In vitro and in situ study of postharvest apple blue mold biocontrol by Aureobasidium pullulans: Evidence for the involvement of competition for nutrients

Aureobasidium pullulans strain Ach1-1 was selected for its effectiveness against blue mold caused by Penicillium expansum on stored apple fruit. The possible involvement of competition for nutrients in the biocontrol activity of this antagonistic strain was investigated both in vitro and in situ. For in vitro assays, the effect of strain Ach1-1 on germination percentages of P. expansum conidia was evaluated after a 24 h incubation period in the presence of increasing apple juice concentrations (0-5%) using a system allowing the physical separation of both agents. In the absence of strain Ach1-1, conidial germination was strongly promoted by apple juice whatever the concentration. However, germination was significantly reduced by the presence of strain Ach1-1 except at the highest juice concentration. For conidia previously inhibited at 0.5% juice, germination after 24 h of incubation was partially recovered in the presence of strain Ach1-1 when fresh juice was added to a final concentration of 5%, and completely restored at both 0.5 and 5% juice concentrations in the absence of strain Ach1-1. For in situ assays, strain Ach1-1 was very protective against P. expansum on postharvest wounded apples. However, the application of high concentrations of exogenous sugars, vitamins and most particularly amino acids, significantly reduced such protection. Time-course analysis of apple amino acids at the wound site revealed that these compounds were more depleted in wounds treated with strain Ach1-1 alone and especially in those treated with both agents (strain Ach1-1 and P. expansum) compared to wounds treated with P. expansum alone or to untreated ones. Exogenous amino acids, applied at high concentrations on apple wounds as a mixture of specific amino acid groups or as individuals, significantly decreased strain Ach1-1 efficacy against P. expansum. The present study provides in vitro and in situ evidence that competition for apple nutrients, most particularly amino acids, may be a main mechanism of the biocontrol activity of A. pullulans strain Ach1-1 against blue mold caused by P. expansum on harvested apple fruit.