Colonization of Potato by Colletotrichum coccodes: Effect of Soil Infestation and Seed Tuber and Foliar Inoculation

Abstract

Colonization of potato (Solanum tuberosum) tissue, including roots, stolons, and above and below ground stems, by Colletotrichum coccodes, the causal agent of black dot, was evaluated following soil infestation, inoculation of seed tubers and foliage, and every combination thereof, in field trials over two growing seasons in North Dakota and Minnesota. A total of 107,520 isolations for C. coccodes performed across four site-years allowed for an extensive comparison of fungal colonization of the host plant and disease severity. The black dot pathogen was detected in potato stems at the first sampling date in all four site-years, as early as 14 days prior to emergence. Colonization of above and below ground stems occurred at a higher frequency than in roots and stolons in all four site-years, resulting in significantly higher relative area under the colonization progress curves (RAUCPCs) (α = 0.05). Although fungal colonization and disease incidence were higher in inoculated and/or infested treatments, sufficient natural inoculum was present to result in substantial levels of disease in noninoculated and noninfested plots. However, noninoculated and noninfested plots displayed the lowest RAUCPC values across three of four site-years and those treatments with multiple inoculation events tended to have higher RAUCPC values. Isolates belonging to vegetative compatibility group (VCG)2 and -5 were recovered from plants sampled in 2004 more frequently than isolates belonging to VCG1 and -3. A significant difference in disease incidence on stems was observed only in North Dakota in 2004 and Minnesota in 2003 (α = 0.05). Noninoculated and noninfested plots displayed the lowest disease incidence, whereas those treatments with more than one inoculation and/or infestation event tended to have higher disease incidence. Results of this study, including the disease severity and yield data, provide a better understanding of colonization of potato plants by C. coccodes and its impact.

Response of Colletotrichum Coccodes to Selected Fungicides Using a Plant Inoculation Assay and Efficacy of Azoxystrobin Applied by Chemigation

“…However, when the same samples were cultured in the laboratory, C. coccodes was isolated from roots, stems and tubers. Similar results have been previously reported (Johnson and Miliczky 1993;Otazu et al 1978;Pasche et al 2010). Furthermore, by 102 DAP, 100% of below-ground stems in both treated and non-treated plots were infected but asymptomatic in 2007 and 83% of plants in treated plots and 100% of plants in non-treated plots in 2008.…”

“…Timing of fungicide applications is critical to prevent infection of potato by C. coccodes. Early season protection appears to be a foundation for managing this pathogen and has previously been suggested Pasche et al 2010). The fungicides azoxystrobin, fluoxastrobin, mandipropamid + difenoconazole, and pyraclostrobin were effective in reducing infection in the inoculated plant assay when applied before potato plants were inoculated with C. coccodes.…”

“…coccodes frequently infects potato stems and other tissues early in the growing season but symptoms which include foliar chlorosis and necrosis, and signs of the pathogen in the form of sclerotia are often not expressed until relatively late in the growing season (Andrivon et al 1998;Johnson and Miliczky 1993;Pasche et al 2010). Potato yield and tuber quality can be reduced by C. coccodes (Hunger and McIntyre 1979;Johnson 1994;Mohan et al 1992;Pasche et al 2010;Stevenson et al 1976;Tsror et al 1999); however, the effect on yield is variable and yield losses are not always evident (Kotcon et al 1985;Read and Hide 1995;Pasche et al 2010;Scholte et al 1985). Even though yield losses are not always observed, C. coccodes is a serious threat to potato production (Pasche et al 2010).…”

“…Potato yield and tuber quality can be reduced by C. coccodes (Hunger and McIntyre 1979;Johnson 1994;Mohan et al 1992;Pasche et al 2010;Stevenson et al 1976;Tsror et al 1999); however, the effect on yield is variable and yield losses are not always evident (Kotcon et al 1985;Read and Hide 1995;Pasche et al 2010;Scholte et al 1985). Even though yield losses are not always observed, C. coccodes is a serious threat to potato production (Pasche et al 2010). This is because there is the persistent threat for direct yield losses from C. coccodes, interaction with Verticillium dahliae to cause greater yield reductions (Tsror and Hazanovsky 2001) and reduced crop value from tuber blemish and shrinkage during storage.…”

“…Timing of fungicide application is important for reducing black dot severity ) and more information is needed about when and how to effectively apply fungicides. Early season application of fungicide was suggested as a potential timing for black dot control from a study where infections of various plant organs, disease development, symptoms expression and potato yield were all compared in epidemics in commercial fields in the Midwestern U.S. (Pasche et al 2010).…”