Mark Stennes spent the better part of his career as an arborist and plant pathologist working to stop the scourge of Dutch elm ­disease, which began decimating the regal urban shade trees lining Twin Cities streets in the 1970s.

So when Stennes found himself gazing upon an enormous century-old American elm standing proudly on a hobby farm in Afton in 1997, he recognized it as something akin to a miracle of nature.

This tree, towering 80 feet tall and measuring 175 inches in diameter, had somehow survived the disease while others around it in the St. Croix River Valley were wiped out.

Working with the homeowners and a University of Minnesota horticultural scientist, Stennes helped unlock the tree's biological secret of survival. The team eventually developed a patented cultivar, which will be sold to the public for the first time next spring.

Stennes, of New Brighton, died July 22 from complications of liver cancer. He was 63.

Named the St. Croix elm, to honor the area where the tree has lived for perhaps 120 years, the cloned offspring is believed to be the only known variety to be tolerant of Dutch elm disease.

"He was very passionate about trees and had such broad knowledge," said Debbie Lonnee, of Bailey Nurseries in Newport, which is bringing the St. Croix elm to market. "He recognized the value and that it was a live tree amongst all the dead ones in the valley."

Stennes grew up in Bemidji, and during a 35-year career in the industry became known as an ardent advocate for trees. He led many trade organizations and won numerous awards for his work. Rarely did he turn down an opportunity to educate young arborists or the public.

"He was so passionate about the industry and teaching others," said Gail Nozal of S&S Tree and Horticultural Specialists in St. Paul, where Stennes worked from 2005 until he retired last year. "He enjoyed seeing their fire and interest grow."

Working on diseased elms on the Minnesota State Fairgrounds, Stennes experimented with injecting megadoses of fungicide directly into root systems and discovered that it helped the immense trees battle Dutch elm disease. After years of research, it became the industry standard of treatment throughout much of North America.

Even though Stennes didn't live to see the St. Croix elm tree make it to market, he was able to put one in the ground just the same. Last October, he stuck a shovel in the dirt at the State Fairgrounds and helped plant a St. Croix elm at an event put on in his honor by the Minnesota Society of Arboriculture, the University of Minnesota and friends and family.

Stennes is survived by his wife, Diane; and daughters Julianne, Leah Renner and Mary, all of New Brighton. Services have been held.