Potato Storage Rots

• 15 Steps to a Healthier Potato Crop
• Best Management Tips from the Experts
• Best Management Practices
• Storage Decay Causes Rejection
• Storage Rot Diseases on the Increase
• Common Storage Rot Diseases

Growers need to take measures in summer to manage potato storage rot diseases.

"Pink rot can be a problem for potato producers attempting to store potatoes on Prince Edward Island, and elsewhere in the region. In each of the last three years between four and six hundred samples of potatoes were submitted to the Provincial Plant Diagnostic Lab in Kensington. The percentage of samples diagnosed as having Pink Rot as the main problem ranged from five percent in 1995 to 14 percent in 1996, with an average of eight percent over the past three years." Peter Boswall, Potato Specialist, Prince Edward Island Agriculture and Forestry

"The incidence of Pink Rot has been increasing since about 1990 in New Brunswick. It is more serious in land where rotation and drainage are poor. And it is more serious where susceptible varieties such as Shepody and Russet Norkotah are grown in short rotation." Sandy Perley, Head, Plant Health Potato Horticulture for New Brunswick Department of Agriculture & Rural Development

15 Steps to a Healthier Potato Crop

Establishing a Healthy Crop Canopy:

1. Thoroughly clean and disinfect storage facilities and handling equipment prior to receiving seed potatoes.
2. Clean and disinfect set-cutters and planters between seed lots.
3. Practice crop rotation.
4. Apply a seed piece dressing to minimize disease carry-over.
5. Plant good quality seed potatoes that appear to be disease free.
6. Hill the potatoes to control weeds and to reduce the risk of Late Blight spores washing onto tubers from infected vines.

Use of Fungicides:

7. Spray the vines in early summer, before row closure, when plants are small (about 6 inches tall), with a broad spectrum fungicide.
8. Maintain the general health of the plants by fertilizing, weeding, cultivating, hilling and watering plants properly.
9. Monitor field health. Consult provincial Late Blight forecasting services.

Harvesting:

10. Scout the field for disease symptoms before vine kill. Allow top killed vines to dry completely before harvesting.
11. Allow tubers time to develop a good skin before harvest.
12. Do not harvest potatoes during wet, rainy conditions or, if soil temperatures exceed 18-20 degrees C (65-70 degrees F), particularly if soil moisture levels are high.
13. Adjust the speed of the harvester and equipment to minimize potato injury.

Storage:

14. Cull out diseased potatoes as they are being loaded into storage facilities.
15. Maintain a consistent temperature and proper ventilation of potato storage bins.

Best Management Tips from the Experts

"The economic impact that storage rot diseases have on a potato grower's operation is not well known. Growers with a history of Pink Rot and Pythium Leak can reduce the incidence of these diseases by applying a systemic fungicide in early summer. Fungicides such as Ridomil^® Prepacks, applied early in the season, when the plants are small and flowering, can significantly reduce losses from these diseases" Dr. Gary Platford, Plant Pathologist, Manitoba Agriculture, Food and Rural Initiatives

"Good vigilance doesn't end at potato harvest. Growers then need to keep a close eye on storage bins. At the first sign of disease, increase ventilation rates until the diseased potatoes dry up" Dr. Jim Holley, Post-harvest Scientist, Alberta Agriculture, Food and Rural Development

Best Management Practices

Sanitation & Storage:

• Clean and disinfect storage areas and handling equipment before receiving seed potatoes, using a pressurized sprayer, and then a disinfectant.
• Clean and disinfect potato storages and handling equipment between crops
• Clean and disinfect set-cutters and other handling equipment when changing seedlots.
• Eliminate cull piles, sources of infected and volunteer potatoes and weeds in the nightshade family.
• Lower storage temperatures gradually, maintaining steady levels of humidity and temperature until all of the field heat is removed. Continue to maintain steady levels of temperature and humidity and adequate ventilation throughout storage period. If some decay occurs, completely open outside air intakes and increase ventilation rate until the diseased potatoes dry up.

Seed Selection:

• Chose high quality seed, i.e. Elite, Foundation or Certified grade, with low levels of diseases.
• Plant varieties that are moderately resistant to Late Blight and are relatively easy to manage in storage.

Vigilance/Disease Awareness:

• Become familiar with disease symptoms on the foliage and weather conditions that encourage their development. Avoid harvesting during wet, rainy conditions whenever possible.
• Minimize mechanical damage during harvesting by adjusting the speed of the harvester and the height of all chains and belts used to lift and load potatoes into storage areas. Careless handling is a major contributor to soft rot disease.
• Monitor storages on a regular basis. Failure to control "hot spots" after harvest allows extensive secondary decay to develop in many storage facilities.

