The Contrary Garden: Late Blight on Tomatoes

Matt’s going to have a heart attack when I tell him this, but it turns out that the black lesions that have been working their way up our tomato plants are the dreaded late blight — the blight of potato famine fame.

Late blight or, to be formal, Phytophthora infestans, is a nasty bugger that affects all the plants in the Solanaceae family, of which both potatoes and tomatoes are members.  Late blight is an an obligate parasite, which means that it has to have a living plant host to survive.  That means that I can cut way back on my chances of having this recur next year by destroying all the affected plants this year by burning or freezing.  (I vote for fire).  What is less certain is whether or not the disease has spread to our potatoes.  I planted them very very early and hilled them twice, both of which should have lessened the chance that spores were able to move through the air into the tubers.  We won’t really know until we dig them — and even then they may not exhibit symptoms until they are in storage.  (You’ve no doubt heard the horror stories of Irish families putting up their winter stores only to find them reduced to black slimy mush weeks later.)

I’m not sure how I got the blight.  It may have wafted over to the garden from one of the volunteer potatoes growing in the compost bin, it may have been present in one of the seed potatoes I planted in the garden proper, it may have blown in from a neighboring garden, or it may have been slumbering in the tomato plants I purchased.  At any rate, I think that it is not far enough along that I have to give up.  Organic controls containing copper can slow or halt the progress of the blight and we happen to have some on hand.  Unaffected fruits and tubers are still safe to eat.  I will just have to take care to really soak the affected plants with the copper product and to thoroughly burn all remaining potato and tomato plant material at the end of the season.   Still, the copper is not a silver bullet, as it can build up in the soil and kill naturally occurring microorganisms and the Bordeaux mixture we have in our arsenal is toxic to bees.  For the sake of my beloved honeybees I will spray when they are not very active, probably just before sunset.

— Amanda

Sources: By Alex Stone of Oregon State University:  “Organic Management of Late Blight of Potato and Tomato (Phytophthora infestans)” and Organic Management of Late Blight of Potato and Tomato with Copper Products” (with Brian Baker of the Organic Materials Review Institute), both accessed via the Washington State University Extension website.

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