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Phosphorus acid a strong option for disease control in potatoes

Both foliar or post-harvest application are options, depending on circumstances

Phosphorus acid was a much-discussed topic at Manitoba Potato Production Days in Brandon this year, and for good reason. A variety of phosphorus acid treatments, registered in Canada under the labels Phostrol, Rampart and Confine, are useful additions to growers’ tool boxes for disease control.

Susan Ainsworth, a potato specialist for Syngenta in Manitoba, offered research-based strategies on whether growers should apply phosphorus acid as a foliar treatment, or post-harvest.

In Canada, there are five phosphorus acid products registered for use. Confine, Confine Post and Confine Extra provide suppression of late blight, pink rot and silver scurf. Phostrol offers suppression of pink rot and control of late blight. Rampart provides control of late blight and pink rot.

Phosphorus acid works by suppressing the growth of Phytophthora pathogens, and by conferring resistance through stimulation of the plants’ natural defence mechanisms.

“A large number of Canadian growers is using phosphorus acid,” says Ainsworth.

She says her presentation’s biggest takeaway message is that one method of application, either foliar or post-harvest, is not superior to the other. “Applying foliar is a preventive strategy to reduce the potential infection over the course of the season, and applying post-harvest will protect healthy tubers from becoming infected if they come into contact with infected tubers through the harvest operation,” she says. “The choice should be based on the needs of each individual farm — what works well for one farm might not be the best choice for the next farm.”

Also key to the use of phosphorus acid is correct application — if foliar, it must be applied at the label rate at the right time. For example, if applied when the crop is under stress, foliar application can cause burning. If applied post-harvest, accurate rate and adequate coverage are important.

The fresh market

Tracy Shinners-Carnelley, director of research and quality enhancement for Peak of the Market, discussed the use of phosphorus acid to control silver scurf, a disease which causes blemishes on potatoes for the fresh market.

“We’ve been successful in managing silver scurf for the fresh market to the point where we hardly have it anymore,” she says.

Key to the table market’s success in managing silver scurf is Shinners-Carnelley’s work to have the Confine label amended in 2010-11 to remove a clause saying it was not suitable for use on smooth-skinned potato varieties. Shinners-Carnelley worked in co-operation with Manitoba Agriculture, Food and Rural Development pesticide specialist Jeanette Gautier to generate data for the study.

“A similar study was done by Khalil Al-Mughrabi, who generated a similar set of data,” Shinners-Carnelley says. “The results of those two studies helped remove that restriction from the Confine label. It was clear there was no negative impact to skin finish and we saw a reduction in silver scurf.”

Shinners-Carnelley says an integrated approach to silver scurf management is key, including cultural practices, chemical controls, and regulation of temperature and humidity in storage. “Cleaning and disinfecting and environmental management are important but done alone do not give the effect that the integrated approach does,” she says.

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