Fungicides:

• To be effective, a combination contact/systemic fungicide needs to be applied in two application, beginning in early summer when plants are small (about 6 inches tall), before row closure. Due to varying climate conditions, plants will likely be flowering at this stage in the West. Spraying may need to take place before flowering in Ontario and the Maritimes for maximum effectiveness. Apply a contact fungicide 7 days after the first application of the contact/systemic fungicide. The second application of the contact/systemic fungicide should be applied 10-14 days after the first application of the contact/systemic fungicide. Contact fungicides should then be applied as required to the end of the season.
• Spraying foliage improves plant health and thereby reduces vulnerability to storage rots. Fungicides are most effective when combined with good early management practices.*
• After harvest, if disease is suspected, due to unforeseen weather conditions or other uncontrollable factors, a fungicide such as Mertect can be applied, at the recommended label rate, directly onto the potato skin at the time of harvest. This practice has been shown to be highly effective in combating disease, with the exception being in certain regions where fungicide resistance has developed.
  
  * Overuse of metalaxyl can result in resistance development.

Storage Decay Causes a Significant Amount of Potatoes to be Rejected for Processing

"Extensive storage decays continue to be a significant problem here in Alberta. Probably as many as one in ten potatoes are culled due to disease"  Dave Best, Field Superintendent, Maple Leaf Potatoes

"Based on my own observations, at least five percent of all lots are likely affected. In some cases, as much as 100 percent of the lot has been rejected due to storage rot diseases"  Benny Nabuurs, Manager, Crop Protection, McCain Produce Inc.

Storage Rot Diseases are on the Increase in the East and in the West

"Pink Rot and Pythium Leak continue to cause unacceptable losses in storage and Late Blight is a devastating disease that threatens potato production each year. Control of these diseases and prevention of the pathogens from developing fungicide resistance can be achieved by using broad spectrum fungicides prior to row closure and a systemic fungicide following row closure."  Dr. Larry Kawchuk, Lethbridge Research Centre, Agriculture and Agri-Food Canada

"We're seeing more problems from storage rot diseases now than in the past ten years, and we're seeing a fair amount of Pink Rot and Pythium Leak. Although growers are doing a better job of controlling Late Blight, other storage diseases have become more of a problem." Benny Nabuurs, Manager, Crop Protection, McCain Produce Inc.

Common Storage Rot Diseases

Disease	Symptoms	Conditions Favourable	Prevention
Soft Rot & Dry Rot (Erwinia carotovora pv carotovora & Fusarium spp)	Soft Rot: An unpleasant odour in storage: degeneration of tubers into a rancid pool of slime. Dry Rot: Potato flesh turns dry & grainy, large hollow cavities develop inside tuber.	Bacteria and fungal spores are widespread. However these diseases do not become serious if tubers are healthy, mechanical damage is minimized, and good sanitation is used.	A total disease management program to maintain good tuber resistance. Good storage practices prevent spread of disease.
Pink Rot (Phytophthora erythroseptica)	Wilting occurs, usually late in the season. Aerial tubers form. Brown spots develop on potato skins, the flesh becomes soft and mushy and turns pink when cut. A clear line can be drawn between healthy and diseased areas of infected tubers.	A soil borne disease that develops in very wet areas subject to poor drainage, heavy precipitation or irrigation.	Plant seed tubers in soils with good drainage. Spray with a combination contact/systemic product. Avoid excessive irrigation late in growing season. Minimize mechanical damage during harvest.
Pythium Leak (Pythium ultimum)	Potato skin looks bruised, tubers become soft and the skin moist. After infection, skin ruptures and tuber exudes liquid with slight pressure. A clear line can be drawn between healthy and diseased areas of infected tubers.	Infection usually occurs at harvest as tubers come into contact with spores in the soil. Spores are released into films of soil water when soil temperatures exceed 18 degrees C (65 degrees F).	Spray with a combination contact/systemic product. Delay harvesting until tuber skins are mature. Avoid mechanical injury. Increase ventilation if rot occurs in storage area.
Late Blight (Phytophthora infestans)	Leaves and stems develop brownish spots that spread rapidly under wet conditions. Skin turns purple and looks bruised. Flesh appears grainy and turns to orange, tan or brown.	High humidity (95 percent), for 18 hours or more per day, especially in lush canopies. Failure to identify and treat early can result in significant crop loss.	Follow a total disease management plan. Apply a combination contact/systemic product. Consult provincial Late Blight forecasting services, if available, for recommendations on risk of Late Blight occurrence and fungicide spray intervals.
Silver Scurf (Helminthosporium solani)	Unsightly silver to gold blemishes develop. Skin may thicken, creating a rind.	The fungus which causes the disease is distributed on tubers and survives in the soil.	Regular crop rotation. Good sanitation and culling practices. Proper storage.

This factsheet was made possible with contributions from Dr. Jim Holley, Post-harvest Scientist with Alberta Agriculture, Food and Rural Development, Crops Diversification Centre South; Dr. Gary Platford, Plant Pathologist, Manitoba Agriculture, Food and Rural Initiatives, Soils & Crops Branch; Sandy Perley, Head, Plant Health, Potato Horticulture for New Brunswick Department of Agriculture & Rural Development and with financial support from Novartis Crop Protection Canada Inc.

^®Ridomil is a registered trademark of Novartis Crop Protection Canada Inc.
^®Bravo is a registered trademark of a Zeneca Group Company

For further information, contact your GO representative.