Role of Salicylic Acid in Induction of Resistance against Bacterial Wilt in Eucalyptus urophylla and Changes of Peroxidase and Polyphenol Oxidase

The results from toxicity test indicated that salicylic acid (SA) at 1～20 mmol had no direct inhibition over the growth of Ralstonia solanacearum grown on modified Kelman agar plates. SA within range of 1～5 mmol was capable of inducing systemic acquired resistance (SAR) against eucalypt bacterial wilt by root pouring, but the best induction of disease resistance could only be obtained with 5 mmol salicylic acid 7 days prior to inoculation of the pathogen. Correspondingly, the activities of peroxidase (POX) and polyphenol oxidase (PPO) increased when SA was more than 0.1 mmol, and reached the highest level at the concentration of 5 mmol. Within 3～9 days between SA application and challenge inoculation on decapitated shoot tip with R. solanacearum, 5 mmol SA was able to induce systemic resistance against the pathogen, but the best inducing results could be gained within intervals of? 5～7 days. The activities of POX and PPO were positively correlated to the enhance disease resistance, by increasing to their maxima at the 7th day,with 2 times and 1 time higher, respectively, than those from the controlled plants. Concerning the sensitivity, POX was better than PPO to reflect the physiological change of eucalypt seedlings upon application of SA.Infiltration with SA into lower leaves could not induce SAR against bacterial wilt upon challenge inoculation on wounded shoot tips.