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Late blight lesions and spores. Photo: Cornell Cooperative Extension
Late blight lesions and spores. Photo: Cornell Cooperative Extension

Late blight moving northward

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Late blight, a windborne plant disease that can spread quickly and devastate potato and tomato plants, is on the move. Agriculture officials have been watching since late blight was identified on Long Island earlier this summer. Cooperative extension horticulturist Amy Ivy tells Martha Foley it's now been reported in Syracuse and in Dutchess County, south of Albany.

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Martha Foley
News and Public Affairs Director

Late blight is a disease that affects tomatoes and potatoes. It has been getting closer to the North Country. It was confirmed in Syracuse last Monday, and a newsletter from Hudson Valley just south of Albany said that people had found the blight there as well. According to Cornell cooperative extension horticulturist Amy Ivy, it’s anybody’s guess if the blight will continue moving north.

“It depends on the weather; the spores will be blown in, that’s how they’ll get here at this point. And, you know, it’s just a matter of what the weather decides to do,” said Ivy, who added that the region typically gets storms at the end of the summer that may carry late blight spores. “Even if we don’t get anything as severe as Irene, and let’s hope not ever again, but even just the storms that hit the south do stir up the air and bring those air currents up and we’re like sitting ducks.”

The area has already experienced high winds this summer. Ivy says that the spores are microscopic and the wind carries them easily. “It’s just kind of a crapshoot as to where they get carried and where they drop out, and then they have to find the environment to grow,” said Ivy. “And that’s one good thing about this dry weather, is that if a spore were to land - of any disease - were to land, it would have a hard time growing because the conditions aren’t right.”

According to Ivy, three things have to come together for the blight to flourish. These are the spore, the viable host and the right conditions for the disease to grow. This is called the disease triangle. Ivy said, “So luckily, the weather, this terrible, dry weather we’re experiencing on every other front, has been discouraging to most diseases. Not just late blight, but other ones too.”

Late blight causes distinctive dark lesions to appear on the leaves and stems of tomato and potato plants. It can also be found on the fruit of the tomatoes. Ivy said, “I like to think of it as a watercolor. You know how when you put watercolor on paper, it spreads with very soft edges and it spreads fast? That’s what’s distinctive about late blight. There’s all kinds of leaf-spot diseases that tomato plants get every year, but they have a very distinct border. Some of them make just round spots on the leaves and some make bigger spots, but they definitely have a distinct edge and they don’t grow that fast.”

In the course of a day, late blight will grow from one inch to two or three inches. If a gardener notices it on one of their plants, they’re instructed to put the plant in a plastic bag and bring it to a cooperative extension office. According to Ivy, the horticulturists want to see the plants because there are several strains of late blight blowing around the country. The regional lab for the northeastern United States at Cornell collects the samples and discovers which strain is in.

Cucumber are another vegetable found in the garden at this time of year. Though cucumbers can eat up gardens, Ivy said, “I love to grow cucumbers. They’re just so quick; I’m harvesting already. But I find they tend to not be long-lived in the garden and so don’t despair.”

Ivy says that the cooperative extension office is asked a lot of questions from people about cucumbers that aren’t doing well. They aren’t long lived, and they also suffer from a variety of problems. Diseases that come through the area tend to affect cucumbers first. Ivy said, “Don’t panic too much if your cucumbers start to go down. Even the professional growers have a tough time keeping their cucumbers thriving through August because of the various diseases they get.”

In the winter, when gardeners order seeds, it is important to look for disease resistance. Ivy says that she does not usually give out specific variety names when recommending seeds since they can be difficult to find. “You have to go with what’s available to however you choose to buy your seeds. So what I usually encourage home gardeners to do is look through their favorite sources and read the descriptions and look for that disease resistance listed,” said Ivy.

The powdery mildew that was prevalent in gardens last summer isn’t as common this year since the weather has been much dryer. This mildew resembles a sprinkling of baby powder and it is seen on the surface of plants such as squash and flocks. There are different strains of this as well. According to Ivy, the mildew thrives in humid, though not wet, conditions.

Ivy says that spider mites have been doing well in the hot, dry conditions this summer. “They make a stippling on the leaf,” said Ivy. “They’re so small that you actually see the damage before you see the mites. And the leaves look like little teeny dots, they call it stippling or a salt and pepper look. It’s either white or yellow  or spots on the leaves. Tiny spots, though, just like a mottling on the leaves.”

The mites live on the underside of the leaves, but they are very small. Ivy says that gardeners may be able to see very fine webbing as well. Their location beneath the leaves makes the mites difficult to control. Ivy said, “Using something like insecticidal soap or an oil to control them can be really challenging because you have to hit them with the product for it to work.”

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