Low Oxygen Effect on Penicillium expansum Mould in Stored Apple

Penicillium expansum causing blue mould is a serious postharvest pathogen in all growing pome fruit areas. Physical treatments, including storage atmosphere manipulation, have been developed to control postharvest fungal decay. The present study aimed to determine the influence of oxygen (O-2) levels on growth of P. expansum (P37 strain). Pathogen enzymatic production at low oxygen levels was also tested in in vitro trials by agarose diffusion assay. In in vitro trials, 100 mu l of conidial suspension (10(3) conidia per ml) was spread on MEA dishes, stored for 30 days at 0 degrees C with 0.7, 1.5, 3, 6 and 21% O-2. The concentration of carbon dioxide was 1%. In in vivo trials, fruit were wounded and inoculated with 20 mu l of a conidial suspension (10(3) conidia per ml) of P. expansum and stored with the same oxygen concentrations for five months at 0 degrees C plus 3 days at 20 degrees C. No significant differences were observed, within the different oxygen concentrations, on P. expansum growth in vitro. The percentage of infected fruit was significantly greater at low oxygen concentrations (0.7, 1.5, 3%) than at 6 or 21%. However, the dimensions of decay lesions, on apples exposed to 21 and 6% of O-2, were significantly larger than those of apples stored at 0.7, 1.5 and 3% O-2. In addition, O-2 levels greatly influenced the enzymatic activity. Polygalacturonase, protease and amylase activities were increased by low oxygen level (0.7%), whereas cellulases were reduced by the same oxygen concentration. More investigations are required to better elucidate the interactions between pathogen-fruit-oxygen level and the role of the enzymes in pathogenicity.