Joseph Schwarz explains how he screens trees for resistance to leafrollers during WSU’s field day. Geraldine Warner

Joseph Schwarz explains how he screens trees for resistance to leafrollers during WSU’s field day.
Geraldine Warner

Scientists at Washington State University hope to breed apples with resistance to key apple pests.
Joseph Schwarz, a doctoral student with WSU in Wenatchee, this summer reported progress in identifying apple cultivars that seem less appealing than the standard varieties to codling moth and obliquebanded leafroller.

Genetically resistant varieties could reduce the need for insecticide applications and, in turn, reduce problems with disruption of secondary pests or development of pest resistance to insecticides. They could also help enhance biological control and reduce concerns about ­insecticide residues on fruit.

Schwarz is conducting his research in a genetically diverse block of apples at WSU’s Sunrise research orchard near Wenatchee. Established as a resource for the breeding program, the block includes 236 different accessions from the National Clonal Germplasm Repository in Geneva, New York. It contains apples from wild forests of Kazakhstan and China, heirloom varieties, and material representing popular cultivars of the world.

Schwarz is focusing first of all on screening the trees for natural resistance to leafrollers and codling moth. He then hopes to identify the genes that are associated with the resistance. Eventually, genetic markers would be developed to identify resistance in potential parents used in the breeding program and to screen their progeny.

During WSU’s summer field day at the Sunrise Orchard, Schwarz described how he has been taking leaf samples from trees and putting them in feeding chambers in the laboratory with newly hatched leaf­roller larvae. Leaves of Granny Smith and Gala apples are used as controls. Larvae development is monitored weekly.

Schwarz is looking at when the larvae pupate, how much they weigh, and when they emerge as adults. He is also studying the fecundity and mortality of the adults. He began the work last fall and will continue for another couple of years. Schwarz said some varieties look promising, but the tests will have to be repeated so that seasonal variability can be accounted for. He expects to start next year with experiments with codling moth, exposing them to whole fruit from the various trees in the block.

Dr. Jay Brunner, director of the Tree Fruit Research and Extension Center, who is Schwarz’s advisor, said even if a variety has less than 100 percent resistance to the pests, it could still be useful. “If we can get a slowing of the development of the leafrollers, that allows more opportunity for biological control. If the insects are nutritionally challenged, or affected by some of the genes and are not as large, they will lay fewer eggs.”

Schwarz earned his bachelor’s degree in biology at Kean University in New ­Jersey and a master’s degree in chemical ecology from Simon Fraser University in British Columbia, Canada. He is pursuing a doctorate in entomology.