Fungi in the mix: Unravelling the pathogenicity of Trichoderma pleuroticola, Aspergillus flavus, and Penicillium citrinum in Pleurotus pulmonarius cultivation in Malaysia

Fungal infections on grow bags of Pleurotus pulmonarius, which is more widely known as grey oyster mushroom, have caused massive losses to mushroom growers in Malaysia and other countries. Previous studies have been able to isolate the causative fungal pathogens. Nevertheless, most of these works have mainly focused on isolating the pathogens; their virulence toward P. pulmonarius is less understood. Thus, the present study aimed to isolate, characterize, and evaluate the aggressiveness of the species found in a dual culture assay with P. pulmonarius. Furthermore, we aim to delineate the distribution of each species regionally and evaluate their aggressiveness toward P. pulmonarius. Three fungal species were successfully isolated, namely, Trichoderma pleuroticola, Aspergillus flavus, and Penicillium citrinum. T. pleuroticola was the most prevalent species in most sampled locations. In a dual culture assay where the mushroom was grown with the pathogens, only T. pleuroticola and A. flavus presented a significant increase in their mycelial growth rates (MGRs). The damages were more pronounced with T. pleuroticola, as the pathogen also concomitantly suppressed the MGR of the mushroom. Resistance analysis demonstrated 100 % colonization and damages toward P. pulmonarius, besides the complete loss of resistance toward T. pleuroticola infection. Microscopic analysis revealed a competition for space and mycoparasitism, as T. pleuroticola competed by coiling, wrapping, and lysing the hyphal system of P. pulmonarius. The formation of appressoria-like structures that mediated hyphal penetration was also evident. These pathogens produced a distinctive pattern when grown singly in in vivo bags, which could aid in evaluating the worthiness of treatment on the basis of the pattern presented on the infected bag. The findings of this study could enhance the understanding of how disease presentation can progress to severe, thus, helping growers to better assess the severity of the disease to ensure its viability for treatment.