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© 2013 Plant Management Network.
Accepted for publication 11 February 2013. Published 27 March 2013.

First Report of Powdery Mildew (Microsphaera palczewskii) on Siberian Peashrub in Montana

Erin Lonergan, Graduate Student, and Linnea G. Skoglund, Plant Diagnostician, Department of Plant Sciences and Plant Pathology, Montana State University, Bozeman, MT 59717

Corresponding author: Linnea G. Skoglund.

Lonergan, E., and Skoglund, L. G. 2013. First report of powdery mildew (Microsphaera palczewskii) on Siberian peashrub (Caragana arborescens) in Montana. Online. Plant Health Progress doi:10.1094/PHP-2013-0327-01-BR.

Samples of the introduced ornamental shrub Caragana arborescens Lam. infected by a powdery mildew were submitted to the Schutter Plant Disease Diagnostic lab from Ravalli and Gallatin counties, Montana in 2012. A powdery mildew has been observed on this shrub since 2005 on samples from Judith Basin, Teton, Flathead, Gallatin, and Pondera counties. Siberian peashrub (C. arborescens) is a common ornamental shrub in Montana and is often planted as hedges and in windbreaks. The causal agent of the powdery mildew affecting these samples of C. arborescens was determined to be Microsphaera palczewskii Jacz. [synonym Erisyphe palczewskii (Jacz.) U. Braun & S. Takam.] based on morphological characteristics (3).

On the C. arborescens specimens, white to grayish mycelium was observed covering the leaf surface; minimal mycelial growth was observed on petioles and none was observed on stems. Conidiophore foot cells were hyaline and cylindrical measuring 16-21 × 5-7 µm; conidia were born singly, lacked fibrosin bodies, and measured 25-32 × 7-12 µm. Both immature and mature chasmothecia were observed on leaf surfaces, most abundantly on the abaxial surface of the leaf (Fig. 1). Mature chasmothecia were round, dark brown to black, consisted of irregular polygonal cells, and measured 100-122 µm in diameter. Chasmothecia had 5 to 10 straight, hyaline, dichotomously branched appendages that measured 127-234 µm in length; appendages were branched at a wide angle (Fig. 2). Chasmothecia contained multiple thick-walled asci that measured 48-54 × 34-43 µm (Fig. 3). Asci contained three to six ellipsoid-ovoid, hyaline- pale yellow, single-celled ascospores measuring 14-17 × 9-11 µm (Fig. 4).


Fig. 1. Chasmothecia of M. palczewskii on the abaxial surface of a C. arborescens leaf. Photo by E. Barge.


Fig. 2. Dichotomously branched apex of a chasmothecial appendage of M. palczewskii. Photo by E. Barge.


Fig. 3. Chasmothecium of M. palczewskii releasing multiple asci. Photo by E. Barge.


Fig. 4. Asci of M. palczewskii containing multiple ascospores. Photo by E. Barge.

M. palczewskii was first reported in 1927 from far-eastern Russia and is now widespread throughout much of Europe (3). It was first reported as occurring on C. arborescens in North America in 2003 from collections taken in eastern Washington and northern Idaho (4). In 2005, E. palczewskii was observed infecting both C. arborescens and C. grandiflora DC in Alaska (1). In 2006, the first record of E. palczewskii occurring on C. arborescens east of the Rocky Mountains was reported from Minnesota and North Dakota (2). Due to the geographical distance between the Minnesota and North Dakota collections, Glawe et al. (2006) hypothesized that this fungus may indeed be widespread on C. arborescens in Central and possibly Eastern North America. This is believed to be the first report documenting M. palczewskii on C. arborescens in Montana.

Literature Cited

1. Glawe, D. A., and Laursen, G. A. 2005. First Report of powdery mildew on Caragana arborescens and Caragana grandiflora in Alaska caused by Microsphaera (Erysiphe) palczewskii. Online. Plant Health Progress doi:10.1094/PHP-2005-1017-01-BR.

2. Glawe, D. A., Stack, R. W., and Walla, J. A. 2006. First report of powdery mildew on Caragana arborescens in Minnesota and North Dakota caused by Microsphaera (Erysiphe) palczewskii. Online. Plant Health Progress doi:10.1094/PHP-2006-0117-01-BR.

3. Heluta, V. P., and Minter, D. W. 1998. Microsphaera palczewskii. IMI Descriptions of Fungi and Bacteria 1375:1-3.

4. Nischwitz, C., and Newcombe, G. 2003. First report of powdery mildew (Microsphaera palczewskii) on Siberian pea tree (Caragana arborescens) in North America. Plant Dis. 87:451.