July 30, 2012

Late blight has arrived

By AMY IVY, Cornell Cooperative Extension

---- — Late blight is a devastating fungus disease that wipes out tomato and potato crops with breathtaking speed. 

Now home gardeners and commercial growers need to watch their plants more carefully than ever because late blight was just confirmed on tomatoes in a West Chazy home garden last week.

Anyone who grew tomatoes or potatoes in 2009 knows all too well what late blight does to these crops.

There are only two good things I can think of about this disease: It affects only tomatoes and potatoes, and it does not overwinter in soil or plant debris.

The only way the strains we currently have of this disease can survive from one year to the next is on potato tubers. 

Late blight is of serious concern because it can wipe out entire plantings of tomatoes or potatoes in a matter of days and it spreads rapidly from one garden or planting to the next. 

It usually arrives when spores from infected plants to our south are blown into our area on storm fronts. Because of all the turbulent, stormy weather we’ve had across the eastern United States already this year, pathologists at Cornell are assuming that spores have been spread over most of the state already. But the hot dry weather we’ve experienced discourages those spores from growing. We’ll just have to wait and see if the rainy weather late last week created better conditions for those spores to flourish. Let’s hope not.

Anyone growing tomatoes or potatoes should check their plants daily.

As soon as you see anything suspicious, clip off a few leaves or branches, drop them in a plastic bag, and bring it to any Cornell Cooperative Extension office. There are two common diseases — early blight and septoria leaf spot — that we see every year, and we want to rule those out first. If we suspect late blight, we’ll send it off to the lab at Cornell for verification at no charge.

For the latest information on the occurrence of late blight and to see lots of excellent photos of the disease as well as other information, visit We also have copies of an excellent new fact sheet on late blight with full color photographs of the symptoms. Stop by our office or visit our booth at area farmers markets for a free copy.

If you’re willing to spray to protect your plants, now would be a good time to start using either a copper-based product (this is the approved organic method) or a chlorothalonil product, such as Daconil or Fungonil. Check the active ingredients list on the label, and look for either copper or chlorothalonil. Repeat the application according to the label’s directions, which will be something close to every seven days or after a heavy rain.

Copper is approved for organic growers, but that does not mean it’s not harmful. Copper is very harsh to your skin, and you need to follow the label directions exactly (as you would with any pesticide), especially when it comes to mixing and handling.

Late blight is an aggressive disease. Begin spraying before the disease takes hold, in an effort to slow it down.

Once it is established on your plants, it is very difficult to stop.

At that point, your best approach will be to harvest what you can and pull out the plants. Anyone who has this disease needs to do what they can to stop it from spreading to their neighbors.

Amy Ivy is executive director of Cornell Cooperative Extension, Clinton County. Office phone numbers: Clinton County, 561-7450, Essex County, 962-4810, Franklin County, 483-7403. Website: Email questions to