Timing Pseudomonas chlororaphis applications to control Pythium aphanidermatum, Pythium dissotocum, and root rot in hydroponic peppers

The ability of the microbial agent Pseudomonas (Ps.) chlororaphis Tx-1 to suppress Pythium aphanidermatum and Pythium dissotocum, and control root rot, was investigated as a function of time of agent application and pathogen inoculation in vegetative-stage sweet peppers grown in single-plant hydroponic units. The agent was applied to the nutrient solution at 1× 107 CFU/mL and roots were inoculated by immersion in suspensions of 5 × 103 or 1 × 104 zoospores/mL. When Ps. chlororaphis was applied at 14, 7, 3, or 0 days before roots were inoculated with P. aphanidermatum, treatments timed as follows were significantly, and often markedly, effective: 3 and 0 days against root colonization by the pathogen; 14, 7, and 3 days against brown root tips; and 7, 3, and 0 days against percent discolored roots. In a parallel study of P. dissotocum, each treatment suppressed symptoms, though inconsistently, but not colonization. When Ps. chlororaphis was applied once (day 0) or twice (days 0 and 14) and roots were inoculated with P. aphanidermatum on day 3 or day 17, the single treatment strongly suppressed colonization, brown root tips and percent colonization regardless of inoculation time, but the second treatment did not improve control. Density of Ps. chlororaphis associated with the roots after one or two applications, respectively, ranged up to 1.3–1.5 × 105 and 8.0 × 106–1.6 × 107 CFU/g fresh roots. The density rapidly declined in noninoculated roots but generally remained high in roots inoculated with either pathogen. Collectively, the observations suggest that Ps. chlororaphis treatments should be timed to maintain about 105 CFU of the agent/g fresh roots, that more frequent application is needed to maintain the agent in healthy compared to infected roots, and that an ideal time for initial application is 3 days before an attack by P. aphanidermatum or P. dissotocum.