How To Figure Out The Replant Disease Puzzle

An up-close view of a Geneva rootstock. (Photo credit: Sarah Bauer, ARS)

The new Geneva rootstocks have shown resistance to replant disease, and may prove to be a strong weapon for growers. (Photo credit: Sarah Bauer, ARS)

Replant disease is something all perennial crop growers will face — crop rotation is simply not possible, and prime land for plantings is also at a premium.

It was once believed to be strictly isolated to apples; however, all replanted tree fruit and nut crops will experience some sort of replant issue, Tim Smith, educator emeritus with Washington State University Extension, says.

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Nematodes are sometimes blamed for poor tree growth, they are only one minor part of the replant disease puzzle.

Soil is chock full of living organisms, and they live off what is in the ground. As an orchard is replanted, the new roots are under attack from these organisms.

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“We used to say five years out was a good [period to wait before replanting],” he says. “If you don’t grow another crop on that ground, five years isn’t enough time to wait. Replant doesn’t go away.”

The roots of a replanted orchard will be targeted much more heavily than the trees that used to grow on that land, Smith says.

“When you take that tree out, and stir the soil up, and put a new baby tree in, the roots get attacked immediately at a much higher level than the first time you planted,” he says. “That’s the real cause of replant disease — root death. The tree doesn’t die, it generally staggers along as a stunted plant for some time. It may be 20 years that you try to get that orchard to go.”

Tom Auvil, research horticulturalist with the Washington Tree Fruit Research Commission (WTFRC), says soil type is also a big factor in the prevalence of replant disease.

“Sandy soils and pumice in volcanic soils can be the worst. Areas in central Washington that have biggest threat are the traditional oldest sites, such as Cowiche, Tieton, Naches, Lake Chelan, Omak, Oroville, and along the Columbia River from Bridgeport to the Tri-Cities,” he says.

Auvil notes that Gisela cherry rootstocks have some sensitivity to replant disease in sandy and volcanic soils. Pears also suffer from replant disease and may be complicated by incompatibility of scion wood and rootstock.

Fumigation’s Role
“Fumigation in the last 30 years has been the big breakthrough,” Smith says.

Smith says the comparison between fumigated and non-fumigated replant sites is pretty evident.

“Every tree is different. Look down the row and there’s tall ones, short ones, medium-sized ones, skinny ones and fat ones. They’re all different because they’re all going through their own struggle with the disease organisms,” he says.

Smith says part of the challenge of getting growers on board with fumigation is timing. He strongly encourages growers to pull out trees and work the soil over in the fall and fumigate before the soil temperature dips below 40°F or 45°F. Otherwise, growers will have to wait until the ground thaws and fumigate in the spring. However, Smith says he has heard of some growers fumigating frozen ground when there is an early cold snap.

“If the ground is frozen, that’s one of the possible causes of fumigant failure,” Smith says. “It is partial result, it won’t penetrate the frozen ground you’re putting the product on, but you’re not treating the volume of soil that you think you are.”

However, if a grower is harvesting Pink Lady, Fuji, or Granny Smith, next season’s plantings are far from the grower’s mind.

“If you don’t fumigate in the fall because you run out of the proper condition, and you wait until the next spring, plant your trees when you have to, they’ll be alright by the next October and they will far out-yield anything you put in because you didn’t have the time to fumigate,” Smith says.

Anecdotally, Smith talks of a research trial he did on Radar Hill, north of the Tri-Cities, on Gala, comparing a fumigated block versus a non-fumigated block for five years. He says the yield boost was surprising.

“In the fifth leaf it was 50 to 1 ahead of the cost of fumigated,” he says. “Using 2006 dollars during the life of a 20-year Gala orchard that gross difference and the value of the fruit coming off the acre was $64,000 an acre.

Auvil says his research aligns with what Smith says.

“In all replicated trials with good application technique, fumigation has enhanced tree performance compared to untreated plots, “Auvil says. “When there is not a difference, application has been the factor when asking the question of ‘why is there not a fumigation response?’ On rare occasions planting too close to the fumigation application has been an issue.”

Another Piece Of The Puzzle
Fumigation is just one part of ensuring the success of your replanted trees. Thanks to a combination of management strategies, it has become less of a worry for the WTFRC, Auvil says.

“Only recently with the broader acceptance of Geneva rootstocks, fumigation, and use of drip irrigation and/or fertigation, has replant slid to a medium priority of the advisory committees,” he says.

The availability of the Geneva rootstocks have also provided growers with more options to combat replant disease. There are studies now on cherry and pear rootstocks and replant resistance will be a big factor in that research.

“That real resistance is the best way to approach this problem,” Smith says.

Auvil studied the effectiveness of the Geneva rootstocks and compared them to the replant sensitivity of the popular Malling series, which will induce a slightly larger tree.

“You can’t get a bigger tree with replant, by just planting a vigor-inducing rootstock,” Smith says.

However, another important factor is the care of your young trees. Fertigation and careful watering can help nurture susceptible young roots.

“If you don’t irrigate your young trees very well, and you have replant disease, you have a runt,” Smith says. “If you water carefully and you feed your trees carefully, you’ll have a bigger runt.”

Ultimately, getting your trees off to the best start they can have is one key piece of the replant disease puzzle. It’s vital in high-density orchards where if a canopy isn’t filled by the third leaf, it’s going to be hard to make up for that lost income, Smith says.

“If you haven’t filled your space by year three, four, or five, you’ve lost a lot of money. If you don’t get into full production by year 4 or 5, that’s an indication your orchard is off to a bad start,” he says. “There’s so many things that cost a lot of money on those high-tech orchards. Growers spend more money on wire and posts than on fumigation.”

The Challenge For Organic Growers
Tim Smith, educator emeritus with Washington State University Extension says organic growers have been particularly challenged by replant disease because fumigation is not an option.“Some of the best horticulturists I know are organic growers who take special care on details,” he says. “It was driving them nuts because they couldn’t get a tree to grow well.”

Their only effective remedy has been mustard seed meal.

“That’s effective if you stir it into the ground, let it decompose in the ground a bit, and then plant the tree the next spring,” he says.

Availability on a large scale has been another challenge organic tree fruit growers have been faced with, not to mention cost.

“It takes about 3,000 to 5,000 pounds per acre. You put it in 8-foo-wide strips. We think that is the secret,” he says.

The next challenge will be to make mustard seed meal more effective because with limited availability, all growers are competing for the same limited amount.

Despite these challenges, though, Smith says there is a bright spot for organic growers.

“The [Geneva] apple rootstocks are going to be a huge breakthrough for organic growers. They believe in growing disease-tolerant plants,” he says.

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