Prairie fruit growers fight fire with fire

Fire blight | Producers have learned that controlled burns are an effective way to control this devastating disease

An ancient way is new again.

Forrest Scharf of Saskatchewan Agriculture says burning saskatoon berry plants infected with fire blight is a proven method of controlling the disease.

“The way people are handling fire blight is unique and somewhat new,” the fruit crop specialist told the Saskatchewan Fruit Growers Association conference held last month during Crop Production Week in Saskatoon.

“What they have discovered is if you cut the plant down to the ground and burn overtop, you clean up the orchard. Most of the other diseases that possibly could be infecting the plants will basically be killed off.”

Fire blight is one of the most damaging diseases that affect saskatoon berries. It’s a bacterial disease that favours warm and wet conditions during longer bloom times with severe outbreaks following a six to seven year infection cycle.

“I think 2010 was one of the worst ones that I’ve seen,” said Scharf.

The bacterium, erwinia amylovora, contains plasmids and can propagate itself. Insects are the main vectors.

“Whether it’s pollination (insects) or just flies looking to get something from the nectar, they spread that bacteria from flower to flower,” he said.

The point of infection is through the flower’s pollen tube and into the twigs and branches. It creates cankers, usually on the bark tissue, on which insects can land.

Fire blight is aptly named. Vascular flow in the plant is interrupted, and a lack of nutrients and water to the leaves from infected branches creates a brown, scorched look.

“The dead tissue appears to be burned, and the young branches often keel over at the tip, creating a ‘shepherd’s crook’ appearance,” he said.

Sandy and Ken Purdy of Prairie Berries are well versed on the damage that fire blight can do to saskatoon berry trees. It threatened their entire 10 acre orchard near Marquis, Sask., in 2006.

“It was devastating our orchard and therefore the business that we had built around it,” said Sandy Purdy.

She said the disease reduced their production from 25,000 pounds in 2001, when they first experienced the disease, to 5,000 lb. in 2006.

“We made the decision, as hard as it might sound, to burn the orchard. The logic behind it was the fact that in the wild things burn and then they re-grow. Being a perennial, we thought we could make the same thing happen with saskatoon berries,” she said.

Scharf said the Purdy’s plan made sense.

“Since saskatoons are a plant that used to be subject to grass fires in the natural environment, their root system is adapted to send up new shoots,” he said.

The Purdys cut down the trees so that only the stumps showed. Branches were piled in rows and a thick layer of flax straw was piled on top. A slow back burn was completed on a windy day, and all the stumps burned off.

Purdy said new trees were sprouting by the following spring.

“They weren’t growing off of each of the stumps. They actually came from the soil, from the root system and started individually new plants,” she said.

They grew about two feet the first year and new fruit buds were seen on the stems in the fall. A few flowers appeared the following May, and a small amount of fruit was harvested.

The experiment proved successful the following year when each tree was laden with fruit.

“They were loaded to the extent that it almost doubled the production we were getting on a normal year,” she said.

The success of their trial by error approach has resulted in “a dual strategy” of using a cut and burn technique to manage their orchard’s diseases and old growth.

They operating on a 10-year cycle, cutting down and burning a portion of their orchard each year.

“It takes about 10 years for them to get so tall that we can’t get in with mechanical harvesters,” she said.

“If we take out two acres in a 10 acre section, we don’t really miss the two acres because it’s still capable of the same production that the 10 acres did in the old way because there’s so much production in those first few years after the trees come back to normal growth cycle.”

Purdy said infected plants and a four feet area around them are cut and burned, even in season, if fire blight is observed.

She said the burn method is key to killing spores in the vicinity of the tree, which would otherwise survive if only a cut is done without fire.

Soon after successfully controlling fire blight in 2006 they expanded their operation to 160 acres.

“Typically, we bet the bank on 2,000 lb. an acre,” she said.

“We’ve been pretty consistent. Even in poor years we’ve been able to keep that average up.”