Appearance of Powdery Mildew of Wheat Caused by Blumeria graminis f. sp. tritici on Pm17-Bearing Cultivars in North Carolina.

Pm17 is a gene for resistance to powdery mildew caused by Blumeria graminis (DC.) E.O. Speer f. sp. tritici. The gene was first confirmed in the wheat-rye translocation cultivar Amigo (1). In Amigo, the translocation is T1AL-1RS and the 1RS arm has the gene Pm17. In the mid-Atlantic United States, at least two widely deployed soft red winter wheat (Triticum aestivum L.) cultivars, McCormick (2) and Tribute (3), possess Pm17 inherited from Amigo. Before 2009, low frequencies of mostly intermediate virulence to Pm17 were detected among isolates from research plots of highly susceptible cultivars (4), but Pm17-bearing cultivars remained immune to mildew in the field. In April 2009, moderately severe powdery mildew was observed for the first time throughout plots of McCormick, Tribute, and other cultivars in both Kinston and Raleigh, NC. At Kinston, Pm17 virulence was observed at two research sites, separated by approximately 10 km, throughout plots of Amigo, McCormick, Tribute, and the hard red winter wheat cultivar TAM 303, which also contains Pm17. In the same month, virulence to Pm17 was observed in Raleigh throughout rows and plots of Amigo and TAM 303. In Kinston and Raleigh, ratings of powdery mildew severity on the Pm17-containing cultivars were 4 or 5 on a scale of 0 to 9, with 0 being the absence of mildew pustules and 9 the most severe mildew infection. Mildew was observed on leaves of all ages. Mildewed leaves were collected from field plots of all four Pm17-bearing cultivars, and an assay to confirm Pm17 virulence was conducted in the laboratory. Mixed-isolate cultures were derived from the leaves and a detached-leaf assay was performed using Amigo, which is the standard Pm17 differential (4). All tested cultures were fully to moderately virulent on Pm17 and all were fully virulent on the susceptible control Chancellor. In the field, chasmothecia (sexual fruiting bodies) were observed on Pm17-bearing cultivars. Together with the quantitatively varying Pm17 virulence detected in the laboratory assay, this suggests that multiple strains of Pm17-virulent B. graminis f. sp. tritici may be present in the field, although that has not yet been demonstrated. Pm17 has protected wheat from powdery mildew over a substantial area in the mid-Atlantic United States. The loss of Pm17 is the most important virulence shift in the U.S. wheat powdery mildew population since Pm4a became ineffective around 2002. Isolates virulent to Pm17 can be expected to appear and multiply in wheat-producing states of the mid-Atlantic United States, including Delaware, Maryland, Virginia, North Carolina, South Carolina, and Georgia. Thus, the urgency of developing and releasing wheat cultivars with other sources of effective mildew resistance is heightened. References: (1) B. Friebe et al. Euphytica 91:59, 1996. (2) C. A. Griffey et al. Crop Sci. 45:416, 2005. (3) C. A. Griffey et al. Crop Sci. 45:419, 2005. (4) R. Parks et al. Plant Dis. 92:1074, 2008.