Resistance of Malus plants to Diplocarpon mali infection is associated with the antioxidant system and defense signaling pathways

Marssonina apple blotch is one of the most severe diseases of apple. Using Malus sieversii and Malus prunifolia cv. Donghongguo, we investigated the levels of hydrogen peroxide, activities of both antioxidant and pathogenesis-related enzymes, and expression of marker genes for salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) in pathways for defense regulation during incompatible and compatible interactions. In incompatible interaction, as the key enzymes of antioxidant system coordinated work, a ROS burst and associated SA-signaling were activated within 10 days post inoculation. This was followed by activation of JA-mediated defense signaling. Levels of pathogenesis-related enzymes were induced more strongly by Diplocarpon mali than in compatible interaction. Suppression of the ET pathway was associated with the resistance response. Our findings provide important clues for designing strategies to curb the effects of this disease.