Differential Phytotoxicity of Metabolic By-Products of Helminthosporium victoriae

Abstract

The means by which H. victoriae causes necrosis in oats is discussed. Cultures of the fungus were grown for 30 days at room temp. in flasks, each containing 100 ml. of Richard''s soln., filtered to remove the hyphal mass, and the filtrate passed through a Berkefeld filter to make it aseptic. Boone and Clinton oat seedlings were grown in nutrient soln. for 1 wk., then the soln. was replaced by the filtrate in a series of dilutions from 1:15 to 1:1,800 in tap water. In the Boone var. at dilutions of 1:90 or less the leaf blades became rigid within 40 hrs. and showed a slight twisting after 48 hrs. At dilutions of 1:45 or less the green color of normal leaves changed to a grayish-brown after 52 hrs. The seedlings of Clinton were unaffected by the filtrate in these dilutions. It was concluded that a subst. was formed as a metabolic by-product of the growth of H. victoriae that was toxic to the susceptible var. of oats. The toxin occurs in the cells of the fungus as well as in the nutrient medium, since leaves of susceptible oat vars. were killed when sprayed with a water suspension of sterilized blended mycelium containing no culture substrate. The toxic principle in the culture extract is relatively stable.