MAINE — Researchers walking through the forest during a major spruce budworm outbreak in the 1990s discovered some healthy fir trees unaffected by the deadly pests, and wondered what gave them apparent immunity. They took needles from the healthy trees to study at their University of New Brunswick lab, and found they contained a fungus with a beneficial relationship with the trees. …The discovery attracted the attention of Canadian forestry conglomerate J.D. Irving, the largest owner of forested land in Maine. The partnership, which started in 1999, resulted in a novel way to combat future outbreaks: breeding budworm-resistant trees. Its strategy to use natural fungi — known as endophytes — is at the forefront of science to find more natural solutions. Several universities in the US are researching how endophytes act in other types of trees. …Irving Woodlands is ramping up production of the trees in a newly opened greenhouse in New Brunswick.