Fungal biologists fighting peach tree-killing pathogen


Many a Coloradan looks forward to that first, juicy bite of a Palisade peach every summer. Demand for the famous Western Slope fruit never runs dry, but the state’s $40 million peach industry is under increasing threat from an insidious pathogen that destroys peach and other fruit-bearing trees.

Researchers in Colorado State University’s College of Agricultural Sciences aim to turn the tide against the tree-killing fungal pathogen Cytospora, which for decades has wreaked havoc among peach orchards across the Western Slope, cutting the lifespans of trees in half and costing farmers millions of dollars each year.

Plant and fungal biologist Jane Stewart, assistant professor in the Department of Agricultural Biology, is leading a series of studies to arrive at a deeper understanding of the molecular biology that governs this harmful pathogen, and to devise an arsenal of best practices to ward off infections and help growers stay in the peach business. Such strategies could range from choosing cultivars that are less susceptible to the fungus to changing how trees are cared for and pruned to limit the fungus’s spread.

New funding streams

Stewart and colleagues are recent recipients of three streams of federal funding all aimed at various aspects of tackling Cytospora canker on behalf of fruit growers across Colorado. The National Institute of Food and Agricultural Research is supporting Stewart’s team to study cultural practices currently used in peach orchards. They aim to determine if infected, pruned and mulched branches placed under trees leads to increased incidence of Cytospora infections; to better understand the epidemiology of the pathogen; and to determine Cytsopora species diversity. The telltale sign of a Cytospora infection is a canker wound visible on the bark or branches of a tree.

cytospora canker

And since the Cytospora pathogen is not limited to peach trees, Stewart’s team will look at species diversity of the fungus across peach, apple and cherry hosts and study population genomics of the fungus as it moves around and between orchards. In the long term, this research will lead to management strategies for the Cytospora canker built on an understanding of the biology and epidemiology of the various species of the fungus.

NIFA is also supporting research by Stewart and colleague Dana Hoag of the Department of Agricultural and Resource Economics to provide new insights into the economic feasibility of various management practices. The work will illuminate tradeoffs of different choices like spraying trees with pesticides versus planting new ones.

“We are looking at combatting this disease from a molecular standpoint, as well as from cultural practices of growers,” Stewart said.

Long battle

graduate student stephan miller
Stephan Miller with a peach tree being studied for Cytospora infection.

The work being undertaken builds on a long history of fighting the Cytospora pathogen in Colorado for Stewart, in collaboration with researchers at the Western Colorado Research Center in Grand Junction, in the heart of peach country. While Cytospora has created problems for peach growers since the 1950s, it has exacerbated over time and is now halving the lives of peach trees, which used to be in the ground for 40 years but now average around 20, Stewart said.

Several years ago, Stewart and colleagues collected isolates from different orchards and found that Colorado peach orchards are impacted primarily by only one species of the fungus, Cytospora plurivora, while in other areas of the country, the species prevalence is more diverse.

Stewart and graduate student Stephan Miller then developed a chemical treatment plan for farmers and fact sheets about the pathogen, in an effort to keep infections from killing trees before cankers take hold.

With the new funding, Stewart and graduate student Sean Wright are extending that work by combining expertise in fungal biology and population genomics with observations of farmers’ habits for managing the disease, and how those habits could be changed for the better in areas like watering techniques, chemical treatments and mulching.