In an attempt to better understand the importance of tuber-borne inoculum in black dot development, several potato cultivars were inoculated with various Colletotrichum coccodes isolates. Symptoms developed first on underground organs (starting 2 weeks after inoculation on roots, and later on stolons and tubers) of inoculated plants; stem infections developed only after 7-10 weeks, depending on the cultivar. Infection with C. coccodes resulted in a reduction in numbers of stolons and tubers in cv. Bintje, but not in the later maturing cv. Roseval. Significant isolate by cultivar interactions were detected from the analysis of root symptoms after inoculation of three potato cultivars (Bintje, Spunta and Desiree) with five C. coccodes isolates. Such an interaction was also detected for stolon/tuber symptoms at the latest scoring date (98 days after inoculation), but not at earlier dates (58, 70 and 84 days after inoculation). These results suggest that protocols based on root colonization might be used for investigating cultivar response to black dot and pathogenicity of C. coccodes isolates, and that some specificity exists in the reaction of potato genotypes to this pathogenic fungus.
SummaryA PCR-based kit, Probelia TM, for the detection of Erwinia carotovora subsp, atroseptica (Eca) on potatoes was evaluated at five laboratories in four countries. The kit is based on DNAspecific PCR amplification followed by detection of amplicons by hybridization to a peroxidase-labelled DNA probe in a microplate. Specificity of the PCR primers for Eca, regardless of serogroups, was confirmed by testing against 246 bacterial, fungal and plant species. Detection limits of the assay varied little between six Eca strains in pure cultures (1.3• to 1.5x103 cells ml-l). When Eca-free tuber peel extract from four cultivars was inoculated with known numbers of 15 Eca strains, detection limits were more variable (1.0xl0 l to 6.2x103 cells ml -I peel extract), attributed probably to inconsistency in the recovery of DNA during extraction. When the PCR assay was compared with three current commercial Eca detection methods, using naturally contaminated tubers, results matched most closely those from viable counts on a selective medium, the most sensitive method (88%), followed by enrichment ELISA (72%) and last ELISA (30%), the least sensitive method.
The pathogenicity and ecology of some isolates representative of the four main Streptomyces species (S. scabies, S. europaeiscabiei, S. stelliscabiei and S. reticuliscabiei) identified as pathogenic to potato tubers were investigated. Three pathogenicity groups could be distinguished. Group 1 included all isolates of S. scabies, S. europaeiscabiei and S. stelliscabiei from common scab lesions of potato and other susceptible root crops. All these produced similar symptoms and were pathogenic to potato, carrot and radish. Group 2 included all isolates from S. reticuliscabiei netted scab lesions; they were pathogenic to both tubers and roots of only a few potato cultivars, and did not infect carrot or radish. Group 3 included three isolates of S. europaeiscabiei from netted scab lesions on cv. Bintje, which produced either common or netted scab symptoms depending on the potato cultivar or plant species. In an experiment on a few isolates from each of the three groups, held at various soil temperature regimes, the three from group 1 were most pathogenic at higher temperatures (20ЊC or 20/30ЊC), the two from group 2 were most pathogenic at a lower temperature (17ЊC). The group 3 isolate caused netted scab symptoms on susceptible cultivars at low temperatures (Յ 20ЊC) and deep-pitted lesions at higher temperatures. Since the groups identified differ in ecological requirements, it is important to adapt the control methods to the pathogenic species present in the soil.
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.
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