Potatoes in Canada

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Summer checklist for late blight

June 27, 2019  By Eugenia Banks

Ontario’s potato specialist Eugenia Banks shares early summer, mid-summer, and late summer practices to protect growers against late blight in their potato fields.

Early summer

Start your field scouting program before the crop emerges. This will allow scouts to become familiar with field characteristics such as low spots and changes in soil texture.

Risky areas that should always be monitored are:

  • Low lying areas that tend to be wet for long periods after rainfall.
  • Compacted areas.
  • Rows close to tree lines.
  • Field edges along creeks or ponds.
  • 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. Windborne spores may blow in and infect here.
  • Any area that is protected from the wind where leaves tend to remain wet longer.

The early summer checklist continues:

  • 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 potatoes.

  • 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.

  • Start your fungicide program early.

Begin a fungicide program early, when plants are 12-15 centimetres 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.

  • Be aware of the bi-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 may be common where soil temperatures do not reach -40 C to -60 C four-to-six inches deep during the winter. Use labeled broadleaf herbicides where possible to suppress growth of volunteer potatoes in rotational 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 disk the area. If you decide to disk 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 midseason, rows are closed, and the lower part of the plants remains wet for longer periods after rainfall. 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 neighbouring potato fields. Destroy this weed wherever you find it.

  • Manage irrigation.

If disease pressure is high, try to irrigate early in the morning so that foliage can dry up quickly during the day.

  • Continue the spray program alternating fungicides of different chemical families.
  • Check the spray coverage.

Optimize spray equipment. Use adequate water and pressure to cover plants. By the end of July, use more water than earlier in the season and make sure all nozzles are operating properly. Place sticky paper disks sensitive to pH in the canopy to evaluate spray coverage in the field.

  • Destroy hot spots.

If late blight is found in a localized spot in a field, disk up or kill vines immediately. 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 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 5 years. US 23 is sensitive to Ridomil, but the late blight pathogen might develop resistance to this fungicide during the current season. Larry Kawchuck with Agriculture and Agri-Food Canada in Lethbridge, Alta., can conduct molecular tests to identify late blight strains (no fees charged).

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.

  • 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.

  • Do dig tests to check for tuber infection before harvest.
  • Seal soil cracks with a roller.

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

  • Top kill vines at least two weeks before harvest.

The vines should be completely dead before digging to reduce the risk of tuber infection. Green vines may be releasing spores of late blight that can infect tubers at harvest.

  • Tank mix Reglone with a copper fungicide to kill any remaining late blight infected tissue not killed by the desiccant.

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