Induced Systemic Resistance (ISR) by Pseudomonas spp. impairs pre- and post-infection development of Pythium aphanidermatum on cucumber roots

The effect of induced systemic resistance (ISR) by Pseudomonas rhizobacteri a on the pre- and post-infection development of Pythium aphanidermatum on c ucumber roots was investigated. Cucumber plants (cv. Corona) were grown in vermiculite, roots were split with one side bacterized with Pseudomonas cor rugata strain 13 or P. aureofaciens strain 63-28 (bacterized roots) and the other distant side was treated with water (distant, induced roots). For th e non-induced control, roots on the bacterized side were treated with buffe r instead of the bacterial treatment. Intact, non-split roots were also tre ated with the bacteria or buffer as a control. Cucumber root tissue from th ese treatments were harvested and incubated with a zoospore suspension of P . aphanidermatum for three hours. Most of the zoospores in the suspension w ere stimulated to encyst or germinate. The numbers of germinated zoospores were significantly decreased on distant induced cucumber roots in compariso n to non-induced controls. Germination was also reduced on intact bacterize d roots, compared to controls. There was less attachment, germ tube product ion and penetration on roots bacterized or induced by the rhizobacteria com pared to non-induced roots. Effects were significantly greater on bacterize d roots (roots colonized by bacteria) compared to distant induced roots (ro ots with the opposite side bacterized). Systemic resistance induced by the two Pseudomonas spp. also reduced pathogen spread on split cucumber roots i n planta. Crown infection from induced or bacterized roots was delayed for four to six days in comparison to the non-induced control. Results indicate d that Pseudomonas spp. can exert both an indirect influence on P. aphanide rmatum zoospore behaviour and infection via induced systemic resistance (IS R) and a local influence via antibiosis or local induced resistance.