Post-harvest management of silver scurf and Fusarium dry rot of potatoes in storage with azoxystrobin and fludioxonil
Rick Peters Agriculture and Agri-Food Canada (AAFC)
To assess the efficacy of the reduced risk products azoxystrobin and fludioxonil in comparison with thiabendazole for controlling silver scurf, Fusarium dry rot and black dot of potato; and to survey pathogen strains from across Canada for sensitivity to these control products
Summary of Results
With growing demand for high-quality, blemish-free potatoes, the identification and management of storage disease have increased in importance. The aim of this project was to provide information to support potential minor use registrations of reduced risk products for the control of the common potato storage diseases silver scurf and dry rot, as well as black dot, an emerging disease of potato in parts of the country. Silver scurf is characterized by metallic, silvery patches on tuber skins, which can reduce suitability for direct sales and processing. Fusarium dry rot results in seed tuber decay and losses in storage. Black dot affects stressed plants and can also cause tuber blemishes, resulting in yield loss.
Traditionally, growers have managed these diseases post-harvest with the application of thiabendazole; however resistance to thiabendazole in the silver scurf and Fusarium dry rot pathogens is becoming widespread around the world and in Canada, highlighting the importance of identifying effective alternative treatments. The project also assessed current management practices and resistance levels in pathogen populations to ensure that an integrated approach is taken in managing these important storage diseases of potatoes.
In 2007, samples of seed tubers in spring and daughter tubers following harvest, showing extensive symptoms of decay, were received from growers across Canada (Table 1), and used to identify a number of pathogen isolates.
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Isolates were submitted to chemical sensitivity testing via an amended agar assay to assess levels of resistance of the isolates to three fungicides: azoxystrobin, fludioxonil and thiabendazole.
Trials to assess the efficacy of azoxystrobin, fludioxonil, thiabendazole and phosphorous acid applied as post-harvest sprays were established in 2007 in Prince Edward island (PE), New Brunswick (NB), and Alberta (AB); severity of tuber disease was assessed in spring 2008. The studies were separated into 3 trials, based on the disease of interest, with each trial being conducted separately and stored in its own storage area to prevent cross contamination. Controls were randomized along with the other treatments in the same room to allow proper comparisons.
Fusarium dry rot was a disease of significance in seed and daughter tubers across Canada in 2007. Isolation of pathogens from diseased tubers yielded 113 isolates of Fusarium spp., 25 isolates of Helminthosporium solani (silver scurf pathogen) and 3 isolates of Colletotrichum coccodes (black dot pathogen). In terms of Fusarium, F. sambucinum was the most common species isolated from decayed lesions. However,isolates of F. coeruleum, F. avenaceum, F. oxysporum, F. graminearum and other minor species were also obtained. Half of the isolates of F. sambucinum tested (total of 20) were resistant to fludioxonil, which represents the first report of resistance to this compound anywhere in the world. Approximately half of the F. sambucinum isolates were also resistant to thiabendazole, thus multi-class fungicide resistance as observed. The other Fusarium species tended to be sensitive to fludioxonil and thiabendazole with the exception of F. oxysporum, which was always sensitive to thiabendazole but resistant to fludioxonil. Azoxystrobin is not effective against Fusarium spp.
Further resistance testing revealed some insensitivity of the silver scurf pathogen to both fludioxonil and thiabendazole, and of the black dot pathogen to thiabendazole. No resistance to azoxystrobin was observed and this remains a viable product option for control of these pathogens in Canada. Resistance to phosphorous acid was not assessed, as this type of fungicidal agent is generally not associated with a risk of resistance development in pathogen populations.
All treatment combinations tested showed comparable efficacies when applied post-harvest for control of silver scurf in storage, reducing the percentage of surface lesions from approximately 20% in untreated tubers to between 5% and 10%. Going forward, combinations that include azoxystrobin and phosphorous acid are anticipated to be particularly effective, if populations of H. solani with resistance to thiabendazole and fludioxonil continue to affect stored potatoes in Canada.
Results were more variable in the Fusarium dry rot trials, where a combined treatment of azoxystrobin, fludioxonil, and thiabendazole proved most efficacious in controlling disease severity in storage. Combined treatment with azoxystrobin, fludioxonil and phosphorous acid was also highly efficacious, as was treatment with fludioxonil alone. All other products assessed, either alone or in combination, showed some measure of disease control. These results must be tempered however, that they were obtained using a fungicide-sensitive isolate of F. sambucinum. Given the increasing incidence of fungicide-resistant strains of F. sambucinum in Canada (strains with resistance to both fludioxonil and thiabendazole), new management tools will be needed both at planting and post-harvest in order to better manage these strains.
These findings will be important in designing a new, integrated approach for controlling potato storage diseases, and will serve to support minor use registrations of azoxystrobin and fludioxonil for post harvest use on potatoes in Canada. Shortly after this project was conducted, Canadian registration was granted for the use of the phosphorous acid product CONFINE for management of potato storage diseases.
The results of this study were used to prepare a factsheet for the AAFC’s Pest Management Centre entitled Biofungicides Provide Post-harvest Disease Protection in Potatoes.
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