Late Blight of Potato and Tomato

Late Blight of Potato and Tomato

This information sheet is intended primarily for home gardeners. It shows you what to look for and gives suggestions on ways to prevent or deal with the disease.

Late blight, caused by the fungus Phytophthora infestans, is the most destructive disease of potatoes throughout the world.

The fungus originated in the Peruvian Andes, where potatoes came from, or in Mexico and spread from there to the rest of the world in the 1830's & 40's. It caused the "Irish Potato Famine" during which at least one million people died and another 1 1/2 million emigrated from Ireland in 1846 & 47. In the words of one Irish potato farmer, "On August 6, 1846...I shall not readily forget the day...I rode up as usual to my mountain property, and my feelings may be imagined when, before I saw the crop, I smelt the fearful stench, now so well known and recognized as the death sign of each field of potatoes...I saw the crop fast disappearing and melting away under this fatal disease..."

At the time, the role of fungi in causing plant disease was not understood. Research over the next 20 years confirmed that the fungus was indeed the primary cause of the devastating disease. The modern discipline of plant pathology was born during that period.

Recently, late blight on both potato and tomato has become more difficult to control worldwide. This is believed to be due to the migration of new strains of the fungus from Central Mexico during the 1970's. Included in the new strains was the A2 type, previously known only in Mexico, which can mate with the A1 type to produce a whole range of new races. Mating of the two types also produces thick-walled spores. These resistant spores can live for many years in dead vines or even in the soil.

Infected potato tuber showing external symptoms Infected potato tuber showing internal syptoms

On Potato

The first indication of the disease on potato foliage usually appears after several days of wet weather. Blackened shoots, dark lesions on stems and water-soaked or black areas on the leaves indicate late blight. For confirmation, look for glistening white spores on the undersides of the infected leaves. If wet weather continues, all of the foliage may be destroyed in a matter of a week or so. Tubers may also rot slowly from the outside if heavy rains wash spores from leaves through the soil or if the tubers contact diseased vines during harvest.

Potato leaf infection Potato stem and shoot infection

On Tomato

Leaf and stem infections on tomato are very similar. If wet weather continues after the disease appears, the fruit also becomes infected. The fruit rot is slow developing but eventually destroys the fruit before it ripens. Green fruits harvested in the fall turn black and leathery before they ripen if they were infected in the field.

Tomato stem infections

Control Without Fungicides

In damp climates, it may be difficult to grow potatoes or tomatoes without any fungicides. However the following steps will improve your chances of success and are often all that is required in drier areas.
Remove all dead and dying potato and tomato vines from the garden at the end of the season. Do not place them in the compost. Remove them from your property or bury them in a deep hole.
Buy new, certified seed potatoes each year and check them carefully for any signs of tuber rot. Do not plant any with rots or dry, sunken lesions.
Avoid highly susceptible potato varieties such as Bintje, Norchip, Norland, Norgold, Norkotah, Red Pontiac, Russet Burbank, Sangre, Shepody and Warba. If available, plant more resistant varieties such as: Brador, Chieftain, Desiree, Dorita, Epicure, Kennebec, Nooksack, and Sebago.
Plant potatoes in a different part of the garden each year, not close to tomatoes or in shaded areas.
Plant tomatoes against a south wall where they can be protected from the rain by the overhanging eaves or by a temporary lean-to.
For tomatoes grown in the open, be prepared to erect a temporary shelter of polyethylene or similar material above them to keep the rain off. Do not enclose the plants completely or the high humidity will encourage the disease.
Do not overfertilize or overwater. Dense lush foliage stays wet longer and is more likely to become infected.
Water the ground only. Don't use sprinklers on either potatoes or tomatoes.
Monitor your garden on a regular basis and quickly remove any suspicious leaves or shoots. Put them in a plastic bag and seal.
For potatoes to be stored, remove tops two weeks before digging the tubers. Harvest in dry weather.
Harvest all of the tubers so there are no "volunteers" the next year.

Control With Fungicides

Careful use of fungicides will allow the growing of more popular potato varieties and help to ensure success in the battle against this disease. Keep in mind:
The product that you use must contain one of the following ingredients: chlorothalonil, copper oxychloride, tribasic copper sulphate, copper hydroxide, maneb, mancozeb or zineb. The copper materials are the most "natural" and are generally available in garden shops.
For best results, the first spray must be applied before any infections appear. For potatoes, start spraying when the plants are about six inches high. For tomatoes, start as soon as they are planted out.
The fungicides must be re-applied every 5-10 days. Read the label for rates of application and days to harvest limitations.
Spraying can be suspended during dry weather in mid-summer but start again before the onset of fall rains.
These fungicides are not systemic so residues can be washed off tomato fruits with warm water and a little detergent. Rinse in cold water and dry. For canning tomatoes, peeling the skin off will remove any residues.

Late blight life cycle diagram

This is a joint publication of the Crop Protection Program, B.C. Ministry of Agriculture and Food and the Food Production and Inspection Branch of Agriculture and Agri-Food Canada.
Supported in part by the Canada-British Columbia Green Plan for Agriculture, July 1994.

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