New oat lines ready for active duty against crown rust disease

A team of Agricultural Research Service (ARS) and university scientists has released two new oat germplasm lines to shore up the cereal crop’s defenses against its most devastating fungal disease, known as “crown rust.”

The team specifically created the oat lines so that they can be crossed with elite commercial varieties to fortify them with new genetic sources of resistance to crown rust, which is caused by the fungus Puccinia coronata f. sp. avenae. Crown rust is a plague of oat worldwide and can inflict grain yield losses of up to 50% in unprotected crops.

The team announced its development of the resistant oat germplasm lines—dubbed CDL-111 and CDL-167—in the May 2024 issue of the Journal of Plant Registrations, culminating more than 25 years of germplasm screening, plant genetic mapping, selective breeding and evaluation in greenhouse and field trials.

“Currently, the majority of the oat varieties with rust resistance carry a gene or two for resistance (often referred to as seedling resistance) to a specific isolate of crown rust,” said Shahryar Kianian, a co-author on the journal paper and research leader of the ARS Cereal Diseases Laboratory in St. Paul, Minnesota.

However, the crown rust fungus is a genetically diverse pathogen and highly adept at evolving into virulent new forms, called races. This can happen so quickly that the average productive life of an oat variety with seedling resistance is between three and five years, necessitating the use of chemical fungicides in conventional production systems.

Unchecked, the fungus infects the lower leaves and sometimes the sheafs of vulnerable oat plants, forming round- to oval-shaped pustules packed with masses of orangish spores that can be carried away by wind or rain. Damage to leaves can diminish photosynthesis and disrupt the movement of sugars from the leaves to developing grain, shriveling it and reducing feed value.

To even the odds in the oat plant’s favor, the team resorted to a plant breeding strategy called “gene stacking” (or “pyramiding”). A key part of that strategy involved making a series of crosses between a cultivated oat variety and wild relatives, one known as lopsided oat, which carry genes for “adult plant resistance.”

“Adult plant resistance, sometimes referred to as ‘slow rusting,’ provides the oat plant some immunity—but not complete immunity,” Kianian said. “In this case, the selection pressure on the pathogen to change is reduced, and the plant is not damaged much so that it can still produce and yield grain for the growers.”

All told, the team stacked offspring plants derived from crosses with three genes for adult plant resistance to crown rust. They then subjected the offspring plants to a trial by fire, of sorts, starting in 2020. In essence, this involved growing them in nursery plots of common buckthorn, a secondary host for crown rust and known source of outbreaks. In the plots, under intense pressure from the disease, two lines of offspring plants consistently fared better than the others, namely, CDL-111 and CDL-167.

The sturdy oat lines have since been propagated for their seed, which is available for use in variety development programs under a material transfer agreement with ARS, Kianian said. This is to ensure the effectiveness of the gene-stacking strategy if the oat lines are crossed with commercial varieties—regardless of whether they already possess seedling resistance to crown rust.

By adhering to this requirement, plant breeders can arm elite oat varieties adapted to particular production regions with a one-two punch against the crown rust fungus—a “jab” via seedling resistance and a “right hook” with adult plant resistance.

“For this, we are also providing molecular markers linked to the three genes that can be used in selecting the lines that carry them,” added Kianian, who collaborated with Eric Nazareno and Kevin Smith—both with the University of Minnesota—Melanie Caffe (South Dakota State University), Roger Caspers (ARS), Howard Rines (ARS, deceased) and Marty Carson (ARS, deceased). Carson started some of the oat work 20 years ago, continuing much of it after retirement, Kianian noted.