First occurrence of potato late blight in Ontario in 2015

Eugenia Banks, Potato Specialist, OMAFRA

On July 9, late blight was found in a potato field near Orangeville. This is the first occurrence of potato late blight in Ontario in 2015. Infected plants were removed, and the grower is following an aggressive fungicide spray program, preventative sanitation practices and close field monitoring. All these management practices help to slow the rate of disease spread and to reduce the incidence of late blight. Samples were sent to Dr. Lawrence Kawchuck from Agriculture and Agri-Food Canada in Lethbridge. Dr. Kawchuck will identify the strain that caused this late blight outbreak.

Late blight is one of the most devastating diseases of potatoes. If weather conditions are favorable and no effective fungicides are applied, late blight can destroy a potato field in 4 or 5 days.

Fungicides must be used properly to control late blight. Base spray applications on weather conditions, crop growth and disease pressure. Frequent spraying, every 3 to 4 days, is necessary when late blight has been found and the weather is cool and damp. Intervals can be extended when the weather is warm and dry. Fungicides with different modes of action must be rotated to avoid the development of resistance. Good coverage is essential for fungicides to be effective. Spray coverage should be even, with no skips or areas left untreated. It is critical to keep new growth protected because new growth is a good target for late blight, especially the growing point where water persists for longer periods after a rain.

Late blight is a community disease. Once the disease starts, wind can carry late-blight spores long distances affecting many farms and many fields. A coordinated approach to disease control is best. Make sure there are no volunteers or cull piles on your farm. A single cull pile can cause an epidemic with serious economic losses for many growers.

Risky areas that should always be scouted carefully 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 for longer periods than farther out on the boom.
• 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.

Late blight symptoms on potatoes

• Symptoms on leaves. Look for small light or dark green spots that look water-soaked. Lesions expand rapidly during cool, wet weather to form large dark brown or black spots. Usually the spots are surrounded by a light green halo. A white fungal growth develops on the underside of affected leaves.
• Symptoms on stems. Dark brown irregular lesions form on stems. These lesions usually initiate at the point of leaf attachment. The white fungal growth may develop on stems under favorable weather conditions.
• Symptoms on tubers. Tuber infection is characterized by irregularly shaped, brown to purplish areas on the skin. These infected areas appear slightly depressed. A dry, granular reddish- brown rot develops under the skin.

Fungicides registered for potato late blight

Consult OMAFRA Publication 838, “Vegetable Crop Protection Guide” for the list of fungicides registered to control late blight in potatoes.