AN INVITRO SYSTEM FOR STUDYING THE EFFECTS OF PYTHIUM-ULTIMUM METABOLITES ON PELARGONIUM X HORTORUM

An in vitro bioassay was developed to investigate the toxicity of metabolites of Pythium ultimum on geranium (Pelargonium X hortorum). Unrooted plantlets from two geranium cultivars were subcultured on a rooting medium supplemented with 2.5% (v/v) culture filtrates of P. ultimum. After 10 days of growth on the treated medium, the emerging roots of the plantlets appeared thinner, shorter, and darker than the controls. Several necrotic lesions developed on the root surface, and microscopic observations showed an absence of root hairs and a collapse of cortical and epidermal cells. In addition, vegetative growth of plantlets was stunted for both cultivars, even in the absence of foliar symptoms. Symptoms observed in vitro were similar to those associated with infections of P. ultimum on the same cultivars in the greenhouse. The in vitro sensitivity of the cultivars to the fungal metabolites also was related to their susceptibility to the pathogen. This study supports the hypothesis that P. ultimum metabolites are involved in the pathogenicity on P. X hortorum.