Late blight disease (2011).

In a rather roundabout way, I recently received a bulletin from the Government of Alberta regarding Late Blight disease, and as this is something that can potentially affect gardeners all across the province this season, I thought I would post the gist of the message in the hopes that we can stem the spread of this extremely nasty problem.

Last autumn, a handful of lines in a local rag proclaimed “Rotten tomato season in Alberta.”  And you’ve probably noticed that Alberta-grown tomatoes and potatoes have been kind of scarce for awhile now.  The culprit is Late Blight disease, caused by a fungus called Phytophthora infestans (it even SOUNDS ghastly).  It’s actually the same thing that caused the Irish potato famine in the mid 1800’s…well, combined with the poor idea to plant only one potato variety, which was subsequently completely wiped out.  (Monocultures are bad, folks!).   We haven’t had a major outbreak of Late Blight disease in Alberta since 1993, and while the fungus usually sticks to attacking potatoes, for some reason in 2010, tomatoes were severely affected as well.  (Tomatoes and potatoes are part of the same family:  Solanaceae or nightshade, so it naturally follows.  Veggies and flowers such as eggplant, peppers, and petunias might also be harmed).   Late Blight can occur whenever conditions are just humid and cold enough, and the weather of last summer definitely co-operated in that regard.  Unfortunately, the problem wasn’t detected early enough, either, and spread rapidly and rampantly.

Phytophthora infestans cannot survive freezing temperatures, so any spores left in the soil will not have overwintered, BUT infected potatoes in storage may still be carrying the bacteria.  If these potatoes are used for seed (or are stored near new seed potatoes), there will be a problem.  Watch for lesions on potato tubers (and on tomato fruit over this summer.  If you still have fresh tomatoes left over from last year, I have some other questions for you).  Plants growing in the garden this summer may exhibit lesions around the tips of mature leaves that promptly turn yellow and then brown and crispy – these lesions are patchy and aren’t contained within the veins of the leaves like another disease called Early Blight.  Leaves might also show evidence of sporulation:  fuzzy white goop on the undersides, spores that easily transfer by wind or rain down into the soil and subsequently infect plant tubers.

The key is not to panic, and not to avoid planting nightshade crops altogether, but to watch for the signs and symptoms of Late Blight disease and take action if you notice it.  The Government advises burying or freezing affected plant parts (I’m a little bewildered by the “burying” tip – I guess you have to make sure you bury the infected bits well away from healthy plants in the garden so that the spores don’t spread around).  If  you feel you have to compost the pieces, you’re going to have to forgo use of the entire compost pile for awhile and cover it completely with a tarp so that it doesn’t give the spores a chance to spread.  Once the pile has frozen over the winter, it is safe to use it again.

Oh, and hope for a hot, dry summer.  That’s our best ally against Late Blight.


Check – FAQs for more information.  Remember that this disease isn’t only affecting commercial farmers and market gardeners – many cases last year were found in urban residential gardens.  Community gardens may also be at risk.  Be vigilant!

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