Tomato and Potato Late Blight Disease Alert,
September 2010

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

The disease has been reported in British Columbia, Manitoba and eastern Canada earlier this year. With our cooler, wetter summer, it has begun to appear in Alberta. The disease could 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 Disease affects the plant family, Solanaceae, which includes the potato, egg plant, pepper, Datura or Brugmansia, petunia and deadly nightshade. It flourishes in humid or 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. We know it can be managed sensibly now, unlike the time of the famous potato famine in Ireland.

It is spread by spores that rapidly complete their cycle. 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. The late blight can kill a plant and its fruit rapidly.

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. Clothes, gloves, etc. should be cleaned.
• Destroy all infected plant material by burning or bagging for the landfill. Do not compost the material, as spores can survive. Avoid piling diseased plants, including fruit and tubers, as they continue to grow spores and spread. Kill or remove potato vines and then wait 2 to 3 weeks to harvest the potatoes. Do not harvest the tubers when the soil is wet. Check the tubers and if any show signs of infection, destroy them immediately. Any potatoes put into storage should be kept untouched from each other, 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 overwater your plants. Do not water the foliage, 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 and eggplants next to each other, as they are all hosts of the disease spores.
• Practice crop rotation.
• Watch regularly for early signs of infection, and particularly after a time of high humidity or cool and rainy weather.
• If signs of disease do surface, kill all vulnerable plants in a 15' radius.
• Fungicides are used by commercial growers, although that is not always effective and it is not a cure.  Some strains have also become resistant. This would not be an option for an organic grower.