Friday, September 3, 2010

Bad Luck O' the Irish

We got this alert today from the Edmonton Horticultural Society:

If your Tomato or Potato plants show signs of dying, or have turned yellow and brown prematurely this year – READ THIS:

There is ‘late blight disease’ affecting the tomatoes and potatoes at the St. Albert Botanic Park. It is very important to reduce the effects of this disease by harvesting crops and cleaning up the affected plant material immediately. It is important that we work together to eliminate all potential plant materials that can be affected by this disease.

NOTE: This problem will persist in the soil if you bury affected plants, and may also persist in your backyard compost pile. We have been lucky here in Alberta that this hasn’t been a problem in the past – but it could become one if proper precautions are not taken.

Please tell all fellow gardeners.

This is the disease responsible for the Irish Potato Famine. It is a serious and devastating disease that can cause a 100% loss and it is very difficult to control; consequently, it is a community concern.

The disease flourishes in cool and moist conditions and spreads rapidly. It appears as water-soaked spots that 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 kills the plant rapidly, and water can carry the disease into the soil and tubers. If the tubers are infected they will show dark patches and the potato will rot. This can happen quickly or it can happen slowly in storage.

Diligent things can be done to manage the disease:

Sanitation is paramount. Bag affected material immediately and put out for waste collection.

Check the tubers and if any show signs of infection destroy them. Potatoes put into storage should be kept separate from other potatoes, kept dry and well ventilated, and checked regularly as the infection can show at a later date.

Do not harvest the tubers when the soil is wet.

Do not use any of these potatoes for seed potatoes - even if you think they look okay.

If signs of disease do surface, kill all vulnerable plants in a 3 metre radius.

more news here...


Use only registered healthy seed potatoes, preferably ones grown in low blight areas like northern Alberta.

Plant blight resistant varieties of potatoes, although no variety is absolutely immune to the disease.

Do not plant any seed potato that shows a blemish, and cut the seed potato to ensure the inside is healthy. If the potato has a problem, then destroy it and disinfect the knife before cutting another potato.

Plant healthy tomato plants only.

Do not over-water 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.

Regularly watch for early signs of infection, and particularly after a time of high humidity or cool and rainy weather.

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.


  1. Maria sent me an email saying:

    So if we disappointed gardeners are all bagging our diseased plants, how is the city composter going to handle it? Or does the composter get to temps that can kill all those pathogens? (I’m not sure I believe that, as I got some compost this year that had more quack grass in it than there should have been...) Should we be doing like in Saskatoon and spreading diseased plants out to be frozen, then bag them in the spring?


    Please DO NOT spread diseased material out for the winter. Some thoughtless people say to do this, and yes, the winter will probably kill the protist, but in the meantime it will only spread the disease to your neighbours (and I don't have it yet!)

    Our composting piles get hot enough for long enough to kill most disease, and since the piles will be turned a few times over winter, exposing the vines to freezing temperatures, that should help things along.

    You didn't mention whether your quack grass emerged from roots of seeds - do you know? MOST of the windrow reached killing temperatures that should have killed living plant material. If you got some seeds then there is a chance they blew into the field during curing. I will ask Allan about that...

  2. ...hang on a minute...

    When I said "Our composting piles will..." I referred to the windrows at EWMC.

    If I led you to believe that our backyard bins will get hot enough for long enough to do the job, I apologise. Temperatures that high are difficult to achieve and maintain at this time of year.

    If you have put this material into your backyard bin, the problem may persist. To kill this once and for all, spread all the material from your bin onto planting beds in November or December, let the organism die over the winter months, and dig the material into the soil in spring.