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 isolatecollection. The virulence was tested on detached leaves of susceptible Red Delicious cultivar and Am-1 showedhighest virulence. Two greenhouse trials were conducted using two year old grafted seedlings of Red Deliciousapple cultivar to assess the efficacy of seven SAR activators, 2,6-dichloroisonicotinic acid (INA),benzothiadiazole S-methyl ester (BTH), ?-aminobutyric acid (BABA), K2HPO4, K3PO4, Ca(OH)2 and CaCO3applied 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 oncontrol plants. All the SAR activators significantly lowered the disease intensity as compared to control. BABAwas most effective with least disease intensity before and after pathogenic inoculation. Penconazole provedsuperior to all the SAR activators, except BABA. The application of SAR activators before pathogeninoculation showed significantly lower disease intensity (12.71%) in comparison to SAR application afterpathogen inoculation (14.77%). This induced resistance exploiting natural defense machinery of plants could beproposed as a non-conventional and eco-friendly approach for plant protection.