USDA researchers have developed two new oat germplasm lines to support the cereal crop’s defenses against one of the most devastating fungal diseases, known as “crown rust.”

A team of Agricultural Research Service (ARS), the USDA’s chief scientific in-house research agency and university scientists have created the oat lines so that they can be crossed with 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.

Germplasm screening 

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 Registration. This culminated in more than 25 years of germplasm screening, plant genetic mapping, selecive 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,” says Shahryar Kianian, a co-author on the journal paper and research leader of the ARS Cereal Diseases Laboratory in St. Paul, Minnesota, US.

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.

If left unchecked, the fungus can infect 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 reduce photosynthesis and disrupt the movement of sugars from the leaves to developing grain, shriveling it and reducing feed value.

Crossing cultivated and wild varieties

To even the odds in the oat plant’s favor, the team resorted to a plant breeding strategy called “gene stacking” (or “pyramiding”), which involves making a series of crosses between a cultivated oat variety and wild relatives, one known as lopsided oats, 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 notes. “In this case, the selecion 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.”

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 a 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 states. 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.

Arming elite oat varieties 

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,” adds Kianian.