Co-infection of Trichothecium roseum (Pers.) Link and Penicillium italicum Wehmer on sour orange fruit and pH-mediated differences in pathogenicity

A novel dual-colored mold, pink at the margins and blue in the center, was observed on sour orange fruits (Citrus aurantium L.) in Algiers, during the winter of 2021 and 2022. Two fungi were isolated from symptomatic fruits and identified, based on morphological and molecular analyses, as Penicillium italicum and Trichothecium roseum. Pathogenicity tests were carried out by wound-inoculating these fungi on four citrus cultivars, singly or in combination. T. roseum alone caused limited external lesions and cultivar-dependent internal decay, whereas P. italicum induced typical blue mold with surface and internal soft rot, particularly in lemon and sour orange. Co-inoculation reproduced the dual-colored mold and led to host-dependent changes in disease severity: external lesions were significantly larger only in co-inoculated sweet oranges, while in sour orange and tangerine severity was similar to P. italicum alone and in lemon it was slightly reduced, indicating that P. italicum is the main driver of rot progression. To investigate the role of pH, tissue pH was monitored in flavedo, albedo and endocarp. Penicillium italicum maintained or further acidified infected tissues, whereas T. roseum progressively alkalinized them, with co-inoculated fruits showing intermediate pH values. In vitro dual-culture assays at different pH values confirmed that the interaction between the two pathogens is pH-dependent. This study provides the first report of T. roseum on sour orange and of mixed infections by P. italicum and T. roseum on citrus, and shows that pH modulation by acidifying and alkalizing pathogens contributes to the outcome of co-infections in citrus fruits.