Beating back blight and disease in the garden 9/25/08

Sept. 25, 2008
By Greg Duggan
Observer staff

Summer has passed and the growing season has but a few weeks remaining — if blight or disease hasn’t already put a stop to local gardening and farming.

If allowed to spread through a garden or farm blight and disease can turn veggies into a rotting mess, and they were not uncommon this year in Vermont.

One disease that appeared in the state, late blight, primarily affects potatoes and tomatoes. In fact, it’s the same one that caused the Irish potato famine in the mid-1800s.

Another disease that plagued Vermont gardeners and farmers this year, downy mildew, attacks cucurbits, which include squash, pumpkins and cucumbers. “It’s a blight on the potato leaf,” Ann Hazelrigg, a Williston resident and plant pathologist with the University of Vermont Extension, said of late blight. “You’ll see a greasy black spot on the foliage. If you turn it upside down in the morning or when humidity is high, you’ll see a white fungus sporulating on the underside. … It rapidly moves through the whole plant if you have the right weather conditions.”

She described the “right weather conditions” as moist, with lots of rain.

Hazelrigg, who also runs UVM’s Plant Diagnostic Clinic to help commercial growers and home gardeners with disease, insect and weed diagnosis and identification, first identified late blight in Lamoille County after receiving a call from a grower.

Though Hazelrigg said the blight hasn’t been identified in Chittenden County, she figures it has probably made an appearance.

“It can wipe out potatoes very quickly,” Hazelrigg said. “Most conventional growers are putting on fungicides to protect tissues from disease. It’s trickier in a home garden when you’re not protecting tissue with fungicide.”

Regardless of protection method, Hazelrigg suggested the best preventative at this point in the season is to simply harvest the spuds.

“If growers are seeing it at this point in the season … the potatoes are probably sized up. They should go ahead and cut the tops and harvest the potatoes rather than trying to keep the foliage healthy,” Hazelrigg said.

Even if the blight has infected the tuber — the potato itself — Hazelrigg said there’s no health risk. The infected spot will be a brown or purple discoloration on the skin of the potato, and below the skin will be what Hazelrigg described as “a dark, reddish brown, corky rot.” Cutting away the rot makes the potato edible; the problem worsens when the blight is allowed to fester from year to year.

“That’s how the Irish got in trouble. They harvested the potatoes and stored them in trenches. When they opened the trenches, it was just a mass of rotting tissues,” Hazelrigg explained.

But even if the late blight has spread to Chittenden County, it’s unlikely it would ever cause problems anywhere near the level of the Irish potato famine. For one thing, the Irish essentially relied on potatoes as their sole food source, and their storage methods — as well as several years of cool, wet weather — allowed the blight to fester.

Furthermore, the spores that cause the blight cannot survive the winter in Vermont — Hazelrigg said they don’t have the right mating types.

Downy mildew

Downy mildew has caused problems in Chittenden County this year.

“If you notice a lot of die back in cucumbers, it’s likely this fungus disease,” Hazelrigg said.

The disease causes small, angular spots on a plant’s foliage, and turning a leaf upside down reveals a “velvety, brownish purplish sporulation,” said Hazelrigg. Downy mildew can overtake plants in a matter of days, and also spread to pumpkins and winter squash.

“Fruit turns white and rots in the middle of the field. Powdery mildew, downy mildew, there are several diseases there,” said John Adams, owner of Adams

Apple Orchard & Farm Market in Williston. “Some you can spray for. We don’t spray much in our field. We’re not organic, but close to it.”

Adams said a lot of vegetables, particularly squash, rotted on the ground. The market managed to save most of its pumpkins, which were brought in from the field once they were ripe to avoid disease and potential frost.

June Jones, a Vermont master gardener from Williston, said gardeners can treat downy mildew organically by spraying plants with a mixture of one gallon of water and three tablespoons of both baking soda and horticultural summer oil.

Hazelrigg said she first identified downy mildew in Addison County in early August, and has heard more widespread reports of downy mildew than of late blight. She’s even lost the foliage in her personal garden’s pumpkins, winter squash and tomatoes due to the mildew, and said many growers will likely miss the last planting of cucumbers due to the disease.

“It can be devastating for commercial growers. It means they may have lost the last planting of cucumbers,” Hazelrigg said, but added that most Vermont farms grow a variety of produce to avoid problems caused by losing one crop.

“For home gardens its more of an annoyance, because we’re not relying on this to keep us healthy, to eat through winter or make money,” Hazelrigg said.

Planning for next year

Though neither late blight nor downy mildew can winter in Vermont, according to Hazelrigg, they can appear in any particular year as spores are carried on storm fronts.

“It’s good for people to plan for next year,” Jones said.

As the current growing season draws to a close, Jones and other master gardeners are cleaning their plot in the Williston Community Garden. Jones said all vegetable matter needs to be taken to a landfill, rather than put in a compost pile, and the ground needs to be rototilled once now and again in a month or so.

Those steps will help prevent disease from remaining in the soil. Jones also suggested spraying any equipment, including tools and tomato cages, with a bleach solution.

When next year’s planting season arrives, Jones and Adams suggested rotating crops so veggies won’t be in the same soil that held similar, diseased plants the previous year.

“You don’t want to plant the same thing in the same area, or blights or various diseases develop,” explained Adams, who said his farm rotates crops as much as possible in its relatively small growing space.

Also, many gardening stores sell disease resistant varieties of vegetables.

Jones’ other suggestion was to start thinking about disease before it even arrives.

“The first thing, in the spring, when you put new plants in, is to start spraying them with fungicide before you even see anything,” Jones said. “A lot of people wait until they see (disease), then start spraying. And that’s kind of too late.”

Learning in the garden

➢    Questions about a garden? The University of Vermont Extension’s Master Gardener program has a helpline to offer advice on trees, plants, bushes, bugs and more. Call 800-639-2230 with a question, and someone will call back with an answer.

Ann Hazelrigg is also available at the University of Vermont Plant Diagnostic Clinic at 802-656-0493.

➢    The Master Gardeners are offering a Master Composter Course starting next month. The course runs from 6 p.m. to 9 p.m. on Thursdays, from Oct. 23 through Nov. 20. A Williston course will be held at Vermont Interactive Television, and cover topics including the biology of composting, worm compositing and trouble shooting.

Tuition is $45. Enrollment deadline is Oct. 3. For more information call 656-9562.