Characterization of Fungal Species Isolated from Cankered Apple Barks Demonstrates the <i>Alternaria alternata</i> Causing Apple Canker Disease

Apple canker disease, also named as apple <i>Valsa</i> canker, is one of the most destructive diseases for apples (<i>Malus domestica</i> Borkh.). <i>Cytospora</i>/<i>Valsa</i> spp. are the dominant causal agent of this disease, but many studies have revealed that fungi from some other genus can also cause typical apple canker symptoms. In this study, we performed fungal pathogen isolation from cankered ‘Fuji’ apple barks. Six representative morphologically different fungi (Strain 1–6) were further subjected to <i>ITS</i> sequencing and evolutionary analysis. Molecular identification results revealed that Strains 1–6 are <i>Cytospora mali</i>, <i>Fusarium</i> cf. <i>solani</i>, <i>Alternaria alternata</i>, <i>C. mali</i>, <i>Diplodia seriata</i> and <i>F. proliferatum</i>, respectively. All these fungi have been reported to be causal agents of apple diseases. By inoculating fungal plugs onto trunks of ‘Fuji’ apple trees, the pathogenicity of the six fungi were accessed. Only the inoculations of the two <i>C. mali</i> strains (Strain 1 and Strain 4) and the <i>A. alternata</i> strain (Strain 3) resulted in typical apple canker symptoms in trunks. It is worth noting that Strain 1 caused much more severe canker symptoms and higher pathogenicity incidence than the other two fungi. <i>A. alternata</i> has been identified as a pathogen causing diseases on apple fruits and leaves. By further assessing its pathogenicity on apple fruits and leaves, we verified that it can also cause typical fruit rot and leaf spot symptoms. To the best of our knowledge, this is the first report on apple canker disease caused by <i>A. alternata</i> in China. Our present study can provide a theoretical foundation for the prevention and control of apple canker disease.