THE TOXIC SUBSTANCE PRODUCED BY THE EYE-SPOT FUNGUS OF SUGAR CANE, HELMINTHOSPORIUM SACCHARI BUTLER

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Abstract
Tests for a toxic reaction of susceptible sugar-cane leaves were conclusively negative when tested with filtrates of cultures of H. sacchari in pure sucrose solutions and in beef-extract bouillon media. Tests on sugar-cane leaves with filtrates of cultures in Richard''s solution showed the presence of a toxic substance. This toxic substance was shown to persist after heating to an extent which would destroy enzymes and coagulate most proteins. The presence of considerable concentrations of nitrites was shown in the filtrates of cultures in Richard''s solution and to a lesser extent in an asparagin medium, but such nitrites were not evidenced in filtrates of cultures in a peptone bouillon medium nor in a beef-extract bouillon, nor in uncultured Richard''s solution as a control on the test for nitrites. Susceptible sugar-cane leaves evidenced a toxic reaction from filtrates of the cultures in Richard''s solution, to a lesser extent in an asparagin medium and to a much slighter extent in a peptone medium. There was no correlation between quantity of growth in these media, and quantity of the toxic substance produced. The reaction of cane leaves to these filtrates of cultures in media with various N sources was closely correlated with the presence of nitrites in such filtrates. Dilute solutions of KNO2 brought about the same reaction in susceptible sugar-cane leaves as filtrates of cultures of the eye-spot fungus in media containing the lower N compounds. This reaction, in dilute concentrations, was a yellowing of leaves either totally or in streaks, and sometimes the formation of brownish-red streaks up and down the leaves. Leaves in these filtrates wilted and lost the glossy appearance typical of the control leaves. The results indicate a strong relationship between the virulence of this disease and N nutrition of the host plant. The attempt to apply these results to commercial crop production by using organic N fertilizers for the nutrition of the host plants depends for its success on the N from such fertilizers reaching the leaves in a N form other than as NO3. Field results with organic N fer-tilizers in the case of eye-spot disease indicate as large an increase of the disease as that which follows applications of inorganic N fertilizers. A practical application has been developed for minimizing this disease which in principle is dependent on avoiding nitrogen fertilizers during seasonal periods in which climatic conditions favor fungus infection.

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