Cornell research battles shrub willow leaf rust

A shrub willow with leaf rust disease.

Just as shrub willow has garnered national interest as a key sustainable bioenergy crop, a pervasive fungus has threatened to undermine its potential as a stable addition to the portfolio of renewable energy sources. A Cornell research project backed by a $1 million federal grant aims to elucidate the genetic underpinnings of resistance in shrub willow to a potent leaf rust disease, thereby strengthening the plant’s standing as a reliable feedstock for bioenergy and bio-based products.

Funding for the project, a collaboration between Larry Smart, associate professor of horticulture, and Chris Smart, professor of plant pathology and plant-microbe biology in the School of Integrative Plant Science, is part of a joint initiative by the U.S. Department of Agriculture and the U.S. Department of Energy to conduct fundamental research in biomass genomics.

In the past, breeding programs at Cornell and elsewhere have focused largely on increasing yields of shrub willow (Salix), a fast-growing woody plant preferred in part for its ability to grow on marginal land. Larry Smart says maintaining the consistency of those improved yields now requires a concerted effort to understand at the genetic level the resistance of shrub willow to the leaf rust pathogen, considered the most serious disease affecting plantations in North America and Europe.

Infection by the fungus Melampsora results in pustules that appear on the underside of willow leaves beginning in midsummer. The pustules eventually burst, scattering thousands of spores to the wind. Infected plants can become completely defoliated by late summer, stunting growth months prior to the normal autumn leaf drop.

To protect against the disease, farm managers routinely plant a mixture of cultivars throughout their fields to minimize the risk of widespread infection. But the fast-evolving nature of the pathogen threatens the long-term health of the perennial plant expected to produce for up to a quarter-century.

“It’s a serious threat to have this pathogen that can potentially overcome what was initially a resistant cultivar within that 25-year time frame,” said Larry Smart.

As an obligate parasite, leaf rust requires living tissue to grow, making it difficult to cultivate strains to be studied in the laboratory. The first-phase of the project now underway at Cornell’s New York State Agricultural Experiment Station requires Chris Smart’s ingenuity to design ways to isolate and propagate many strains of the pathogen for genetic investigation. Once isolated, molecular marker technology will be deployed to explore the genetic diversity of Melampsora rust population.

On another front, Larry Smart and his team will explore the genetic basis for rust resistance in shrub willow. By using cutting-edge genetic mapping approaches to identify the genes involved in rust resistance in willow, the team can generate molecular markers to be used by breeding programs in the early selection of resistant seedlings. The improved shrub willow cultivars potentially could lead to greater yields, wider adoption of willow bioenergy crops and increased production of renewable energy.

“Shrub willow can be a key component to the nation’s renewable energy future,” Larry Smart said. “The focus of the project is to ensure the long-term sustainability of the crop, maintaining yields and continuing to breed improved cultivars.”

Matt Hayes is managing editor and social media officer for the College of Agriculture and Life Sciences.

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