The persistence of Colletotrichum coccodes and Mycosphaerella ligulicola in soil, with special reference to sclerotia and conidia

Abstract

In vitro tests with air-dried agar cultures and greenhouse tests in sand and soil revealed that the maximum longevity of sclerotia of Colletotrichum coccodes was generally at least 2.5 times that of sclerotia of Mycosphaerella ligulicola. After 83 weeks' burial in natural soil, which had been subjected to the normal greenhouse watering routine, 53 % of the sclerotia of C. coccodes sampled were viable, whereas all sclerotia of M. ligulicola sampledafter 30 weeks' burial were dead. Of twenty-eight sclerotia of C. coccodes retrieved from soil after 84 weeks, three infected tomato test plants, but sclerotia of M. ligulicola lost the ability to infect chrysanthemum cuttings after only 8 weeks in compost. The numbers of conidia of both fungi declined rapidly after a few days' burial in natural soil and none survived for 3 weeks. The sclerotia of C. coccodes and M. ligulicola are considered to have ‘strand’ and ‘loose’ types of formation respectively. For sclerotia in general, there is no evidence that morphological complexity is indicative of persistence value in soil. The results are related to what was previously known about the life-histories of both pathogens under greenhouse conditions.