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Cracking Pecan's Problems

By Laura Beil, Writer

Posted Dec. 28, 2016

One of the first Americans to appreciate the flavor of pecans was George Washington, who planted the stately trees on the lawns of Mount Vernon in 1775. Yet true cultivation of the nuts wouldn't begin for about another century, when entrepreneurs established orchards in Texas, Louisiana and Georgia.

Today, pecans remain as all-American as ever. They live naturally nowhere else in the world, thriving in the moist but well-drained soil along the riverbanks of Texas and Oklahoma, stretching as far north as Illinois. Over the past few decades, production has expanded far outside a pecan's native range, to 15 states. And while the trees have changed little over the centuries, modern demand has. Spurred largely by a rising appetite for pecans in China, production has gone nuts over the past decade. Now about 300 million pounds of pecans are produced annually.

But the trees are paying a price for their newfound popularity. Crops are increasingly attacked by pecan scab, a disease that is caused by a fungus. "We're pushing the trees harder, and that makes it worse," said Charles Rohla, Ph.D., director of the Center for Pecan and Specialty Agriculture (CPSA) at the Noble Foundation. The scab spores only strike actively growing tissue. Therefore, when the trees maximize production, they also become more vulnerable to disease.

A close up of a pecan leafA close up of a pecan leaf reveals a small piece of the intricate nature of researchers' work to better understand the plant.

More than two thirds of pecan varieties are now affected by this disease, especially those grown in warm, humid states like Georgia. Plus, to get the most economical use out of their land, growers will plant trees close together, which limits air circulation and enhances the moist, muggy conditions the fungus likes. Some southeastern growers spray fungicide multiple times a year to keep the disease under control.

This real-world production problem has become the focus of a new Noble Foundation research project that draws together the organization's expertise from the laboratory to the orchard. The effort received a boost in October, when the National Institute of Food and Agriculture awarded a historic, $4.3 million grant to Noble and five other institutions to develop resources for pecans and study some of the toughest problems facing pecans.

Pecan scab fungusPecan scab fungus grows in a petri dish for research purposes.

Answering the Fungus Question

Along with new ways to tackle pecan scab, Noble Foundation scientists are discovering other ways to keep trees productive and healthy for decades by investigating pecan root structure, biology and genetics.

These scientific efforts have already led to at least one surprising discovery about the pecan scab culprit. Traditional wisdom maintains that the fungus reproduces asexually – meaning it grows from genetically identical spores dispersed by the wind and rain.

Yet a team led by Carolyn Young, Ph.D., an associate professor in the Forage Improvement Division, has found evidence that the fungus can also reproduce sexually in laboratory experiments, recombining its genetic material with other scab isolates. The next step is to determine if and when this occurs in the wild.

Once confirmed, this research will help explain why pecan scab has preferences for certain types of pecans and how fungicide resistance can spread. "Everybody we talk to says it's an asexual fungus," Young said. "But one of the fundamental assumptions may not be true. This impacts our understanding of the pathogen's biology and may influence how we manage the disease."

Will ChaneyWill Chaney, a research associate in the Center for Pecan and Specialty Agriculture, moves young pecan trees that will become part of studies into a shaded area to protect them from direct sun.

Fingerprinting Pecans

Some varieties of trees are naturally less vulnerable to scab. It's just a matter of identifying them. The laboratory of Maria Monteros, Ph.D., an associate professor who leads a genomics laboratory in the Forage Improvement Division, is working to establish a genetic profile or "DNA fingerprint" unique to each variety. Currently, she says, growers tend to identify varieties based on observations of the nut characteristics including size and shape. But visual identification can be challenging, given that the water or nutrient availability, or the presence of disease, can change the appearance of the nuts. Also, young trees don't produce nuts at all for several years.

A more precise – and objective – way to identify a tree is through its DNA. As of now, Monteros is building a genetic catalog of about 60 different tree samples. "The focus of this work is to provide growers with a tool to determine which trees they have in their orchard," she explained. "If certain trees are susceptible to pecan scab, knowing this information would help develop orchard management practices. Also, if a grower has a tree that was exceptionally productive, and wanted to get more of them, we could develop a genetic fingerprint of that tree." The concept of DNA fingerprinting has already been applied to other crops, she said, including coffee. In that case, coffee grains are visually difficult to distinguish, but those with better flavor are in higher demand and often get a price premium. Her team is also working on identifying DNA segments that provide resistance to pecan scab disease.

In addition to genetic exploration, the laboratory of Elison Blancaflor, Ph.D., a professor in the Plant Biology Division, is starting to look at ways by which microscopy equipment at the Noble Foundation can provide insight into root development in pecans, an area where very little is known. Propagation of elite pecan varieties requires grafting of the upper part of a tree to a compatible rootstock. Blancaflor and Rohla want to understand how the root system of pecans contribute to healthy and more robust rootstocks.

Other questions about fundamental pecan biology also remain. Rohla is exploring why all trees cycle in unison between low and high production on the same years. A heavy production followed by lighter bearing the following year is a hallmark of the crop's natural rhythm. But quantity sacrifices quality. "Whenever we have a big crop, it takes so many nutrients from the tree resulting in poor quality," Rohla said. "The following year the crop is smaller as a result of the stress the tree endured the year before."

That makes sense, but what's not known is why each tree, no matter what cycle it begins with, eventually produces in sync with every other tree in the country. If production years were mixed, farmers could have more predictable prices – especially as worldwide demand rises. "Because they are native to North America, most countries don't even know what a pecan is," Rohla said.

They soon will. As more parts of the world discover health benefits and tastiness of the pecan, Noble researchers will continue to support a food with roots deeper than America itself.

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