Relationships of Pre-Inoculation High Temperature to Root Browning Caused by Pythium Aphanidermatum in Hydroponically-Grown Sweet Pepper

Episodes of high root-zone temperature (33 degrees C) were investigated in relation to susceptibility of sweet pepper to root rot caused by Pythium aphanidermatum. Pepper plants were grown in aerated nutrient solution in hydroponic units positioned in temperature-controlled water baths. Root zone temperature was 23 degrees C except during high temperature treatments. Roots were exposed to 33 degrees C for 0 to 216 h, inoculated with P. aphanidermatum and assesseda at intervals for colonization by the pathogen and for severity of browning (necrosis). The pathogen colonized all roots within 12 h following inoculation. The roots turned brown earlier when exposed to 33 degrees C for more than 9 h prior to inoculation compared to those exposed for 0 to 6 h. Browning developed progressively earlier as the period at 33 degrees C was increased from 9 to 144 h, and was 4 days earlier for exposures of 144 to 216 h compared to 0 to 9 h. Browning was precocious also when inoculation was delayed as long as 216 h following exposure at 33 degrees C for 72 h. We conclude that high temperature predisposes pepper plants to precocious browning caused by P. aphanidermatum and that predisposition lasts for at least 9 days.