Phenolic responses of resistant and susceptible Malus plants induced by Diplocarpon mali

In China, Marssonina apple blotch, caused by Diplocarpon mali (anamorph: Marssonina coronaria ), is one of the most severe diseases of apple ( Malus × domestica ). Using M. sieversii and M. prunifolia cv. Donghongguo, we monitored the concentrations of 18 types of phenolic compounds, transcripts for key enzymes in the phenylpropanoid pathway, and activities by phenylalanine ammonia lyase (PAL), polyphenol oxidases (PPO), and peroxidases (POD) when plants interacted with D. mali . Gallic acid production was induced in disease-resistant M. sieversii but decreased in the susceptible M. prunifolia . In both species, infection with D. mali enhanced their concentrations of epicatechin and reduced their levels of ρ-coumaric acid. Upon inoculation, PAL, PPO, and POD activities were induced to different degrees in each species. Our findings shed light on the mechanisms underlying resistance to D. mali and provide important clues for designing strategies to curb the effects of this disease.