Influence of ecological conditions on Aspergillus carbonarius germination

Aspergillus carbonarius is confirmed worldwide as the main species responsible for ochratoxin A (OTA) production in grapes and consequently in processed products based on vine fruits. Several studies have provided information on the ecology of A. carbonarius, in particular for growth and OTA production, in relation to water activity (aw), temperature, pH and modified atmosphere. To date, no detailed studies have been addressed to quantify the germination rate of this fungus, in particular on grape skin and flesh. Therefore, the objective of this study was to compare the ability of A. carbonarius spores to germinate in vitro and in situ on grape skin and grape flesh in relation to temperature (15-40°C) and relative humidity (100-85% ERH) regimes, in time course studies (4-36 hours). A spore suspension of A. carbonarius (106 spores ml-1) was inoculated onto the skin and on the flesh of cut berries of white organic grapes. For comparison, spores were also spread plated onto an artificial grape juice medium, modified to the same range of 0.85-0.995 aw. The time to 5% germination was significantly shorter on grape flesh than in vitro and on grape skin (6 vs 9 vs 24 hours in optimal conditions). Fifty percent germination was reached in a specific range of conditions on grape skin; 25-30°C and ERH≥90%.This suggests that damaged skin may be the main pathway which is conducive to A. carbonarius spore germination providing an easier and more rapid infection route. Data on the combined effect of temperature and aw on spore germination were fitted with Bete and polynomial equations that provided a good fit of the biological processes investigated (R2 ≥ 0.98). These functions are useful to quantify the initial steps in the infection cycle and can contribute in a predictive model to estimate the relative risk of OTA occurrence in grapes.