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Glomerella leaf spot (GLS), a fungal disease caused by Colletotrichum fructicola, severely affects apple (Malus domestica) quality and yield. In this study, we found that the transcription factor MdWRKY71 was significantly induced by C. fructicola infection in the GLS‐susceptible apple cultivar Royal Gala. The overexpression of MdWRKY71 in apple leaves resulted in increased susceptibility to C. fructicola, whereas RNA interference of MdWRKY71 in leaves showed the opposite phenotypes. These findings suggest that MdWRKY71 functions as a susceptibility factor for the apple—C. fructicola interaction. Furthermore, MdWRKY71 directly bound to the promoter of the salicylic acid (SA) degradation gene Downy Mildew Resistant 6 (DMR6)‐Like Oxygenase 1 (DLO1) and promoted its expression, resulting in a reduced SA level. The sensitivity of 35S:MdWRKY71 leaves to C. fructicola can be effectively alleviated by knocking down MdDLO1 expression, confirming the critical role of MdWRKY71‐mediated SA degradation via regulating MdDLO1 expression in GLS susceptibility. In summary, we identified a GLS susceptibility factor, MdWRKY71, that targets the apple SA degradation pathway to promote fungal infection. [ABSTRACT FROM AUTHOR]

Valsa mali causes Valsa canker, one of the most destructive diseases on apple trees, leading to severe losses for the apple industry in China. Considering the development of fungicide resistance and the harmful effects of chemical residues, it is urgent to identify alternatives to control this disease. Trans‐cinnamic acid (t‐CA), a compound with good antibacterial, antitumour and anti‐inflammatory properties, is widely used in food, medicine and other industries. However, the antifungal activity of t‐CA against V. mali and its regulatory role in apple defence against biotic stresses are unclear. Based on a metabolomic assay, we found that Malus yunnaensis (Valsa canker‐resistant) twigs infected with V. mali dramatically accumulated t‐CA. Exogenous application of t‐CA effectively inhibited V. mali growth on potato dextrose agar. The EC50 value of t‐CA inhibiting mycelial growth was 200 μg/mL. Malus prunifolia (Valsa canker‐susceptible) leaves and twigs pretreated with t‐CA had significantly enhanced V. mali resistance. The t‐CA application increased salicylic acid but reduced jasmonic acid levels in leaves and twigs. Moreover, the contents of phenolic acids and flavonoids increased in t‐CA‐treated samples. In addition, t‐CA increased the activities of phenylalanine ammonia‐lyase, β‐1,3‐glucanase and chitinase. These results indicate that t‐CA plays a significant role in inhibiting V. mali growth and priming apple defence. [ABSTRACT FROM AUTHOR]