First Report of Leaf Rust on Southern Highbush Blueberry caused by Thekopsora minima in California

Leaf rust symptoms have been intermittently observed on southern highbush blueberry (Vaccinium corymbosum L. x V. darrowii Camp) in California (CA). Rust symptoms were first seen in 2010 near Carpinteria, CA (M. Kong, pers. comm.) and then in July 2016 on leaves of three blueberry cultivars Emerald, Jewel, and Snow Chaser in a field trial in Watsonville. In early 2017, leaf rust was observed again in Watsonville on the same cultivars and on various 3-year-old selections. Symptoms were found primarily on older leaves. Purplish-brown necrotic spots were present on the adaxial leaf surface, while the abaxial leaf side exhibited orange to yellow colored uredinia. Under a light microscope, the uredinia were clustered on the leaf surface, ovoid in shape, and approximately 100 – 400 μm in diameter. Certain rust species possess ostiolar cells projections near the ostiole of the uredinium, which aid in identification, however, ostiolar cells were not observed. Each uredinium contained numerous ellipsoid urediniosp...