 min) at different intervals (5, 12, 24, 36, 48 and 72 hr) after wounding and inoculation. When the infection was advanced (48-72hr) the mycelium was visibly damaged, while conidia were not.