Virulence variation in Alternaria mali (Roberts) and evaluation of systemic acquired resistance (SAR) activators for the management of Alternaria leaf blotch of apple

Alternaria leaf blotch, caused by Alternaria mali, is an economically important disease of apple (Malus xdomestica Borkh.). Twenty one isolates of Alternaria mali (Am-1 to Am-21) were obtained during the isolate collection. The virulence was tested on detached leaves of susceptible Red Delicious cultivar and Am-1 showed highest virulence. Two greenhouse trials were conducted using two year old grafted seedlings of Red Delicious apple cultivar to assess the efficacy of seven SAR activators, 2,6-dichloroisonicotinic acid (INA), benzothiadiazole S-methyl ester (BTH), β-aminobutyric acid (BABA), K2HPO4, K3PO4, Ca(OH)2 and CaCO3 applied 48 hrs before and after spore inoculation. The SAR activators and a conventional synthetic fungicide (penconazole) were evaluated against most virulent isolate Am-1. Distilled sterilized water was sprayed on control plants. All the SAR activators significantly lowered the disease intensity as compared to control. BABA was most effective with least disease intensity before and after pathogenic inoculation. Penconazole proved superior to all the SAR activators, except BABA. The application of SAR activators before pathogen inoculation showed significantly lower disease intensity (12.71%) in comparison to SAR application after pathogen inoculation (14.77%). This induced resistance exploiting natural defense machinery of plants could be proposed as a non-conventional and eco-friendly approach for plant protection.