Larry Pusey has used crab apples for his fireblight studies, as they can be manipulated to bloom year round in the greenhouse.

Larry Pusey has used crab apples for his fireblight studies, as they can be manipulated to bloom year round in the greenhouse.

Geraldine Warner

For almost 20 years, Dr. Larry Pusey has been focused on researching a single problem—­fireblight.

As Pusey, 60, retires this month as plant pathologist with the U.S. Department of Agriculture in Wenatchee, Washington, fireblight remains the most economically devastating disease of apples and pears, but important progress has been made in terms of understanding how the bacterial disease develops and how it can be controlled without using the traditional antibiotic treatments.

“It’s a tough problem,” Pusey reflected, “and there are many researchers before me that have come and gone. It’s a challenging disease that has been researched for more than 200 years.”

After earning his doctorate at Ohio State University, Pusey worked as a postdoctoral researcher at Kearneysville, West Virginia, on a project looking at the use of microorganisms to control postharvest diseases. His discovery of a strain of the bacterium Bacillus subtilis that was effective against brown rot of stone fruit led to the first U.S. patent on the use of a living organism to protect a plant food product from decay.

From Kearneysville, he moved to the USDA’s Fruit and Nut Research lab in Byron, Georgia, where he worked on a “smorgasbord” of peach diseases for ten years, before transferring to the Wenatchee Agricultural Research Service lab in 1993 to focus exclusively on biological control of fireblight. He was research leader at the Wenatchee lab from 1996 to 2002 but stepped down to focus on his research.

Pusey collected large numbers of microorganisms from apple blossoms, adding to an existing collection from pears, and evaluated more than 1,000 strains as potential biological controls for the disease. The bacterium Pantoea agglomerans strain E325, commercialized in 2006 as Bloomtime Biological, proved the most effective at outcompeting the fireblight pathogen Erwinia amylovora. Pusey doubts that further bacterial strains will be discovered that are more effective and says research is needed on ways to make Bloomtime more effective.

An important step forward was a model he developed using crab apple that led to a better understanding of how the disease becomes established in flowers. He looked at how water, temperature, and flower age affected microorganisms and disease development. For his work, Pusey used potted crab apples in the greenhouse, which can be manipulated to bloom year round. “That’s been a handy tool that we can use throughout the year to learn more about this disease and how to control it,” he said, noting that other scientists have used the model also.

When Pusey began working on fireblight, little was known about the ecology of the flower parts, and particularly the stigma, where the fireblight pathogen initially builds up. He has devoted much time to sampling flowers and looking at the interaction between microorganisms on the surfaces of the flower parts.

What Pusey found was that conditions on the stigma are conducive to a large diversity of bacteria, including the pathogen and beneficial microorganisms. Later, when there is moisture from rain or dew, the pathogen moves down into the flower’s hypanthium, a high-sugar environment that is more hospitable to yeasts. A yeast-based product called BlossomProtect, containing ­Aureobasidium pullans, is available as a fireblight biocontrol and could act as a second line of defense against the pathogen, Pusey believes. He has been pursuing the idea of combining Bloomtime with BlossomProtect or other yeast biocontrol and is looking at whether they would be compatible.

Other strategies

Pusey has also been collaborating with scientists around the world on other strategies that might help growers better fight the disease in the future. For example, nanoengineers at the University of Illinois are working on controlled-release microcapsules that might lead to better dispersal and survival of the organism in the orchard. Scientists with Agriculture and Agri-Food Canada in Vineland, Ontario, are exploring the use of bacterio­phages (viruses that reproduce within bacteria) that could attack the fireblight organism, with Bloomtime used as a carrier.

Ultimately, Pusey believes fireblight-resistant varieties will be the answer. Cornell ­University in New York has developed resistant apple rootstocks, an ­important step in preventing infected trees from being killed.

“A lot of the dwarfing rootstocks that are commonly used aren’t very resistant to fireblight, and I’ve looked at samples that have come to me from growers—cases in which the organism moved from the blossom right down to the root and killed the whole tree in a relatively short time,” he said. “This happens with young trees especially. A lot of times, you may not see the symptoms on the scion before the tree collapses.”

Developing a tree that would resist infection would be difficult using traditional breeding techniques, since the variety would also need all the desirable tree and fruit characteristics to make it a commercial success. However, Pusey has been working with Dr. Bruce Barritt, retired apple breeder at Washington State University, and his successor Dr. Kate Evans to develop methods to screen seedlings for fireblight resistance.

“I think, ultimately, resistance would be the answer, but that takes a long time,” he said.

Dr. Jim Mattheis, research leader at the Wenatchee lab, said Pusey’s research advanced the knowledge of fireblight and demonstrated the potential for biological disease management, and he leaves ARS as a member of active collaborative projects that will ­continue the investigations that he initiated.

His position will be filled, though it is not known when, Mattheis said.