Figure 1. Buckthorn leaves with crown rust symptoms. The crown rust pathogen must infect buckthorn first before moving to oats.
Crown rust is the most devastating disease of oats in South Dakota. This disease can develop as early as the jointing stage and can lead to heavy grain yield loss in susceptible cultivars. The pathogen which causes crown rust, Puccinia coronata f. sp. avenae, survives on crop residue and on wild grasses but first must infect buckthorn plants before moving onto oats. Buckthorn is an alternate host of this pathogen which is required in order for this pathogen to complete its life cycle. Buckthorn scouted around Brookings were found with spores of the crown rust pathogen (Figure 1). Infections observed on buckthorn is an indicator of the presence of inoculum for the crown rust pathogen.
Crown rust is favored by mild to warm temperatures between 68-77 F and wet conditions. Wind facilitates movement of spores from oat stubble to buckthorns and from buckthorns to oats. Wind and rain splash also spread the spores within the oat field (secondary spread). Dry and hot weather (>86 F) prevent infection from taking place. Infections which take place before flag leaf emergence cause the greatest yield loss.
Crown rust can be best managed through planting resistant cultivars. Crown rust resistant cultivars are available. However, the crown rust pathogen can change races quickly resulting in a cultivar that was resistant and has now become susceptible. This usually happens after more than five years from the time the resistant cultivar is being grown. Growers should keep records of the cultivars planted and note if a previously resistant cultivar is now showing moderate to severe symptoms. The SDSU Oats Breeding Program is developing oat cultivars with crown rust resistance as one of the traits emphasized.
A well-timed fungicide application is effective against crown rust. Fungicide application should coincide with flag leaf appearance. Earlier fungicide application before flag leaf emergence will leave the flag leaf unprotected yet the flag leaf and the leaf below flag leaf contribute the most to yield. Moreover, because of heavy inoculum in the area, an early fungicide will do little to reduce inoculum. There are not many fungicides registered for oats in South Dakota, but a few that are registered include Stratego YLD, Caramba, Quilt Xcel, and Priaxor.