Sensitivity of Sugarcane Clones to Toxin fromHelminthosporium saccharias Determined by Electrolyte Leakage

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Abstract
Host-selective toxin from H. sacchari was processed to a high level of purity and was used in assays of relative sensitivities of sugarcane clones. An assay based on toxin-induced loss of electrolytes from leaf tissue was more precise than were assays based on visible expressions of toxicity. Electrolyte losses from tissues exposed to toxin for 1 h and monitored for 18 h equaled those of tissues exposed to toxin continuously for 18 h. The rate of loss increased for 2-3 h, then decreased. When toxin concentrations were increased from 0.1 to 0.5 .mu.g/ml, electrolyte loss increased linearly. High concentrations (50 .mu.g/ml) resulted in gradual but complete loss of ability of tissues to retain electrolytes. Sugarcane clone reactions varied from very sensitive (affected by toxin at 0.01 .mu.g/ml) through several intermediate levels to very insensitive (unaffected by at least 100 .mu.g/ml). In general, sensitivity or insensitivity to toxin correlated with susceptibility or resistance of the fungus. However, the correlation did not hold for 3 clones of the 17 tested. Toxin may determine pathogenicity to some clones but not to others; it should be used with caution in screening for disease resistance.

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