Biochemical Mechanisms Underlying Resistance to Alternaria alternata f.sp. mali in Apple Genotypes

ABSTRACT

The present study aimed to understand the biochemical response of different apple genotypes inoculated with Alternaria alternata f. sp. mali , which causes Alternaria blotch, in Himachal Pradesh, India. Three different apple cultivars, Royal Delicious (highly susceptible), Golden Delicious (resistant), and Spartan (moderately resistant), were chosen to understand the impact of A. alternata f. sp. mali on different biochemical parameters. The experiments were conducted in a glasshouse and laboratory conditions of the Department of Plant Pathology, University, in December 2015. The biochemical parameters studied included reducing and nonreducing sugars, phenolics, peroxidase (POX), polyphenol oxidase (PPO), phenylalanine ammonia‐lyase, and ROS. Apple cultivars, whether resistant or susceptible, showed a reduction in reducing and nonreducing sugars when inoculated with A. alternata f. sp. mali . The Royal Delicious cultivar, which was highly susceptible, showed a significant reduction in reducing and nonreducing sugars when infected with A. alternata f. sp. mali . The highly susceptible Royal Delicious cultivar showed a reduction of 109 µg/g in reducing sugars and 15 µg/g in nonreducing sugars. On the other hand, when the resistant cultivar Golden Delicious was inoculated with A. alternata f. sp. mali , there was an increase in total phenolics, POX, PPO, phenylalanine ammonia‐lyase, and ROS. The corresponding values for Golden Delicious were: total phenolics (137 µg g ^{− 1} ), POX activity (0.41 ΔA min ^{− 1} mg ^{− 1} ), PPO activity (0.34 ΔA min ^{− 1} mg ^{− 1} ), phenylalanine ammonia‐lyase activity (0.55 µmol trans‐cinnamic acid min ^{− 1} g ^{− 1} ), and hydrogen peroxide content (0.56 OD at 450 nm), respectively. However, biochemical analysis reported that Spartan exhibited stronger induction of defense‐related compounds following pathogen inoculation, indicating differential resistance mechanisms among cultivars rather than absolute resistance levels. These findings highlight the importance of these biochemical parameters in imparting resistance to Alternaria blotch in apple cultivars. Considering the biochemical responses and their association with disease resistance can contribute to the progress of effective strategies for the management of A. alternata f. sp. mali and enhancing apple production in Himachal Pradesh, India. This research sheds light on the possible avenues for refining apple cultivation by targeting detailed biochemical pathways linked with disease resistance.