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Branch of a Manchurian crab apple showing canker disease symptoms. (Courtesy Tom Auvil)


Orchard A was irrigated with overhead sprinklers; Orchards B and C were drip irrigated. Preliminary data for 2013 harvested fruit . Does not include postharvest evaluation of fruit after nine months in storage. SOURCE: Dr. Richard Kim, Pace International (Aurora Lee/Good Fruit Grower illustration)


A Manchurian crab apple showing disease laden, mummified fruit from the previous season alongside crab apples from the current season. (Courtesy Tom Auvil)


This crab apple pollinizer was pruned quickly with a chain saw to remove wood showing cankers. (Courtesy Richard Kim)

When China reopens its market to U.S. apples, managing crab apple pollinizers in the orchard for diseases is expected to be part of the new ­Chinese export protocol.

Managing the diseases that brought on the market closure will take a two-prong systems approach involving both pre- and postharvest, says Dr. Richard (Yong-Ki) Kim of Pace International, a chemical company and services provider.

Kim, a former postharvest pathologist for Washington State University, is heading a three-year research project that began last year to study disease infection in orchards and packing houses and to develop management strategies for control.

China closed the U.S. market in August 2012 after years of detecting various rots in shipments of Red Delicious apples. Red and Golden Delicious are the only varieties allowed to be imported.

The three fungal diseases detected by China were bull’s-eye rot (Neofabraea sp.) and two crab apple canker-based diseases, Sphaeropsis rot (Sphaeropsis pyriputrescens) and speck rot ­(Phacidiopycnis washingtonensis).

“Reopening the Chinese market for Washington apples is an extremely high priority and impacts future access for all varieties, not only Reds and Goldens,” said Dr. Mike Willett, vice president for scientific affairs, Northwest Horticultural Council. “We’ve lost significant market share since August 2012.”

Getting U.S. apples back into China is a big deal, Willett said. “The crab apple diseases aren’t unique to Red and Golden Delicious orchards. We hope within the coming year to have access for all apple varieties.”

When China began detecting the apple rots several years ago, the relationship between Manchurian crab apple and the diseases was not well understood, he noted. “When we visited the orchards implicated in the detections, we realized there was a serious disconnect in how crab apple trees were being managed.”

Manchurian crab apple, widely planted as a pollinizer in Pacific Northwest orchards, is highly susceptible to speck rot and Sphaeropsis rot and serves as a major source for the inoculum.  An effective alternative to using Manchurian crab apple has not been identified.

“We found that the lack of crab apple management was leading to high levels of decay,” Willett said. “Most growers are farming Manchurians for the blossoms and ignoring the trees for the rest of the year.”

Pruning research

The disease resides in overwintered fruit and pendant wood, explained Tom Auvil, research associate of the Washington Tree Fruit Research Commission. “Mummified fruit left on the tree and orchard floor are loaded with spores,
creating a lot of opportunity to infect the orchard,” he said.

“If the crab apple trees haven’t been pruned, the pendant and flat wood develop cankers and serve as a major source of inoculum. You’re then well on your way to continuing the disease cycle in your orchard.”

Willett, Kim, and Auvil, who shared the podium during a presentation at the Washington State Horticultural Association’s annual meeting last December, are also involved in the research project to learn the effect of crab apple pruning on speck rot and Sphaeropsis rot.

The three-year project, which will conclude next year, has three goals:

• Generate practical information on the impact of pruning crab apples in apple orchards as part of a postharvest decay integrated pest management program.
• Understand in-season fruit infection by speck rot and Sphaeropsis rot after pruning crab apple trees.
• Evaluate the impact of pruning crab apples on the incidence of the two rots in storage.

Data were collected in 2013 from three Red Delicious orchards in George and Royal City that have a history of Sphaeropsis or speck rot incidence. One orchard uses overhead sprinklers for irrigation; two use drip irrigation.

Auvil is coordinating the pruning portion of the project. Three treatments are being studied—no pruning, chainsaw pruning, and detailed pruning. The detailed pruning treatment uses a chainsaw and lopper to remove all diseased wood, especially pendant wood with fruit.

Although the crab apple trees looked rather spindly after his chainsaw work, they responded with regrowth of healthy wood by July.

The project is also monitoring fruit infections on apples adjacent to crab apple trees throughout the growing season. Additionally, fruit held in storage is evaluated monthly for decay for up to nine months after harvest.

Previous research in Delicious and Fuji orchards showed that crab apple trees are a major source of Sphaeropsis rot and speck rot infection from May through November, Kim reported. For both diseases, September was the highest month for infection, with up to 90 percent of the crab apple trees infected. But even in the beginning of the season in May, more than 10 percent of the crab apple trees were infected in some orchards.

Though project results are preliminary as storage data are still being collected, Kim said detailed pruning (using a chainsaw and loppers to remove all visible signs of the diseases) greatly reduced the number of commercial apples infected from nearby crab apple trees. In the three orchards, detailed pruning reduced commercial fruit infection to 5 percent or less. In comparison, when no pruning was done, up to 25 percent of the commercial apples were infected.

Chainsaw-only treatments resulted in 10 percent fruit infection in the overhead sprinkler orchard and 5 percent in the two drip irrigation orchards.

Preharvest fungicides

Research has shown that preharvest fungicides help control the two crab apple diseases. Kim recommends a preharvest spray of Pristine (boscalid, pyraclostrobin), Topsin, (thiophanate-methyl) or ziram one to two weeks before harvest.

Of the three postharvest fungicides TBZ (thiabendazole), Scholar (fludioxonil), and Penbotec (pyrimethanil), Penbotec was the most effective at controlling Sphaerosis rot and provided complete control, said Kim. The same three fungicides control speck rot, with Scholar and Penbotec providing complete control.

“Postharvest treatments are the most effective way to control the two diseases,” Kim said. Although the industry has been using postharvest fungicides for several years, the diseases are still showing up in commercial packing houses.

Why are these two diseases so hard to control?

Kim said spores can germinate in the stem and also penetrate fruit tissue through stomata, cracks, or lenticels. Fungicides are not as effective when the disease gets inside the fruit. “That’s why we need to eliminate all spores.” •