In order to determine the effects of chemical elicitors used for inducing resistance in plants against necrotic leaf spot caused by Alternaria mali, Salicylic acid (SA) with 0-700 ppm, DL-beta-amino-n-butyric acid (BABA) and Acibenzolar-s-methyl + metalaxyl (ASM) with 0-1500 ppm concentrations were tested on PDA. A. mali isolated from Red Jim apple variety and having high virulence was used. The spore germination tests were conducted at 24 degrees C for 12 h on water agar medium included different concentrations of chemicals and the effects on hyphal length were determined. SA decreased the mycelial growth of A. mali with increasing concentration and it was inhibited at 700 ppm completely. SA had the same effect on spore germination inhibition at 300 ppm concentration. BABA had no significant effect on mycelial growth with increasing concentration. However, the hyphal development was decreased with increasing concentration and hyphal lysis occurred just after spore germination at 800 ppm concentration. Mycelial growth of A. mali was decreased by increasing ASM concentration as well, but it was not inhibited completely. However, spore germination decreased by increasing concentration and hyphal lysis occurred at 500 ppm. In vivo tests, plant activators were found effective with increasing concentration. SA, BABA and ASM at 100 ppm concentration reduced disease severity by 89.5%, 87.6% and 87.6% on young apple seedlings, respectively. Plant activators that induce host resistance have potential for the control of necrotic leaf spot disease. (C) 2012 Friends Science Publishers