April 11, 2007 (Revised April 14)

Today I was listening to the most recent podcast of KUOW's gardening segment "Greendays". The folks on that show are all, in general, very knowledgeable - probably far more knowledgeable than I - but I heard one of them propagating a common myth about the disease Late Blight (Phytophthora infestans).

People are becoming more informed regarding this fungus, but with some folks the (incorrect) idea that it can live in our soil just won't die. I suspect there are two reasons for this. First, many common fungal diseases can live in the soil. Second, other Phytophthora species - such as certain tree root rots - are quite happy staying and playing in the dirt.

However there is no good evidence that Late Blight can overwinter in our soil; nor any evidence that it does so anywhere outside of its Mexican home range. Late Blight spores need living tissue to survive around here. It is able to overwinter in the Maritime Northwest, though. Unfortunately, we have many Nightshade (Solanum) species in our area; some of which are perennial. Late Blight can infect at least some Solanums. Additionally, it is well documented that Late Blight can survive quite handily on potato tubers - whether in the soil or in your root cellar - through the winter. I also have no doubt that an ample fresh supply of the fungus arrives every year, as our local mega-nurseries and home centers import tomato plants from mega-growers in the southeastern United States. Since the spores of this fungus can travel many miles through the air, your garden plants have lots of chances to become infected.

From a practical perspective, this distinction probably doesn't matter much to the gardener. Just remember to practice good gardening habits: If you use a sprinkler don't water in the evening (although a low-yield sprinkler going overnight is okay); and consider using drip irrigation. Protect your succeptible plants from rain, if possible, with a PVC hoophouse or some similar sort of cover. To help prevent infection of your potato tubers, don't harvest them until the vines are completely dead - or else cut the vines at ground level a week or so before harvest. Clean up diseased plant material promptly, and don't compost it yourself.

Further reading:

Late Blight FAQ
OSU fact sheet HYG-3102-95: Late Blight of Potato and Tomato
Cornell University IPM Program: What is Late Blight?

All contents © Travis Saling
This page was last updated November 18, 2013