VIEW ARTICLE

Relationship of Pectic Enzyme Activity and Presence of Sterols to Pathogenicity of Pythium ultimum on Roots of Antirrhinum majus. H. M. Mellano, Former Research Assistant, Department of Plant Pathology, University of California, Riverside 92502; D. E. Munnecke(2), and J. J. Sims(3). (2)(3)Professor, and Associate Professor, respectively, Department of Plant Pathology, University of California, Riverside 92502. Phytopathology 60:943-950. Accepted for publication 14 January 1970. DOI: 10.1094/Phyto-60-943.

Culture filtrates of Pythium ultimum exhibited marked pectic enzyme activity and rapidly macerated susceptible snapdragon (Antirrhinum majus) seedling root tissue, but not tolerant tissue. Tolerance to infection by the fungus was correlated with resistance of the root tissues to maceration by P. ultimum filtrates. Presence of β-sitosterol in the culture medium of P. ultimum depressed pectic enzyme activity of the culture filtrates. The time when pectic enzyme activity was reduced coincided with the first appearance of oospores in the cultures. β-sitosterol, campesterol, and stigmasterol were detected in snapdragon seedling roots. Other compounds, presumed to be oxidized sterols but not identified, were isolated from snapdragon and shown to induce oospore production by Pythium. It is proposed that seedling tolerance is based upon interacting physiologic factors. Substances such as β-sitosterol, present in tolerant host tissues, may switch fungal development from vegetative proliferation to reproductive activity. There may be a corresponding depression of enzymes which degrade cell wall material, and hence a lessening of virulence.