Relationships ofPythiumisolates and sweet pepper plants in single-plant hydroponic units

An isolate of Pythium aphanidermatum and five isolates of Pythium dissotocum were examined for ability to colonize and produce symptoms in root systems of sweet pepper grown in aerated nutrient solution in single-plant hydroponic units in a research greenhouse. Repetitions of the study were conducted in April 2000, February 2001, and May 2001. Roots were inoculated by immersion for 30 min in a suspension of 5 × 103 zoospores/mL nutrient solution. All isolates rapidly colonized the root systems and increased density of cortical cells with cytoplasmic granulation in all repetitions. Symptoms produced by a given isolate differed but generally overlapped with those caused by other isolates and variously included zones of root-tip browning of different sizes and hues (all isolates), expansive browning (P. aphanidermatum) or yellowing (P. dissotocum), architectural changes (chiefly stunting, stubbiness, and proliferation of roots with all isolates), root swelling (P. dissotocum), and callus cell proliferation (P. dissotocum). Patterns of increase in incidence of brown root tips and of expansive root discoloration differed among isolates and among experimental repetitions. Circumstantial evidence suggested that higher temperature and light conditions of May 2001 promoted disease caused by P. aphanidermatum and P. dissotocum isolates compared with February 2001 and, in some instances, April 2000. Observations indicated that moderate differences in temperature, light, or other environmental variables among the repetitions markedly influenced the expression of discoloration and of other symptoms in colonized roots. All isolates reduced various growth parameters of the roots and shoots in one or more repetitions; however, relationships of colonization and symptoms to growth were generally unclear. Collectively, the data have applications for evaluating control methods in single-plant hydroponic units and contribute to an understanding of the epidemiology of root rot caused by Pythium spp. in greenhouse pepper.