Exogenous chitosan enhances the resistance of apple to Glomerella leaf spot

• 0.5 g·L −1 CHT effectively alleviated Glomerella leaf spot caused by G. cingulata on apple leaves. • Exogenous CHT elevated the SA-mediated defense response, including up-regulated the expression of PRs and PAL . • Exogenous CHT enhanced the activity of antioxidant enzymes, and the accumulation of amino acids and phenolic compounds. • Exogenous CHT reduced the content of ABA and ACC. Glomerella leaf spot (GLS), caused by the fungal phytopathogen Glomerella cingulata , is one of the most devastating premature defoliation diseases affecting apple production. Chitosan (CHT) is a degraded product of chitin and is used for bacteriostasis and postharvest preservation. Herein, we investigated the effects of CHT on improving the apple resistance to GLS. We identified that spraying with 0.5 g·L −1 exogenous CHT was the optimal concentration to reduce the G. cingulata disease severity on detached leaves of apple seedlings. Further study demonstrated that the exogenous CHT application significantly enhanced the superoxide dismutase (SOD) activity, elevated alicylic acid (SA) and jasmonic acid (JA) contents, increased certain kinds of phenolic compound contents and up-regulated the expression of different disease resistance-related genes in inoculated leaves than plants without CHT pretreatment. Furthermore, the exogenous CHT application significantly reduced the contents of abscisic acid (ABA), 1-aminocyclopropanecarboxylic acid (ACC), and auxin (IAA) of G. cingulata infection. Additionally, the CHT spraying treatment reduced the amino acid contents compared with the control plants. The results indicated that exogenous spraying of CHT improved the resistance of apples to GLS through enhancing plant immunity.