Studies on the Biology and Pathogenicity of Colletotrichum indicum

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Abstract
In Szechwan Province, China, the causal fungus of an anthracnose of cotton cotyledons and bolls was identified as C. indicum Dast. The acervuli are 27-124 [mu] in diam. Setae are interspersed with the conidiophores. Conidia are falcate, obtuse at the ends and measure 16.5-27.5 X 3-5 [mu]. They germinate more rapidly on agar media than in water, forming 1 or 2 uni-septate germ tubes usually with an appressorium at their tip. The opt. temp. for germination was about 32[degree] C, and for mycelial growth on potato dextrose agar about 28[degree] C. The opt. pH for germination and elongation of germ tubes was 5.4, for mycelial growth and sporulation between 5.4 and 7.6. Susceptibility to desiccation was demonstrated by the failure of conidia to germinate 24 hrs. after having dried on a glass slide, whether smeared with their cirri or in water suspension. No evidence of the production of toxic substances by the fungus was found. Seeds of both Gossypium arboreum and G. hirsutum vars. soaked in a conidial suspension showed no significant decrease in germination % although a high % of infected seedlings resulted. Spraying with conidial suspensions induced lesions on young cotyledons, leaves, stems and bolls. Old leaves were apparently more resistant to such treatment. Several other plants also were inoculated. All proved immune in the seedling stage, but fruits of Solanum melongena, Capsicum annuum, Lycopersicon esculentum, and pods of Vigna sinensis and Glycine soja were successfully infected. No ascigerous stage was discovered. The fungus overwinters chiefly inside infected seeds and possibly inside infected host tissue in the field. Moderately high temps. combined with high humidity are most favorable for the development of the disease.

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