Role of salicylic acid in systemic resistance induced by Pseudomonas spp. against Pythium aphanidermatum in cucumber roots

Pseudomonas corrugata strain 13 and P. aureofaciens strain 63-28, applied t o roots, induced systemic resistance against Pythium aphanidermatum in cucu mber roots. Salicylic acid (SA) from bacterial culture or plant tissues was quantified by high performance liquid chromatography. Both strains produce d SA in King's B broth and also induced cucumber root to accumulate endogen ous SA one day after bacterial inoculation. Using a split root system, more SA accumulated in roots treated with bacteria than in distant roots on the opposite side of the root system in the first two days, but this differenc e disappeared after 3-4 days. SA levels were significantly higher in plants treated with bacteria compared to the split control, from one to five days after bacterization. SA did not inhibit mycelial growth of Pythium aphanid ermatum at 100-200 mu g ml(-1) in vitro, but higher levels inhibited myceli al growth. Zoospore germination increased at concentrations of 10-500 mu g ml(-1), but decreased at 1000 mu g ml(-1) compared to lower concentrations. Exogenously applied SA failed to induce local or systemic resistance again st a challenge infection by the pathogen in planta. The results of this stu dy show that exogenous applied SA does not induce systemic resistance to cu cumber root rot caused by P. aphanidermatum, but endogenous SA accumulation in cucumber roots may be involved in induced systemic resistance.