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Oats Crown Rust Inoculum Abundant This Year

Written collaboratively by Emmanuel Byamukama, Melanie Caffe, Shaukat Ali, and Jonathan Kleinjan.

About Crown Rust

Green leaves on a buckthorn tree covered with crusty spots of bright orange crown rust inoculum.
Figure 1. Crown rust inoculum developing on buckthorn leaves.

Crown rust is the most important fungal disease of oats in South Dakota. In years with heavy disease pressure, susceptible cultivars can have over 80% yield loss due to crown rust. The crown rust pathogen requires an alternate host in order for it to complete its life cycle and the buckthorns serve as an alternate host for this pathogen. The presence of crown rust inoculum on buckthorns can be an indication of the likely risk for crown rust to develop during the growing season. Buckthorns scouted recently were loaded with crown rust inoculum (Figure 1).

Unlike other cereal rusts, the crown rust pathogen overwinters in South Dakota on buckthorns. The pathogen survives as hardened spores called teliospores on oats residue and on other grass hosts. In spring, the teliospores produce another type of spores called basidiospores that infect the buckthorn (Figure 1). The basidiospores on the buckthorn produce another type of spores called aeciospores and it is the aeciospores that infect oats. Infection of oats can produce secondary spores called uredospores that infect other plants or leaves. The cool and wet spring weather conditions have favored crown rust inoculum to build-up. The high levels of inoculum present now might imply a moderate to high risk for crown rust to develop on oats.

Management Strategies

Two varieties of oats planted side-by-side. The variety on the left is green and healthy. The variety on the right is brown and yellow, symptoms of severe crown rust.
Figure 2. Comparison of varieties reaction to crown rust. Deon (left) is showing no crown rust symptoms compared to horsepower (right), which is susceptible to crown rust.

The best management practice for crown rust is planting resistant cultivars. Two new cultivars, Saddle and Warrior , released by the South Dakota Agricultural Experiment Station in 2017 and 2018, respectively, are resistant to crown rust. In addition, cultivar Sumo and Deon (Figure 2) exhibit good level of crown rust resistance. Crown rust resistance is one of the traits emphasized by the SDSU oat breeding program. The South Dakota Oat Variety Trial results provide information on disease resistance, agronomic performance, and end-use quality characteristics for cultivars adapted to South Dakota. Growers are encouraged to use this information when making cultivar selection decisions.

The delayed planting this year may coincide with crown rust inoculum accumulation. If the weather continues to be conducive to infection, we expect to see high level of infection on susceptible cultivars this year. In-season crown rust management can be achieved by a well-timed fungicide application at flag leaf growth stage. Earlier fungicide application before flag leaf emergence will leave the flag leaf unprotected yet the flag leaf contribute the most to yield. Moreover, because of heavy inoculum in the area, an early fungicide will do little to reduce inoculum. Since two fungicide applications in oats are not recommended, the best option would be to scout and properly time a fungicide at flag leaf emergence. Previous research at SDSU has shown a gain of up to 60 bushel per acre from properly timed fungicide application in susceptible oat varieties. A few fungicides registered for oats in South Dakota include Stratego YLD, Caramba, Quilt Xcel, and Priaxor. These have good efficacy against crown rust.