Dr. Richard Kim, right, shows how unmanaged Manchurian crab apples can spread postharvest storage diseases during a crabapple workshop near Quincy, Washington, on March 20, 2014. (TJ Mullinax/Good Fruit Grower)

The Northwest Horticultural Council and the Washington Apple Commission have secured a federal grant of more than $1.9 million to fund research needed to secure access to the Chinese market.

Washington apple producers have been shut out of China for the past two seasons because of concerns about fruit infected with three types of decay: bull’s eye rot (Neofabraea perennans), speck rot (Phacidiopycnis washingtonensis), and sphaeropsis rot (Sphaeropsis pyriputrescens). The industry has lost more than $30 million in sales so far.

This year, Chinese officials agreed to reopen the market under a new, more stringent export protocol that requires growers take steps to manage the fungal diseases in the orchard and certify they have done so.

About a decade ago, Dr. Chang-Lin Xiao, then pathologist at Washington State University, discovered that some fruit decay had been misidentified and was actually caused by two organisms not previously known to cause decay in apples: P. washingtonensis and S. pyriputrescens.

He also found that Manchurian crab apple, commonly used in orchards as a pollinizer, was host of the organisms and acted as a source of inoculum that spread to neighboring commercial fruit trees.

Xiao recommended pruning dead limbs and diseased fruit out of the crab apple trees to reduce the risk of infection in the orchard.

Dr. Mike Willett, vice president for scientific affairs at the Hort Council and author of the funding proposal, said many growers had stopped pruning Manchurian crab, which was a logical decision as the wood has sharp thorns that, when dropped on the orchard floor, can damage the tires of orchard vehicles. However, it has resulted in increased fruit decay in some orchards with Manchurian crab pollinizers.

Systems approach

Officials of the two countries agreed during bilateral negotiations last November in Xiamen, China, that, as a condition of reopening the market, the United States would conduct research to validate the systems approach proposed by the United States to mitigate decay. The systems approach involves removing cankers and twigs with dieback symptoms on crab apple trees, applying postharvest fungicides, and inspecting fruit at the packing house to ensure that no decayed fruit arrives in China.

Part of the federal grant, which was awarded by the Technical Assistance for Specialty Crops program, will fund this research. It will also fund research into alternative pollinizers as a longer-term solution to eliminating decay in exported apples.

As neither WSU nor Oregon State University has a tree fruit pathologist, a postdoctoral researcher was to be hired to do the disease management work, which will be a three-year project.

The scientist, based at the Tree Fruit Research and Extension Center in Wenatchee, will also develop extension programs to disseminate results of the research and supporting work done by other scientists.

Alternative pollinizers

Dr. Stefano Musacchi, WSU horticulturist, and Karen Lewis, WSU extension specialist, are heading the alternative pollinizer aspect of the research, which will be a five-year project.

Manchurian was adopted as a pollinizer by Washington apple growers in the 1980s to provide cross-pollination in solid blocks of commercial apple varieties. This eliminated the need to manage another commercial variety in high-density apple orchards and, as crab apples need little space to grow, they could be planted between the other trees.

Manchurian was one of few crab apples that bloomed early enough to pollinate the earliest blooming commercial apples. It is estimated that Manchurian is planted in 60 to 75 percent of the state’s commercial apple orchards at a rate of 5 to 10 percent of the total trees.

Lewis and Musacchi got a head start on the project last spring when they visited nurseries around the country, including J. Frank Schmidt & Son Company in Boring, Oregon, which has a large collection of crab apple germ plasm. They bought some trees, which they’re growing in pots in Wenatchee for evaluation.

Any alternative to Manchurian would need to be compatible with Washington’s commercial apple varieties, be attractive to bees, and bloom annually. Lewis said they’ll be looking for early, mid-season, and late-blooming crab apple trees to pollinate Washington’s range of commercial varieties. WSU’s WA 38, for example, is a late bloomer.

They’ll also need to look at pollen viability and pollen-tube growth in the alternative crabs. The fertilization process begins after pollen is deposited on a flower stigma. Male gametes from the pollen are transported through a pollen tube that grows from the pollen grain down through the flower style to the ovary where the ovule is fertilized, resulting in fruit set.

Research by Dr. Keith Yoder and Leon Combs at Virginia Tech suggests that Manchurian might not be the most efficient pollinizer because of slow pollen tube growth.

“We absolutely think there are more efficient pollinizers,” Lewis said.

Scientists at Virginia Tech are unable to work on the project, so Musacchi’s research associate, Sara Serra, will go to Virginia Tech to learn how to do the pollen-tube growth work.

“We’ll build capacity in Washington,” Lewis said. “In the long run, it’s very exciting that we’re going to be able to do that.”

Willett said that while the disease-control portion of the work is important to resolve the short-term problem, the efforts of Musacchi and Lewis will be extremely valuable to Pacific Northwest apple growers in the future. •