Potatoes in Canada

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OPB’s Eugenia Banks provides 2022 late blight summer checklist

June 30, 2022  By Eugenia Banks, Ontario Potato Board

Keep an eye on low spots for late blight development. All images courtesy of Eugenia Banks.

For the latest in potato disease protection, we turn to Eugenia Banks with the Ontario Potato Board.

Early summer

Start scouting before the crop emerges, in order to familiarize one’s self with field characteristics such as low spots and changes in soil texture.

Areas that should be monitored frequently include:

  • Low-lying areas that tend to be wet for long periods after rainfall;
  • Compacted areas;
  • Rows close to tree lines;
  • Pivot center points and pivot wheel tracks. Look closely at the plants under the first tower of center pivots. This area remains wet longer than areas farther out on the boom. Also, wheel track areas usually remain damp or wet after other parts of the field have dried up;
  • Weedy areas;
  • Windward sides of fields, where windburn spores may blow in and infect; and
  • Any area that is protected from wind where leaves may remain wet longer.

Scouts should monitor late blight-infested fields last. Scouts should wear disposable suits, gloves and boot covers to scout infested fields. The suits, gloves and boot covers should be removed, turned inside out, and stuffed into a garbage bag for disposal.

Pictured: Spraying a preventative late blight fungicide. All images courtesy of Eugenia Banks.

Make sure there are no cull piles in your farm.

Late blight is a community disease. A single cull pile can cause an epidemic with serious economic losses for many growers. Monitor sites where cull potatoes were buried and eliminate any volunteer plant.

Be aware of home gardens with alternate hosts near your fields.

Tomatoes, peppers, eggplants, some petunia spp. are hosts of late blight. Explain to your neighbours that late blight can spread easily from gardens to commercial fields causing serious economic losses. Plants infected with late blight in home gardens should be destroyed immediately after symptoms are visible.

Start your fungicide program early.

Begin a fungicide program early when plants are 12 to 15 cm tall. Alternate fungicides from different chemical groups to delay the development of resistance. Spray cymoxanil (Curzate) at 80 per cent emergence if the field is at high risk of late blight.
Use short application intervals if disease pressure is high. Be aware of the weekly spore trapping results for your area. If late blight spores are detected by spore traps, include late blight specific fungicides in your spray program.

Keep new growth protected.

New growth is a good target for late blight, especially the growing point where water persists for longer periods after a rain.

Try to achieve good spray coverage.

Calibrate your sprayer and use a volume of water that will ensure even coverage of the canopy.

Monitor last year’s potato fields for volunteers.

Volunteer potato plants are common where trash or snow keeps the soil from freezing (it takes 120 hours below -3oC at 10 cm depth for tubers to freeze). Use labeled broadleaf herbicides where possible to suppress the growth of volunteer potatoes in cereal crops.

Destroy hot spots.

If late blight is found in a localized spot, destroy all diseased plants plus a 6-foot border of surrounding plants. Pull and bag the diseased plants, spray them with an herbicide, or disc the area. If you decide to disc the hot spot, spray the crop with a fungicide before doing it to avoid spreading spores on farm equipment while driving out of the field. Pressure wash the equipment when finished.


Continue scouting your fields.

Keep scouting fields regularly, at least twice a week. By mid-summer, rows are closed, and the lower part of the plants remains wet for longer periods after rain. This allows late blight spores to germinate and start infections. Check plants at random in risky areas and examine the base of the stems for late blight. Pay particular attention when scouting after cool, rainy periods.

Check for alternate weed hosts.

Hairy nightshade is a host of late blight. This weed is becoming more common in Ontario and may be found growing at the edges of potato fields. Infected nightshades spread spores to neighboring potato fields. Destroy this weed wherever you find it.

Manage irrigation.

Avoid pivot overlaps and do not irrigate when there is dew on the crop. Try to irrigate when leaves will dry quickly, in less than eight hours. This would be early in the morning so that the foliage can dry quickly during the day.

Continue the spray program, alternating fungicides of different chemical families.

Check the spray coverage.

Optimize spray equipment. Use adequate water volume and pressure to get even coverage on the canopy. Place sticky paper disks sensitive to pH in the canopy to evaluate spray coverage in the field.

Hose down farm equipment after leaving an infested field.

Destroy hot spots.

If late blight is found in a localized spot in a field, top kill the spot plus a six-foot border of surrounding plants. Surrounding plants may have no symptoms but may be infected. It takes at least three to four days for symptoms to become visible after infection.

Send samples of infected foliage/tubers to a laboratory for strain identification.

New strains of the late blight pathogen develop fast. The US 23 strain has prevailed in North America for at least seven years. US 23, the tomato strain, is sensitive to metalaxyl-M (Ridomil), but the late blight pathogen might develop resistance to this fungicide during the current season.

Late summer

Keep scouting your fields.

Attention should be given to scouting after cool, rainy weather. This is particularly important to detect infections late in the season that can result in tuber rot.

Top kill infected fields if heavy rain is forecast.

Rain washes spores down cracks in the soil into the tuber zone. If heavy rain is forecast, consider top killing infected fields before it rains to reduce the risk of tuber infection.

Seal soil cracks with a roller.

This practice is very effective to reduce tuber infection before harvest.

Do test digs to check for tuber infection.

Map the areas where you found infected tubers; it is advisable not to harvest areas where tubers are infected.

Top kill vines at least two weeks before harvest.

The vines should be completely dead before digging to reduce the risk of tuber infection. Infected green vines may be releasing late blight sporangia that can infect tubers at harvest. Tank mix Reglone with a copper fungicide to kill spores produced on infected tissue not killed by the desiccant.

Phosphites as a post-harvest treatment of tubers, reduce the spread of late blight in storage.

Remember: an ounce of prevention is worth a pound of cure!

Note: The technical name of the spores of the late blight pathogen (P. infestans) is sporangia Sporangia germinate releasing swimming spores called Zoospores.

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