Distribution and fungicide sensitivity of Colletotrichum coccodes in French potato-producing areas

Black dot, caused by Colletotrichum coccodes, is of potential concern to potato production in France as part of the tuber-blemishing disease complex. The lack of information about the actual distribution of the pathogen in potato-producing areas led to a survey of the occurrence of the disease. Black dot symptoms were observed on roots, stems and/or tubers of the 82 potato cultivars examined in 1994. A baiting bioassay, using cuttings of potato cultivars Bintje and Urgenta, revealed the presence of the pathogen in all 37 soil samples tested, which had been collected throughout the main French potato growing areas. In vitro, growth of five C. coccodes isolates recovered from diseased potatoes grown in western and southern France was severely affected by imazalil, tolchlofos-methyl and, to a lesser extent, mancozeb and thiabendazole. Conversely, iprodione, flutolanil and pencycuron were ineffective in reducing the growth of these isolates. These data indicate that C. coccodes is widespread in French potato cropping areas, that currently popular cultivars are susceptible to the disease, and that at least some of the fungicides commonly applied to seed tubers are ineffective against the pathogen. A better diagnosis of the disease, but also the insensitivity of the pathogen to several chemicals frequently used on seed tubers for controlling black or silver scurfs, might thus provide explanations for the apparent increase in black dot occurrence in recent years.