AgronomySummer Checklist for Late Blight

Summer Checklist for Late Blight


When it comes to late blight, an ounce of prevention is worth a pound of cure. The following is a comprehensive list for the control and management of late blight during the summer.


• Field scouting 

Start your 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 centre 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. Wind-borne spores may blow in and infect here. 
  • Any area that is protected from the wind where leaves tend to remain wet longer. 

• 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 neighbors 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 cm tall. 

Alternate fungicides from different chemical groups to delay the development of resistance.  

Spray cymoxanil (Curzate) at 80% 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 -4 C to -6 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 dikc 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-season, 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 neighboring 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 6-foot border of surrounding plants. Surrounding plants may have no symptoms but may be infected. It takes at least 4 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. 

Dr Larry Kawchuck with Agriculture & Agri-Food Canada in Lethbridge can conduct molecular tests to identify late blight strains. (no fees charged). His address is: 

Dr. Larry Kawchuck 

5403 – 1 Avenue South Lethbridge, Alberta 

T1J 4B1 


• 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 2 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


Trending This Week

Covering the Bases with Cover Crops

Welcome to the inaugural edition of “Another Season’s Promise”, a regular column in Spud Smart on potato agronomy. The name comes a lyric from...

Finally: A New (Desperately Needed) Solution for Potato Wart?

If you’re a potato producer who has never laid awake in the middle of the night worrying about potato wart, count yourself lucky. Potato...
Potatoes in storage

There’s Still Lots of Storage Season Left to Go…

Bad news first. It’s been a fairly challenging storage season so far in some regions with many producers reporting real difficulties in getting their...

Loving the Challenges of Growing Potatoes

February is my favourite month of the year because it’s Potato Lovers Month, which has made me think about what exactly I love about...

Canadian Farm Sector Income Expected to Reach New Records

Agriculture and Agri-Food Canada has completed its farm income forecast for 2023 and 2024, with results showing that overall Canadian farm income is expected...