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STORAGE-RELATED DISEASES: Research Recap

Each year, disease causes economic losses for many growers, but more options are on the way for the management of storage-related diseases.

In Canada, numerous projects are under way to study various post-harvest diseases and to evaluate products, some established and some yet to be released, for the prevention and suppression of storage-related diseases, with the aim of determining the best options for growers.

Fusarium

Collaborative projects are under way in Alberta and Prince Edward Island on the management of fusarium dry rot. Research efforts are aimed at determining the best integrated programs to control the pathogen—some strains are showing increasing resistance to a number of commonly used products.

Potato infected with pink rot.

Potato infected with pink rot.

Various strains of the fusarium pathogen were tested for sensitivity to thiophanate-methyl (Senator PSPT), fludioxonil (Maxim PSP, a potato seed piece treatment) and thiabendazole (Mertect SC, a post-harvest treatment). Approximately 50 to 65 per cent of the isolates of Fusarium sambucinum, the major dry rot pathogen, recovered in surveys across Canada in 2008 and 2009, were resistant to all products. Most other strains were sensitive to these products. Therefore, the strain present in a tuber lot plays a large role in the efficacy of the treatment.

“Ultimately, the management of fusarium dry rot and seed piece decay depends upon an integrated approach that takes advantage of a number of control options, and information generated by research studies,” says Rick Peters, a research scientist with Agriculture and Agri-Food Canada in Charlottetown.

One post-harvest treatment under evaluation is a bio-control product called Bio-Save, manufactured by Jet Harvest Solutions. “It’s a post-harvest applied product that goes on the tubers—the bacteria colonizes the wounds where fusarium can establish itself and, basically, reduces the disease pressure by occupying those spaces before the pathogens do,” says Peters. Canada’s Pest Management Regulatory Agency approved Bio-Save for use on potatoes in storage this July. The product is also being tested in other trials at New Brunswick’s Potato Development Centre to assess its efficacy against dry rot and silver scurf.

Silver Scurf

Silver scurf is showing signs of resistance to some commercially-available fungicides. One post-harvest treatment being evaluated is a phosphorous acid-based product, which is showing promise for disease management in a number of areas. The product, registered in Canada under the name Confine, is effective for the suppression of late blight and pink rot. Recent studies by research scientists in Canada also proved it to be effective in the suppression of silver scurf. This research has resulted in the addition of silver scurf to the Confine product label. “That’s been exciting, and offers growers some more options for silver scurf management,” says Peters.

Late Blight

Late blight has become the main area of focus for many of Canada’s potato disease research programs over the past year, following AAFC’s decision to designate it as a disease of priority concern in 2010.

“Late blight is certainly on the front burner because we have had a lot of late blight in the east and west again this year, and certainly that raises concerns because of spores getting to the tubers in the field and then causing post-harvest problems,” says Peters.

Prior to 2010, there was little interest in late blight from a research standpoint, which concerned Khalil Al-Mughrabi, a provincial research scientist at the Potato Development Centre in Wicklow, N.B., who for many years had pushed for late blight strain identification and late blight prevention programs to be established in Canada. Finally, with support from the Canadian Horticultural Council, Al-Mughrabi, together with interested scientists from seven other provinces, established a national late blight monitoring network. The network, which has since been joined by other researchers across the country, is working primarily on a number of three-year-long late blight research projects.

Additionally, a nationwide late blight strain identification program has been established, which began collecting samples from across the country last year, and as a result, discovered some new strains of late blight in the western provinces. Samples are sent to AAFC research facilities for analysis in London, Ont., Lethbridge, Alta., and Charlottetown. “We now have a national late blight program looking at the genetics of the pathogen across the country,” says Peters. “The strains have been changing in recent years, and that changes the management too.” Strain identification results for 2011 should be available this fall.

