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George A. Zentmyer

Retired but never quite finished

by: Lisa Dunlap   (June 2000)

It started, he recalls, with a summer trip to Washington state and some fishing with a friend.

Professor Emeritus of Plant Pathology George A. Zentmyer begins the narrative of his career by remembering summer days 70 years before when he traveled from his home in Los Angeles to spend a few months with his mother’s brother, a soil scientist for the U.S. Department of Agriculture.

The uncle took him to work some days. It was there that the 15-year-old Zentmyer met a USDA forest pathologist who became his friend. The two young men fished on weekends and traveled into the forests on workdays to examine the effects of smelter fumes on pine trees.

“I decided then — I was 15, 17 — to go into forest pathology,” Zentmyer says.

The rest, as they say, is UCR history. Zentmyer devoted his college studies and career to tree diseases, eventually becoming the world’s foremost expert on certain species of Phytophtora, soil-borne fungi that can destroy about 1,000 different types of crops and trees worldwide. In 1979, he received the premier recognition given to an American scientist when he was elected to the National Academy of Sciences (NAS).

That honor joined a list of many others, including a NATO Senior Fellowship, a Guggenheim Fellowship, and election as a fellow of the American Association for the Advancement of Science and the American Phytopathological Society.

In addition, in 1981 he became the third person in the California Avocado Commission’s 67-year history to receive the group’s honor medal with leaf cluster for outstanding contributions to the industry.

The 86-year-old Zentmyer has been what he terms “partially retired” from UCR for 19 years. He no longer teaches or is involved in field research, but he regularly attends seminars by graduate students or visiting lecturers.

He also is working on writing projects from his office in Boyce Hall and his home a few miles from campus. It’s his goal to publish all the research findings of his career now that he has time to write.

“You never really finish the job,” he says. “There’s so much research to do that writing it all up gets put aside.”

A list of his approximately 200 existing publications shows a career dedicated to examining the biology and control of Phytophthora cinnamoni. This fungus is a major cause of rot diseases for about 250 different types of subtropical and tropical plants, including avocado.

“It wasn’t so much that I chose to focus on this, but that research was needed in this area [when I arrived at the Citrus Experiment Station in 1944],” Zentmyer explains. “It was a major cause of disease and still is to some degree. When I came here, avocado root rot was a rather obscure disease. The fungus had just been isolated the year before, and scientists thought at the time that it was mainly water-related. It was very difficult to control, and the damage it caused was just terrific.”

At the time Zentmyer was conducting research, an estimated 5,000 acres of avocado in California had been put out of production by the pathogen.

When he came to the experiment station subtropical trees and fungi were not yet Zentmyer’s specialty. After earning his bachelor’s, master’s and doctoral degrees from UCLA and Berkeley, he became a forest pathologist for the USDA in San Francisco. For three years, he researched the effects of pollution on trees.

Then he joined the Connecticut Agricultural Experiment Station in New Haven to work on treatment of Dutch Elm disease.

“The (Connecticut) Agricultural Experiment Station was one of the most advanced in existence, and the position I took was, I think, the first in the country to work on chemotherapy as a treatment for plant diseases.”

During this four-year period, Zentmyer also made two of the most important associations of his life. He met and married his wife, Dorothy, a social worker and the mother of their three daughters, Elizabeth, Susan, and Jane; and he worked for Dr. James G. Horsfall. Horsfall would later recommend him for NAS membership and also suggested he consider the Citrus Experiment Station position in Riverside.

Zentmyer took his advice and got the job.

It was then that he began focus on investigating the biology and ecology of P. cinnamoni and identifying avocado and related plants of the Persea genus that might provide resistance to the fungi. He traveled extensively through Latin America, South America and California to collect disease-resistant Persea rootstocks. He single-handedly developed a Persea collection for UCR, which totals more than 1,000 specimens that continues to be utilized by researchers today. He started a certified nursery stock program and he evaluated many different control methods such as fungicides, suppressive soils and chemotherapy, and identified the temperature, soil, nutrient and irrigation conditions most favorable to the fungi.

One of his major contributions was a 1944 article in Science that theorized that certain fungicides were effective because of their chelating, or metal precipitating, properties.

According to Zentmyer’s research, fungicides tied up minor mineral elements in the soil, making them unavailable to the fungi for their growth. This theory was later used to develop effective treatments for people with lead poisoning. In 1971, Science published his finding on chemotaxis of P. cinammoni, a phenomenon where zoospores of the fungi are attracted to amino acids and other substances emitted by avocado trees. This study proved that non-host plants did not produce the same substances.

Although best known at UCR for his avocado work, he also developed an international reputation for research on cocoa plants, which can develop black pod disease due to the related fungus P. palmivora.

“Worldwide, cocoa is a much larger commodity than avocado,” says Zentmyer, “and black pod is a very terrible disease, literally capable of wiping out half of a cocoa plantation.”

His research on P. palmivora led him to South America, Malaysia and Africa. But it wasn’t only the travel that was appealing to him about this research.

“It was fun to talk to people,” he says. “Sometimes people don’t understand the science or care about fungi, but most people are interested in chocolate.”

As Zentmyer looks back on his life, satisfaction also comes from his personal life. He enjoys spending time with his children and five grandchildren, all living in California. He also gets satisfaction in knowing that daughter Susan, a UC Davis science graduate, is a marketing manager with Tri Valley Growers.

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