Late Blight: cue Mass freak out

Today late blight, the bane of tomatoes, potatoes, and the Irish, was confirmed by UMass Amherst out in Franklin County. This disease, caused by the pathogen Phytopthora infestans, was the microscopic culprit behind the famines that decimated Ireland in the mid-1800’s. While it has the potential of being every bit as devastating now as it was then, we are lucky to have a much more diversified diet than our forebears (though not nearly diverse enough), as well as various lethal weapons like copper sprays and other vicious stuff that stops, or at least slows, the disease in its tracks. However, despite the odds that are in our favor, we’re not quite able to control the weather, and any cool and rainy summer days can ignite a few late blight spores hitchhiking in the wind and rain. After a few get going it doesn’t take much for the blight to cause a major and often heartbreaking infestation.

To illustrate the nature of late blight, let me offer a brief lesson in contrasts. Tomatoes are beautiful–

Heirloom maters

and late blight is not.

More late blight lesions

Late blight lesions on tomato stem, leaf, and petiole

Roma tomatoes affected by late blight

Mugshot of affected Roma tomatoes. Credit: Previous blight photos from Luscious heirlooms from

Like lots of deadly things, late blight is pretty fascinating.

Early blight tomato

Ew gross, hybrid tomato all eaten up by early blight. Credit:

As opposed to early blight, a true fungus (that you can read more about here), late blight was “‘banished’ from the fungal kingdom” and brought into a new classification, the goofily named Oomycota in the Stramenopila kingdom. This new placement made it distantly related to such disparate organisms as diatoms and the giant kelp that waves along Californian coasts. Late blight was kicked out of the fungi clan in part because its spores, after racing through the air, then swim–swiminto a tomato’s (or potato’s) leaves. Apparently that is just not a very fungal thing to do.

I’m imagining little Michael Phelps’s lazing into leaves, laying on their backs and lounging around on a cloudy, green day before wiggling into the stomata of a leaf. Which is remarkably unscientific. So let me quote somebody that knows what they’re talking about.

In humid conditions, Phytophthora infestans produces sporangia


and sporangiophores


on the surface of infected tissue. This sporulation results in a visible white growth at the leading edge of lesions on abaxial (lower) surfaces of leaves. As many lesions accumulate, the entire plant can be destroyed in only a few days after the first lesions are observed.

(Click on text and photos for credits.)

Sporangia, sporangiophores, sporulation; sort of sounds like Dr. Seuss had some V8 before sitting down to write a tomato tragedy. It’s real life, though, and these tiny, 0.03 mm long lemon-shaped sporangias can kill a field of potatoes in a matter of days.



Please keep a look out. Any infected plant material should be reported (in Massachusetts call the UMass Plant Diagnostic lab at 413-545-3208) and discussed with your fellow gardeners. Leaves, stems, and severely damaged plants should be destroyed (ie, pulled up or pruned with sharp, clean tools and thrown out–not composted). To track the spread of late blight and other pests, diseases, viruses, etc, go to your local extension service’s website or to (recommended by UMass).

I want to add that late blight (and lots of other nasty stuff) can be avoided by feeding and treating your tomatoes well. My personal recipe has worked well thus far (and yeah, I knocked on wood while I typed that). I plant them with a shovelful or two of the calcium-rich Lobster Compost which is made by Coast of Maine. I also feed them about once a month with a few tablespoons of granular fertilizer–ProGrow in my case–as well as a watering can’s worth of diluted fish and/or seaweed emulsion which I apply weekly (Neptune products are great). I also mulch with hay, strive to remove any and all dead and diseased plant material completely (even if it doesn’t look like late blight), and spray with neem oil once a week. (Don’t use neem when bees are around as it is toxic to them.) Tomatoes are a bit prissy but if you treat them well they’ll give you what you want.

For the sake of your sungolds, Brandywines, Cherokee purples, beef masters and early girls, please give your tomatoes some extra good loving throughout the next few months! Because as interesting, historic, and lemon-shaped as late blight is, it really doesn’t need to be in your garden or your friendly farmer’s fields.

margaret thatcher tomato

Don’t cheat yourself of a beaut such as this! Credit:

One response

  1. Pingback: Crop of the month: Potato | Science on the Land

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