Agriculture, Pêche et Aquaculture
Department of Agriculture, Aquaculture and Fisheries
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  Late Blight of Potato and Tomato in the Home Garden

Late Blight of potatoes is a fungal disease that originated in Mexico and spread to Europe and North America about 1845. Late Blight caused disease and famine in Ireland where people were dependent on the potato crop. Late Blight of potatoes was first reported in the Maritimes between 1845 and 1847.

Recently, new strains of the disease have appeared in the Maritimes. These new strains are more aggressive than the old strains, infect plants earlier in the season, cause more severe disease and persist during periods of dry weather.

Late Blight is caused by the fungus Phytophthora infestans. Three conditions must be met for an infection to occur: 1) there must be a source of infection; 2) there must be a susceptible host plant; and 3) there must be the right environmental conditions

Sources of infection

Late Blight survives from one season to the next in diseased tubers. Potatoes in storage, tubers surviving the winter unfrozen in the garden or infected tubers in a compost are all potential sources of new infections. Infected tomato fruit discarded in composts in late winter or early spring may be a source of infection if they do not freeze. Spores are carried from these sources by wind to the new crop of potatoes or tomatoes. Spores can be transported 80 km or more from a source and 8-16 km is common.

Late Blight on tomatoes

Susceptible host plants

Members of the solanaceae family are susceptible to Late Blight. This family includes potatoes, tomatoes and peppers, all of which may be infected by Late Blight. Some potato varieties are less susceptible to Late Blight than others, but no variety is immune to the disease. Recent advances in genetic engineering may soon provide us with improved varieties which possess a high level of resistance to Late Blight.

Environmental conditions

Late Blight development and spread is favored by cool, humid conditions or prolonged periods of rain. Night temperatures between 10-15oC, day temperatures between 15-20oC, relative humidity greater than 90% and a cumulative rainfall of 30 mm or more during the last 10 days are conditions favorable for the development and spread of Late Blight.

Stem tip lesion on potato


The spores land on leaves, stems or fruit and in the presence of free moisture infect the plants. Infections develop into visible lesions in 4-5 days. Infected leaves develop dark green, water soaked spots that enlarge and turn brown with a light green margin. A white, fluffy growth, which produces late blight spores, can be seen around the outside of the lesions on the underside of the leaves. Late Blight spores develop in 7-10 hours at 90% relative humidity. Dark lesions and fluffy white growth also appear on the stems, often starting where the leaves attach to the stem.

Lesion on a potato leaf


Tubers are infected with Late Blight when spores are washed down through the soil by heavy rains. Infected tissue is granular, reddish brown in color and may extend up to 2 cm into the tuber. Fluffy, white growth may be seen on the tubers under high humidity in storage.

Tomatoes develop similar symptoms on leaves and stems. Infected fruit first show dry pale brown discolorations on the surface which later enlarge into brown, wrinkled lesions. These are most often found on the upper surface of the fruit.



Do not use potatoes from your own garden or from grocery stores for seed. Buy seed potatoes which have been inspected and certified by Agriculture and Agri-Food Canada.

Late Blight symptoms on potato tuber

Seed potatoes should be inspected before planting and any seed showing signs of rot should be removed.

First infections of Late Blight often appear in early to mid-August. Avoid the disease by ensuring you have an early crop:

* Plant as early as possible.

* Very early varieties such as Caribe, AC Chaleur, AC Belmont, Jemseg and Eramosa provide good yields in 60-70 days.

* Green sprout your seed to be able to start harvesting 2 to 3 weeks earlier than normal.

Do not leave old potatoes or tomato fruit exposed in a compost pile. Pull and bury any plants which start to grow in the compost pile.

Remove and destroy volunteer potato plants which grow in the garden.

Keep potato and tomato foliage as dry as possible by allowing good air circulation:

* Do not plant too closely in the row (suggested spacing 30 cm).

* Leave enough space between rows (suggested spacing 122 cm), or alternate rows of potatoes with rows of carrots or other short crops.

* Do not water potatoes or tomatoes during the evening as the foliage may stay wet all night.

* Avoid planting in shaded locations.

Do not use excess nitrogen fertilizer as it promotes top growth, reduces resistance to disease and delays maturity.

Transplant tomatoes as far away as possible from potatoes in the garden.

Build a big hill over the potato plant to provide better protection against spores washing down through the soil and infecting the tubers.

If you choose to use them, weekly applications of fungicides on potatoes and tomatoes, started early in the season, will help delay the onset and progress of the disease.

Green sprouting seed potatoes


Remove all infected leaves or stems each day to reduce spore production and disease spread. This method may be effective if the weather turns dry and slows the progress of the disease.

If you cannot keep ahead of the disease, choose a dry sunny day and remove all potato tops by standing on the hill with your feet around the stems and pulling on all stems at once. Take care not to expose the tubers. Place the foliage in garbage bags and seal. The tubers may safely be left in the ground until fall. Allow at least 2 weeks between pulling the tops and harvest.

Agriculture, Aquaculture and Fisheries
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