Poster Session: New and Emerging Diseases-Fungi and Oomycetes

401-P

Observations on blueberry leaf rust, caused by Thekopsora minima, in Michigan.
A. C. SCHILDER (1), T. D. Miles (1), J. M. Gillett (1), R. W. Sysak (1)
(1) Michigan State University, East Lansing, MI, U.S.A.

Over the past few years, leaf rust has become widespread in highbush blueberries in Michigan, sometimes resulting in premature defoliation. Symptoms are most common in late summer and fall. Lesions are small and yellow at first and then turn dark brown with yellowish orange uredinia on the abaxial surface. Uredinia are dome-shaped, erumpent, and 100 to 400 µm in diameter. Urediniospores are yellow, obovate, and 19 to 25 × 16 to 20 µm (ave. 22 × 18 µm). Spore walls are hyaline, echinulate, and 1 to 1.5 µm thick with obscure germ pores. Based on morphology and DNA homology with known isolates, the pathogen was identified as Thekopsora minima. This fungus has also been reported on highbush blueberry in Japan, South Africa, Delaware, and New York. In Michigan, the pathogen was previously assumed to be Pucciniastrum vaccinii. Hemlock (Tsuga canadensis), the alternate host, is a common and valuable conifer in the Michigan landscape. No aecia were seen on hemlocks near infected fields but it may have been too late in the season to observe them. Susceptibility of 12 blueberry cultivars was assessed in a young planting, Cultivar Bluecrop was the most susceptible, followed by Liberty, Rubel and Jersey. Aurora, Berkeley, and Huron were the most resistant. A fungicide efficacy trial showed that Pristine (pyraclostrobin + boscalid) had excellent activity; Indar (fenbuconazole), Orbit (propiconazole), and Quash (metconazole) had good activity. Serenade (Bacillus subtilis), Sonata (B. pumilis) and Bravo (chlorothalonil) had moderate activity.