Canadian growers are facing six major potato storage diseases this season, says Khalil Al-Mughrabi, a pathologist at the New Brunswick Department of Agriculture, Aquaculture and Fisheries’ Potato Development Centre; they are late blight, pink rot, pythium leak, fusarium dry rot, soft rot and silver scurf.

“These diseases can potentially be problems in storage. However, if growers manage their crops well, not only in the field but also in storage, they can reduce their chances of developing them,” he says.

Fungal spores causing Pythium leak enter tubers through wounds.
Fungal spores causing Pythium leak enter tubers through wounds.

In general, if the temperature is high during harvest, tuber pulp temperature and the potential for heat-related issues, such as pythium leak, might be a concern, says Al-Mughrabi. Dry soil at harvest can potentially increase the risk of lumps and subsequent bruising of potatoes, which could increase their predisposition to fusarium dry rot. However, the storage disease of most concern this year is late blight. “The reappearance of late blight this year in some regions across Canada raises a concern about potential storage losses from this disease,” he says.

Late blight is a serious potato disease resulting in severe damage and storage losses. Although late blight most commonly occurs in cool, wet climates, it can occur in any region where irrigation or wet conditions combine with cool temperatures to favour disease development. “The late blight fungus does not require stressed plants in order to thrive and cause disease,” says Al-Mughrabi. “Under weather and crop conditions favourable to late blight, a field of potatoes can be defoliated in two to three weeks. In storage, infected tubers will rot and infect other healthy tubers, causing substantial economic loss.”

The predominant strain of late blight in Eastern Canada is the US-8 (A2) genotype, and in Western Canada, the predominant strain is the US-23 (A1) genotype, explains Al-Mughrabi. At this time, scientists do not have enough data to predict how each genotype will behave in storage.

The US-8 strain is familiar to us, he says, noting that it is less aggressive than other strains. Meanwhile, the US-23 strain is new and scientists don’t yet know how it will behave in storage.

The storage disease of most concern this year is late blight, shown here.
The storage disease of most concern this year is late blight, shown above.

Next to late blight, bacterial soft rot is the most serious cause of potato losses in storage, according to Al-Mughrabi. Soft rot bacteria infect potato tubers that have been damaged by mechanical injury or have been infected by other diseases. Bacterial soft rot develops much faster when potatoes are wet. “Tubers may be wet when put into storage, or become wet by excessive application of a storage fungicide, or excessive respiration and water loss in storage,” he says, adding, “Wet conditions in storage allow the soft rot to spread from one tuber to another. Soft rot can be kept to a minimum if potatoes are kept dry.”

Pink rot, a fungal disease that occurs sporadically in soils that grow potatoes, also thrives in wet conditions. It develops in soils approaching saturation from poor drainage, excessive precipitation or irrigation. “Infected tubers are usually found in wet, low-lying areas during harvest, and symptom development occurs soon after tubers are placed in storage facilities,” says Al-Mughrabi. “In storage, infected tubers deteriorate rapidly and cause economic losses.”

Another fungal potato storage disease, pythium leak, lives in the soil and can enter tubers only through wounds. Therefore, infection usually occurs at harvesting, grading or, less frequently, at planting. Serious crop loss can take place if bruised, immature tubers are harvested during hot, dry weather. “The rot that develops is greatly aggravated by relatively high temperatures and poor ventilation, but may be completely arrested under cool conditions,” says Al-Mughrabi.

Fusarium dry rot of potatoes.
Fusarium dry rot of potatoes.

Potatoes are also plagued by Fusarium species, which cause dry rot. Fungal spores present in the soil on the surface of harvested tubers are the source of dry rot infections, he says. The pathogen enters through wounds and bruises acquired during planting, harvesting, grading or transport operations. According to Al-Mughrabi, it cannot infect the tuber if the potato skin is not damaged. Wounds and bruises that are well healed are not infected by fusarium dry rot.

Growers also need to be on the lookout for silver scurf, which only affects tuber skin after harvest when the potatoes are in storage, says Al-Mughrabi, noting further infections of healthy tubers may take place at that time. “Disease levels increase in storage and further tuber infections will occur at high humidity and temperatures,” he says.

Frequent and excessive rain during the growing season is conducive to the development of some diseases, such as late blight, pink rot and pythium leak, especially in low spots and in poorly drained areas, says Al-Mughrabi. To avoid the spread of these diseases, growers are encouraged to take the following steps prior to and during harvest:

  • scout fields and identify low spots with flags prior to harvest;
  • top-kill at least two weeks prior to harvest to allow time for infected tubers to rot and promote tuber maturity and thicker skin at harvest;
  • stop irrigation well in advance of harvest;
  • avoid harvesting infested fields when soils are especially wet or soil temperatures are below 10°C (50°F) or above 18.3°C (65ºF);
  • windrow potatoes to allow the surface of tubers to dry before harvest; and
  • during harvest, adjust equipment properly to keep harvester chains fully loaded and minimize drops to six inches or less to help avoid skinning or bruising tubers, which provides direct entry points for diseases.

“Growers know that if they work hard during the season it will pay off during storage,” says Al-Mughrabi. After harvest, growers are advised to grade out as many infected tubers as possible before placing tubers in storage, he says, and also to submit samples to local plant disease diagnostic laboratories or local extension offices.

Growers should also apply a post-harvest fungicide registered in Canada for protection against the spread of pink rot, late blight and silver scurf to healthy tubers, says Al-Mughrabi, noting the crop should be ventilated with high volumes of air at low humidity until it is dry in storage. He recommends the post-harvest application of mono- and di-potassium salts of phosphorous acid (Confine) to potatoes to prevent disease development and spread in storage. “This is an excellent product that protects potatoes against pink rot, late blight and silver scurf,” he says.

Phosphorous acid protects healthy tubers as long as the fungal spores are on the surface of the tubers, he says, noting once the tuber is infected, the product does not cure it. No other products are registered in Canada for post-harvest use against pink rot and late blight. Recently, a study revealed the post-harvest application of phosphorous acid to tubers suppressed the development and spread of silver scurf. However, Al-Mughrabi says phosphorous acid is not recommended for use on seed potatoes.

If a large portion of the crop has infected tubers, Al-Mughrabi recommends storing infected lots separately from healthy ones, and placing them near the front of the storage facility. “It’s not recommended to store any lots that have five per cent rot or greater,” he says. “Lots with significant amounts of disease should be marketed as soon as possible as they will not store well.”

“Potato growers are encouraged to use all of the management tools available to them in order to protect their potato crop from storage diseases,” says Al-Mughrabi. Overall, he is seeing less storage loss and believes this is a result of farmers using more effective strategies and tools to mitigate their losses.

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