Tomato and Potato Late Blight Disease

Photos supplied by the St.Albert and District Garden Club – click to enlarge photo in a new tab

Tomato and Potato Late Blight has has begun to appear in Alberta. It may have been spread by wind-blown spores or water droplets containing spores, but likely also by diseased plants brought in from infected sources.

Tomato and Potato Late Blight affects the plant family Solanaceae, which includes the potato, eggplant, pepper, Datura or Brugmansia, petunia, and deadly nightshade. It flourishes in humid and in cool and moist conditions. Once established, it can spread rapidly by spores and will destroy affected plants.

Gardeners (especially vegetable gardeners), should be extra vigilant about appropriate gardening practices in wet and humid conditions, even if there is no sign of this particular disease in their plants. However, the disease is not something to panic about. It can be managed sensibly now, unlike in the time of the famous potato famine in Ireland.

It appears as water-soaked spots that quickly manifest to brown/black lesions on leaves and stems, and will often develop a whitish mold on the underside of the lesions. Late blight can quickly kill a plant and its fruit.

Water can carry the disease into potato tubers. An infected tuber will show dark patches. These patches can eventually go inside the tuber, and the potato will rot and likely smell. This can happen quickly or more slowly in storage.

Tomato and Potato Late Blight Disease is a serious disease that can cause a 100% loss. It is important that gardeners work to control it. It can be especially concerning in community gardens where so many of the same plants grow in a small area.

The disease overwinters and can by spread by wind. The spores can survive in soil, in potato tubers left in the soil, and in tubers in storage. Rain and water runoff will also spread the contamination. The late blight problem in the world has become worse of late as more aggressive strains are developing.

Good Gardening Practices to Manage the Disease:

• Sanitation is important. Keep the area clean of diseased material and disinfect tools and equipment. Clean clothes, gloves, etc.
• Destroy all infected plant material by burning or bagging for the landfill. Do not compost it, as spores can survive. Avoid piling diseased plants, including fruit and tubers, as they continue to grow and spread spores. Kill or remove potato vines and then wait 2 to 3 weeks to harvest the potatoes. When the soil is wet, do not harvest the tubers. Check the tubers and if any show signs of infection, destroy them immediately. Any potatoes put into storage should not touch each other, be kept dry and well ventilated, and checked regularly.
• Do not use any of these potatoes for seed potatoes, even if you think they look okay. Use only registered healthy seed potatoes, preferably ones grown in low-blight areas like northern Alberta. Do not plant a seed potato that shows a blemish. Cut the seed potato to ensure the inside is healthy. If infected, clean the knife afterwards.
• Plant healthy plants only and/or select disease resistant varieties.
• Do not over-water your plants. Do not water the leaves, water under the foliage on the ground.
• Keep air circulation open between the underside of the foliage and the ground.
• Hill potato plants at a sharp angle to facilitate runoff.
• Do not over fertilize.
• Avoid planting potatoes, tomatoes, peppers or eggplants next to each other; they are all hosts of the disease spores.
• Practice crop rotation.
• Check regularly for early signs of infection, and particularly after a time of high humidity or cool and rainy weather.
• If signs of disease do appear, kill all vulnerable plants in a 15′ radius.
• Commercial growers use fungicides, although that is not always effective and it is not a cure. Some strains have also become resistant. Fungicides are not an option for an organic grower.