High temperature predisposition of sweet pepper to Pythium root rot and its remediation by Pseudomonas chlororaphis
Pythium root rot caused by Pythium aphanidermatum, a destructive disease of sweet pepper and other hydroponic crops, is characterized by root browning (necrosis) and reduces growth of roots and shoots. Serious losses in crop productivity are common, in part for lack of adequate control measures. Severe root rot has been previously associated with episodes of high temperature, but whether this is due to high temperature effects on the host, the pathogen, or their interaction remains unclear. To clarify these relationships, and to provide a basis for predicting rapid increases in root rot, quantitative experiments were conducted to determine how episodes of high root-zone temperature are associated with root browning. Pepper plants were grown separately in hydroponic units containing aerated nutrient solution positioned in temperature-controlled water baths. The root zone temperature was 23°C except during high temperature treatments. Browning developed progressively earlier in roots that were maintained at 33°C for 9 to 144 h immediately before they were inoculated with zoospores of P. aphanidermatum, and in all instances earlier than in control plants maintained continuously at 23°C. The data demonstrated unequivocally that high root-zone temperature can predispose pepper plants to Pythium root rot. Browning also developed earlier when root inoculation with P. aphanidermatum was delayed as long as 216 h following exposure at 33°C for 72 h, indicating that predisposition of the host by high temperature episodes can persist for at least 9 days. The ability of Pseudomonas chlororaphis strain 63-28 to suppress Pythium root rot and promote plant growth was investigated in pepper plants grown in the hydroponic units and predisposed to the disease, i.e. plants were predisposed to high temperature (33°C for 72 h ending at 3 days before inoculation) or not predisposed (constant 23°C). When P. chlororaphis was applied in the nutrient solution at a final density of 107 CFU mL-1 7 days before the high temperature episode, the agent delayed root browning, re-mediated predisposition to root rot, and increased growth of plants that were and were not inoculated with P. aphanidermatum. It is concluded that high temperature predisposed pepper seedlings to root rot and that strain 63-28 has substantial potential for managing root rot regardless of predisposition by high temperature.