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Oat Crown Rust Risk Low This Year

Oat leaf with very low crown rust.
Figure 1. Oat leaf with very low crown rust. Oat was starting to head in this field, hence it may not be significantly impacted by crown rust.

Written with contributions by Emmanuel Byamukama, former SDSU Extension Plant Pathologist, Melanie Caffe and Shaukat Ali.

Oat in South Dakota ranges from flag leaf to heading. Oat fields scouted were found with very low levels of crown rust (Figure 1). This is primarily due to warm and dry weather conditions that we are currently experiencing. 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 oat growing season. Buckthorns scouted recently had very low levels of the primary crown rust inoculum compared to previous years (Figure 2).

Crown rust inoculum observed on buckthorn leaves during two different years.
Figure 2. Crown rust inoculum observed on buckthorn leaves on June 8, 2021 (A) compared to about the same time on June 10, 2019 (B).

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 2). The basidiospores on the buckthorn produce another type of spores called aeciospores and it is the aeciospores (primary inoculum) that infect oat. Infection of oat can produce secondary spores called uredospores that infect other oat leaves. The dry and warm weather conditions have limited crown rust inoculum to build-up hence the low levels of this disease currently in oat.

 A fungicide at flag leaf is usually recommended to manage crown rust in oat. Given the low levels of the disease and the advancement of the crop, a fungicide may not be needed in oat this year. Planting a crown rust resistant oat variety is the best practice to manage crown rust. Two new cultivars, Saddle and Warrior are resistant to crown rust. In addition, cultivars Rushmore, Sumo and Deon exhibit good level of crown rust resistance. Ratings of oat varieties against crown rust for 2020 can be found in the latest Oat Variety Trial Results.

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