Abstract: Slash pine dieback caused by Sphaeropsis sapinea (Fr.) Dyko & Sutton was inhibited by pretreating the pine shoots with both salicylic acid (SA) and paclobutrazol solution. Salicylic acid manifested more effects than paclobutrazol in preventing the disease. The mechanism of action of the two chemicals was different. Paclobutrazol mode of action has shown similar to that of fungicide and it inhibited spore germination and mycelium growth of S. sapinea, whereas no inhibitory effects of SA on the spore germination and mycelium growth were detected in vitro. Therefore, the reduction in disease severity in SAPretreated shoots was due to SAinduced resistance of shoots of slash pine to dieback. Superoxide dismutase (SOD) activity in both salicylic acid and paclobutrazol treated shoots was significantly higher than that in control shoots. There is a positive correlation between SOD activity and resistance to dieback in slash pine. Phenylalanine ammonialyase (PAL) activity in the shoots treated with higher concentration of SA and Aspirin decreased markedly, which implied that excessive exogenous SA and Aspirin induced feedback inhibition to PAL activity.