Canada: Potato processor warns against pink rot as harvest gets underway

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One bad potato can spoil the batch, which is why processor J.R. Simplot is reminding producers to beware of pink rot as they begin harvest.

“If you know there’s rot in there, tell your harvest operator, ‘when you come to that low spot, pick up the harvester, drive over, and don’t harvest those certain areas,’” said Scott Graham, Simplot’s raw agronomy manager.

Why it matters: Tubers may get infected while in the field or if damaged during harvest but, if it’s wet enough, the fungus causing pink rot can thrive and spread in the bunker through infected soil.

The fungal disease has been an issue for a number of Manitoba producers in recent years, the company said. Those problems led Simplot to shine the spotlight on the pathogen this year, in an August newsletter to growers.

“Once they’ve dug it and they’ve put it in the pile with their healthy potatoes, they’re in trouble,” Graham said. “The biggest thing is to avoid harvesting and mixing rot areas of the field with healthy areas of the field.”

Spoilage is the ultimate problem when it comes to the pathogen. From the outside, producers will notice dark brownish-gray discolouration on infected potatoes, which is easy to mistake for late blight. The most telling sign, however, happens after slicing. Upon exposure to air, the inside of the potato will take on a rosy hue, which gives the disease its name.

According to the Ontario Ministry of Agriculture Food and Rural Affairs, this first colour change occurs about 20 minutes after cutting. Within about an hour, the colour is much less appetizing. Tuber flesh darkens to gray (also described as brown by some extension materials), and then black.

The same Ontario department warns that post-storage spread can occur “when liquid oozes from rotting tubers creating favourable conditions for soft rot.

“Infected tubers should be graded out to avoid poor crop emergence in the spring and storage losses,” government materials suggest. “Monitor storages closely to determine the development of hot spots.”

“Often the infected tubers will have secondary rot in storage and are often graded out and so not used as seed,” Manitoba Agriculture potato and horticulture crop pathologist Vikram Bisht said in an email to the Co-operator. “However, in storage the spores on infected tubers could contaminate other tubers and become source of inoculum.”

Simplot’s newsletter warns that some potato varieties naturally turn pink after slicing, even without pink rot infection.

Potatoes show the greyish exterior and post-slicing colour shift indicative of pink rot. photo: Vikram Bisht, Manitoba Agriculture

In the field

It’s too early to know if Manitoba potato growers will face pink rot again, but several August rains combined with warm temperatures could create ideal conditions, Bisht said in an interview.

The pathogen causing the disease tends to thrive in warm, poorly drained soils.

Within those soils, the thick-walled spores can live for more than four or five years, according to Bisht, and can become active when the soil is saturated or nearly saturated, the Simplot newsletter says.

It can also travel on tillage equipment or in irrigation water, or by natural spreaders like flooding, said Bisht.

An end-of-year disease, stunted or wilting plants late in the growing season might be the first clue of the underlying threat. Materials put out by Michigan State University cite symptoms like leaf yellowing, drying and defoliation, while stems and roots may also discolour or turn black.

Late in the season, when plants are dying back anyway, that may be hard to single out.

Tuber decay starts near the stem, the university article adds. In early stages, when infected tubers are cut, the rotten portion will be delineated by a dark line.

An ounce of prevention…

During the growing season, producers have a few options to prevent pink rot. Rotation is one method, said Bisht. While avoiding problematic areas is best, sometimes that’s not possible. Extending the rotation to every four years can dramatically reduce spore load in the soil.

Good quality seed potatoes are another way to reduce chances of rot, he added. Drainage, especially later in the season, is another.

There are also several fungicides available, such as metalaxyl or mefanoxam-based fungicides for in-furrow treatment, Bisht said. The fungus can become resistant to these products and in that case, producers can try phosphorus acid-based treatments.

If producers find potatoes with symptoms of pink rot, they can be lab-tested for fungicide resistance.

Potatoes treated with post-harvest phosphite (left side of image) show visibly fewer pink rot issues than untreated tubers to the right. photo: Vikram Bisht, Manitoba Agriculture

Another management strategy is to allow the potato skins to set in order to reduce bruising and wounding during harvest, Bisht said. This is done through top-killing the plants in low-lying or wet areas.

In Manitoba, potatoes for processing are not top-killed, Bisht noted. The skin remains soft and loose and can be easily scrubbed away, which can give ports of entry for the fungus. Potatoes for fresh use are top-killed, creating a thicker, harder skin.

Bisht agreed with Graham that producers could also avoid harvesting low-lying, problematic areas. He also suggested farmers avoid harvesting when the pulp temperature is greater than 20 C.

Once potatoes are out of the ground, other lines of defence include having pickers or graders look for infected tubers before they go into storage.

Potatoes can be treated with phosphorus acid as they go into storage, Bisht said. This won’t cure infected tubers but will reduce spread of the fungus.

Bisht said curing potatoes at greater than 90 per cent relative humidity can promote wound healing. He also suggested reducing tuber pulp temperature as quickly as possible to 10 C or lower. Additionally, he suggested high airflows to prevent water condensation in the pile.