Crown rust (Puccinia coronata Corda f.sp. avenae) can devastate oats (Avena sativa). Oxidative stress is part of the resistance mechanism in several pathosystems, but in the oat–crown rust system, it is unclear, especially regarding partial resistance. We evaluated the effects of P. coronata on oxidative stress in oat cultivars: URS 21 (partially resistant), Leggett (race‐specific resistant), URS22 and Clintland 64 (susceptibles). Seedlings and plants were inoculated with P. coronata uredospores. Cultivars were assessed for antioxidant enzyme activity and the reactive oxygen species (ROS) hydrogen peroxide and superoxide. Due to the importance of the partial resistance of URS21, this cultivar and URS 22 were also appraised for total phenolics and the relative expression of oxidative stress genes. Postinoculation, Leggett and URS 21 showed no increased peroxide levels. The susceptible cultivars increased ROS and ascorbate peroxidase activity. Clintland 64 increased also catalase activity, whereas URS 22 increased glutathione reductase and the expression of genes encoding antioxidant enzymes. URS 21 showed almost no antioxidant enzyme induction. Shortly after inoculation, URS 21 showed increased expression of genes encoding lipoxygenase and peroxidase. Cultivars URS 21 and Leggett accumulated cell wall fluorescent compounds, phenolics being detected in the former. Oxidative stress appears not to cause the hypersensitive response in this pathosystem, but late ROS accumulation did occur in the susceptible cultivars. Cultivar URS 21 may, differently from other known mechanism to date, reduce ROS accumulation by increasing the level of phenolics, resulting in later pathogen and cell death, showing non‐specific resistance to races of the pathogen also at seedling stage.

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