Help Banish Late Blight!

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Q. Mike!: My tomato plants were hit by the Late Blight Plague of 2009; and I seem to remember you saying that the spores can't survive over the winter. I looked at your previous "Question of the Week" on late blight, and was not able to find this reassurance. Am I correct that you said this? If so, wonderful; I'll be counting the days until May 15th! But if last year's spores can affect this year's tomatoes, what should I do? Dredge all of the compost (yes, it's mostly compost!) out of my raised beds and make a trip to our township site to pick up a fresh load? Guidance please!

---Christopher in Norristown, PA

A. Well, Chris; it is my pleasure to inform you that your ears were correct; late blight spores cannot survive winter above ground in Northern gardens.

Now, that reassuring information IS in our archives; it's contained in a follow-up article we did last year about late blight preparations for this season. But you are one of many who have been emailing us lately with concerns about the possibility of another lousy tomato year. And our chief source on this story, Cornell University Plant Pathologist Dr. Meg McGrath (no relation that she or I know of) has asked if I can once again get some important information out to America's backyard tomato growers. So let's review.

Late blight has previously appeared in Northern gardens, but it generally doesn't show up until very late in the season. Typically, the disease begins in the deep South, where lack of adequate killing frosts allows the pathogen to survive year-round. As the weather warms and The Summer Wind blows, late blight spores slowly make their way up the Coast, infecting tomatoes and potatoes as they go. These spores can eventually reach the North (but not nearly as often as many people seem to think), and gardeners do need to keep an eye out for it, even in a year where no massive outbreaks have been reported.
Last season was very different; a perfect storm. A large-scale grower introduced already blighted plants into big box stores up and down the East Coast at the very beginning of the season; then the weather stayed cool and damp, which makes tomato plants more vulnerable to disease and helps keep the spores alive. Hopefully someone in the ag department is keeping an eye on this year's baby plants to prevent a reoccurrence. But if a cost-cutting grower releases pre-infected plants into the trade again, we could have another nightmare tomato season.
A warm, dry summer will hinder the spread of virtually all plant diseases; another cool wet one will mean a below-average to average tomato year at best, even with no reoccurrence of 'early late blight'. (A term I just made up.)
Although late blight can't survive winter above ground, it CAN survive inside of the previous year's potatoes. As I explained repeatedly on our show last year (and on a recent interview with Michele Norris on "All Things Considered") late blight on tomatoes is the same organism that wiped out potato crops in Ireland in the 19th Century, and the pathogen attacks both kinds of plants. It CAN survive Northern winters on infected tubers; that's how it remained so deadly active for so many deadly years in the nation my ancestors finally had to flee for their lives. In today's tomato growing North America, the big risk is potatoes, warns Dr. Meg.
If you didn't grow potatoes last year, you're fine. If you did grow potatoes, don't replant any of your harvested crop, don't plant tomatoes or potatoes in the spots where last year's potatoes grew, and destroy any volunteer potato plants that sprout up. These 'volunteers' are, to me, a HUGE risk. As anyone who's grown potatoes knows, you never find all of the buried treasures; and 'free' plants always seem to sprout up in your garden the following year. This year, destroy any such plants immediately; don't wait for signs of disease. I will, and so should you. Dig it all up and throw it in the trash. You can certainly grow potatoes if you wish, but this year, please be super-conservative and use only new, certified disease free 'seed potatoes'.
Be vigilant! Keep a close eye on your tomatoes this season; even if all the big plant companies DO behave, and even if we have a delightful year. Dr. Meg warns that there are a couple different strains of this pathogen, and if they mate, the resulting spores might begin to develop the ability to survive Northern winters above ground. So, she says (and yes, I do now quote): "Allow no pathogen sex in your garden!" (Ok, so maybe we ARE related...) Home gardeners are urged to immediately report and then destroy any blighted plants they find.
Behave! If you choose to ignore the tell-tall signs of late blight in the selfish hope that you can coax a few more fruits out of the plants, you may be the butterfly flapping its wings that causes a hurricane half a world away. Or as Dr. Meg puts it, "The Typhoid Mary of Tomato Growers".

You'll find Dr. Meg McGrath's late blight information sheets for 2010, and detailed description of what late blight looks like at this Cornell University site.

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