Trichothecium roseum enters ‘Fuji’ apple cores through stylar fissures

Apple core rot, an economically important disease worldwide, appears both before and during harvest. Current gaps in understanding of the infection cycle impede progress toward more effective management of this disease. The fungus Trichothecium roseum is the main pathogen of core rot on apple in China. In this study, we used fluorescent labeling to trace colonization of T. roseum in floral tissues, characterizing routes of penetration to the core of 'Fuji' apples. T. roseum infected petals, anthers, filaments, stigmas and separated styles of flowers, and floral debris served as inoculum for core infection. In field inoculations, T. roseum entered styles initially through stylar fissures and colonized pluricellular hairs of these fissures during early stages of fruit development. Subsequently, hyphae grew along the extending fissures, which are continuations of stylar fissures located between stylar bases and carpel cavities. The hyphae remained in the extending fissures from mid-June to late July. When fruit developed an open sinus in late July, the sinus eventually fused with extending fissures and carpel cavities in late August, hyphae invaded carpel cavities, and ultimately fruit flesh via cracks on carpel cavity walls. Our results revealed for the first time the routes by which T. roseum penetrates apple fruit, and provided significant insights for strategic management of core rot.