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Breeding Better Black Walnut Trees

By University Communications

Abundant in Missouri, black walnut trees are famous for their edible nuts. But most of these trees grow in the wild, making the quality and yield of nuts unpredictable.

A Missouri State University research team is addressing this issue. Led by Dr. Chin-Feng Hwang, department head of environmental plant science and natural resources, team members use DNA marker technology to breed improved varieties of trees quicker.

Better trees have traits such as higher disease resistance and nuts with thinner shells and larger kernels.

“Our aim is to speed up the walnut breeding process to produce new, commercially viable varieties of trees,” Hwang said.

This research is a collaborative project with University of Missouri’s Southwest Research Center (UMSRC) in Mt. Vernon. It is possible thanks to a two-year grant from the U.S. Department of Agriculture. The grant is worth almost $300,000.

According to Hwang, the first step in molecular breeding is creating mapping populations. A mapping population consists of about 200 offspring from a cross of two parents with desirable traits. It allows researchers to look for DNA information that associates with desirable physical characteristics.

For this project, the team crosses Sparrow and Football trees. Sparrow has a high resistance to anthracnose, a fungal disease. It produces larger quantities of nuts, but they tend to ripen unevenly. The tree is also weaker, and may break down under heavy nut loads.

In comparison, Football is a stronger tree that produces better quality nuts. It has a high kernel percentage and ripen evenly. However, this tree has low resistance to anthracnose, which affects its nut production.

As black walnut trees are large, controlled pollination is tough. So, the team relies on wind pollination of the trees at orchards near UMSRC.

Research team member Sadie Land said she and her teammates have collected around 1,000 walnuts. They planted them at MSU’s Fruit Experiment Station in Mountain Grove, and more than 700 germinated.

“But we don’t know which ones are hybrids of Sparrow and Football. That’s why we use DNA markers,” said Land, a graduate student in environmental plant science.

In the lab, researchers isolate DNA from each seedling by grinding up a small piece of leaf tissue to look for the markers. These markers are genes or DNA sequences with a known location on a chromosome.

They compare the markers to each parent’s DNA to see which seedlings have a combination of both parent trees. The ones that do are true offspring or hybrids. So far, the team has identified 91 hybrids and planted them in a new orchard at the Fruit Experiment Station.

Hwang said this process does take time and resources upfront. But it is more efficient and sustainable in the long term compared to traditional breeding.

“It may shorten the whole process of developing new varieties by several years,” he added. “This will help existing growers and attract new growers to establish orchards or adopt agroforestry practices.”