Zenaida Ganga, a research scientist at Cavendish Farms in Summerside, P.E.I., is leading another project to assess the fungicides that are currently available on the market, and some that are soon to be released, for their effectiveness in controlling late blight. The trial compares treatment programs of low, medium and high rates of foliar-applied phosphorous acid-based fungicides (Confine and Phostrol) on the control of late blight. Other fungicides included in the study are Bravo ZN, Manzate, Quadris, Revus Top, Allegro, Polyram DF, Zampro, Headline, Curzate, Tanos, Lance, and Reason. The treatment programs are made up of combinations of fungicides in order to determine their effectiveness in an integrated disease management program. “We are trying to assess what the best combinations are to provide an effective treatment program,” says Ganga.

The project is now in its third year and Ganga says because late blight was more prevalent this year during the growing season, her work, which includes field and storage evaluation of late blight incidence and severity, should yield some interesting results and will be released toward the end of 2011.

Gefu Wang-Pruski, of the Nova Scotia Agricultural College, is also leading a project that involves Confine. She notes that phosphorous acid-based products have been used for the control of many plant pathogens for three decades, and has now been introduced in Canada to control late blight in potatoes, both as a foliar and post-harvest treatment.

Potato infected with pink rot and late blight.

Potato infected with pink rot and late blight.

Wang-Pruski and collaborators at Cavendish Farms completed three-year field trials using Confine and found that under certain conditions the fungicide can delay the spread of the infection for more than two weeks. “From a production point of view, when you have a two- or three-week delay in onset of the disease it’s really significant,” says Wang-Pruski.

Since the molecular mechanisms through which phosphorous acid controls late blight are not clear, Wang-Pruski’s research program is focused on studying its mode of action. Data obtained so far indicate that Confine treatments trigger wide metabolic changes associated with the activation of various defence mechanisms, says Wang-Pruski.

“It is likely these broad changes promote an enhanced immune system and enable potato plants to respond better to late blight infection,” she says.

In addition, the efficiency of Confine treatments for suppressing late blight development in potatoes extends to tubers as well. Data obtained by Wang-Pruski and collaborators indicate that tubers harvested from field plants treated with Confine during the growing season gained significant resistance to late blight during storage.

Phosphorous Acid Applied Post-Harvest

Confine has also been used successfully as a post-harvest treatment to help prevent pink rot and late blight tuber rot in storage, but only on potatoes that are not already infected with disease pathogens prior to entering storage. Research carried out  in New Brunswick and Prince Edward Island with phosphorous acid-based products showed that when they were applied as a post-harvest spray to potatoes entering storage, complete suppression of late blight tuber rot and pink rot, caused by spores that had been splashed from diseased tubers to healthy tubers during harvest operations, could be achieved. The spray had to be applied within six hours of harvest to prevent the spores from germinating and infecting the tubers.

Research led by Peters and Al-Mughrabi has revealed that Confine is most effective as part of an integrated disease management program that combines foliar and post-harvest treatments. When used in combination with other protectant fungicides, the rate of foliar-applied protectant can be reduced by 50 per cent and still achieve disease control.

When applied post-harvest, Confine works by directly inhibiting the growth and reproduction of the disease pathogen. There is also evidence that applying it to foliage can stimulate the production of natural defence chemicals within the potato plant, making it less susceptible to disease.

“Following post-harvest inoculation, healthy tubers from plots receiving phosphorous acid during the field season were significantly more resistant to the development of pink rot and late blight tuber rot in storage,” says Peters in his research findings. “As well, tubers from plants receiving phosphorus acid in the field were less susceptible to disease than control tubers when we inoculated the tubers with the pathogens after harvest. In this way, tubers from plants receiving phosphorus acid in the field enter storage with a significant level of disease resistance.”

Canadian scientists, through research and collaboration, continue to work on solutions to control storage-related diseases. Research results from ongoing and past studies on post-harvest diseases of potato crops in storage offer farmers and storage managers more options for the protection of their crops from storage losses, and in turn, loss of profit.

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