Late Blight - 2010

Ok. First, the take home message…. Tomatoes growers should not expect the 2009 epidemic of Late Blight to return in 2010. And… the disease organism that causes late blight has not survived in soil, pots, stakes or other non-living tissue in Pennsylvania. So, chill out on all of those elaborate plans to sanitize the garden. If Late Blight interests you, read on. If you grew potatoes last year, and have any left over, be sure to read to the end of this blog because you could cause a problem.

Last week I spent an entire day, in one room….learning about tomato production at the Mid-Atlantic Fruit and Vegetable Growers Conference in Hershey, PA. I was in charge of making sure the projector worked and the lighting was conducive to learning, otherwise I would have slipped out to catch that session on horseradish, among others, down the hall.

A lot of time in the tomato marathon was devoted to late blight, the disease that caused widespread destruction of tomatoes in the Northeast US last summer. Two things were responsible for that epidemic. One… infected transplants were sold throughout the Northeast thru the “Big Box" stores. The stores were supplied by a large greenhouse business that had late blight (a disease caused by a fungus-like organism) in their production system. I am sure that neither the stores nor the greenhouse business intended to create such an epidemic. But the end result was distribution of infected plants over the Northeast... a devilishly effective Step One: spread a very contagious organism over a wide geographic area. Late Blight spreads by spores which can blow 30-40 miles in moist air. It can infect tomato and potato as well as some weeds in the tomato family (and petunia, I learned).

Thanks to an eagle-eyed plant pathologist from Cornell, the problem was diagnosed every early. He blew a whistle and everyone paid attention, otherwise, it could have been worse. He said it may have been the most constructive thing he has done in his long career.

Step Two was Mother Nature. She picked 2009 to provide excess moisture and cooler than normal temperatures from June until September. Perfect for the Late Blight organism. Hey, whose side are you on Ma, the tomatoes or some pathogen? I guess we know now. Seems she loves all of Her creations, including Late Blight. Hmmm.

The good news is that the organism that causes Late Blight has no history of overwintering in the Northeast U.S. Once infected plants die, so does the disease. Plant pathologists are concerned about a situation in which late blight does develop the capacity to overwinter here, but so far that has not occurred (as far as they know).

So again, the really good news is that both gardeners and farmers have no reason to expect late blight to be any more of a threat in 2010 than it was in any other year. And 2009 can be considered an unusual year for the disease. Since the organism requiures a living host, there is no need to plan elaborate crop rotations or go to extremes in trying to kill what is already dead. Dead tomatoes equal dead late blight. Unless the spuds get us….

Now about those potatoes…. Recall that late blight can survive in living tissue and since the same organism infects both tomatoes and potatoes … do you know where your spuds are? The concern is that infected potatoes are laying cull piles or worse, stockpiled for planting. Don’t do this! Buy certified, clean, potato planting stock!. It ain’t worth starting the epidemic of 2010 to save a few bucks on seed potatoes. And that means your precious heirloom varieties, too. If you grew potatoes in 2009 and experienced late blight, be alert for volunteer spuds next spring and destroy them.

No problem with saved tomato seed.

I located an excellent summary of late blight and its management at UMass extension. It provides and excellent overview and advice for gardeners, including those using the organic approach.

For a utube segment by PSU's own plant pathologist, Beth Gugino, see this.