ONION-LEAF ANTHRACNOSE

Abstract

(1) An anthracnose disease of the leaves and bulb scales of the onion is described. (2) Market onion (Allium cepa L.) and a multiplier variety of Allium cepa L. often referred to as false shallot are affected. (3) There does not exist any resistance in the yellow, red or white varieties to the disease. (4) Tropical varieties of onion are as susceptible as the temperate- zone types. (5) Plants of all ages are equally susceptible. (6) The symptoms on the leaves may be described as elliptic or oval spots. On red varieties a red stain appears on young lesions. (7) The pathogene is described as a new species, Colletotrichum Chardonianum. (8) Growth of the causal fungus in carbohydrate media was excellent. It was especially good in starch-containing media. (9) Infection experiments under control conditions demonstrate the ability of C. Chardonianum to produce disease. Infection occurs primarily through the stomata but the germ tubes may penetrate through the cuticle. (10) Conidia germinate in water at a temperature of 21° C. in 3 1/2 to 4 1/2 hours. During germination of conidia, primary, secondary and even tertiary appresoria are produced. Also secondary conidia. Temperatures of 18° C. and 33° C. seem to inhibit germination, while conidia germinate readily at 25° C. In synthetic culture media the fungus grows best at a temperature of 27° C. In the field the disease appears during seasons when temperature is 23° C- 30° C. (11) The pathogene apparently lives over from one season to the other as mycelium in debris on the soil. (12) The conidia seem to resist the action of the sulphur fungicides. An effective action of copper fungicides, particularly colloidal copper, has been observed in laboratory experiments.