Penicillium bialowiezense Causing Blue Mold on Bag-Cultivated Shiitake (Lentinula edodes) in China: Morphological, Molecular and Pathogenic Characterization

Lentinula edodes (shiitake) is a major edible and medicinal mushroom and a key component of the horticultural mushroom industry in East Asia. During April–June 2024 cropping season, a widespread blue mold outbreak was observed on bag-cultivated shiitake in Xixia County, Henan Province, China. Affected cultivation rooms showed extensive blue-green sporulation on the exposed surfaces of substrate blocks and on developing and mature fruiting bodies, leading to rapid loss of marketability. To clarify the etiology of this disease, we coupled field surveys with morphological, molecular, and pathogenicity analyses. Fifty-five Penicillium isolates were obtained from symptomatic cultivation bags. Three representative isolates (LE06, LE15, and LE26) were characterized in detail. Colonies on PDA produced velutinous to floccose mycelia with blue-green conidial masses and terverticillate penicilli bearing smooth-walled, globose conidia. Sequencing of four loci—the internal transcribed spacer (ITS1-5.8S-ITS2), β-tubulin (benA), calmodulin gene (CaM), and RNA polymerase II second largest subunit (rpb2)—followed by multilocus phylogenetic analysis placed all three isolates in a well-supported clade with the ex-type CBS 227.28 of Penicillium bialowiezense. Inoculation of healthy shiitake cultivation bags with conidial suspensions (1 × 106 conidia mL−1) reproduced typical blue mold symptoms on substrate surfaces and fruiting bodies within 40 days post inoculation, whereas mock-inoculated controls remained symptomless. The pathogen was consistently reisolated from diseased tissues and showed identical ITS and benA sequences to the inoculated strains, thereby fulfilling Koch’s postulates. This is the first confirmed report of P. bialowiezense causing blue mold on shiitake, and it expands the known host range of this species. Our findings highlight the vulnerability of bag cultivation systems to airborne Penicillium contaminants and underscore the need for improved hygiene, environmental management, and targeted diagnostics in commercial shiitake production.