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2005 Plant Management Network.
Accepted for publication 27 September 2005. Published 17 October 2005.


First Report of Powdery Mildew on Caragana arborescens and Caragana grandiflora in Alaska caused by Microsphaera (Erysiphe) palczewskii


Dean A. Glawe, Plant Pathologist, Puyallup Research and Extension Center, Washington State University, 7612 Pioneer Way East, Puyallup 98371-4998; and Gary A. Laursen, Research Professor, Institute of Arctic Biology, University of Alaska, 305A Bunnell, Fairbanks 99775-6100


Corresponding author: Dean A. Glawe. glawe@wsu.edu


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.


During August 2004, the authors surveyed sites in and near Fairbanks, AK for powdery mildew diseases and found a powdery mildew previously unreported in Alaska on two specimen plants of the introduced ornamental species Caragana arborescens Lam. (Siberian pea tree) and C. grandiflora DC. The causal agent was determined to be Microsphaera palczewskii Jacz. This report provides the first documentation of M palczewskii in AK and includes information on the morphology and taxonomy of this species.

Signs of the disease included effuse to dense patches of white to grayish-brown mycelia on leaf surfaces. The fungus formed superficial hyphae with lobed appressoria (Fig. 1); conidiophore foot cells (Fig. 2) were cylindrical and measured (15-) 15.5-21.5  (-21.5) (4.5-) 5.5-7 (-8) m; conidia were short-cylindrical to barrel-shaped, formed singly, lacked fibrosin bodies, and measured (22.5-) 24.5-34.5 (-36.5) (10-) 11-16 (-17) m. Mature chasmothecia were found in the specimen collected from C. arborescens and were dark brown to black, convex, 100-125(-155) m in diameter; dichotomously-branched chasmothecial appendages (Fig. 3) measured (95-) 155-270 (-325) m; chasmothecia contained multiple asci that were short-stipitate to saccate and measured (54.5-) 56-63 (-64.5) 31.5-43.5 (45) m. Asci (Fig. 4) contained 6 to 8 ovoid, pale yellowish ascospores measuring (15.5-) 16-22 (-24) 10-14.5 m. Voucher specimens were deposited with the Mycological Herbarium of the University of Alaska, Fairbanks (ALA).


     
 

Fig. 1. Appressorium (arrow) on leaf surface of M. palczewskii.

 

Fig. 2. Conidiophore of M. palczewskii forming a single conidium.

 

     
 

Fig. 3. Dichotomously branched apex of chasmothecial appendage formed by M. palczewskii.

 

Fig. 4. Ascus with ascospores formed by M. palczewskii.

 

The host genus and morphological characteristics of the fungus matched the features described by Braun (1) for M. palczewskii, also designated as Erysiphe palczewskii U. Braun & S. Takamatsu (2). The fungus was described in 1927 from the Russian Far East and some 50 years later was noted in Eastern Europe where it is now widespread (3). The first report of this fungus in North America was made in 2003 (4) from C. arborescens in northern ID and eastern WA. The present account appears to be only the second report from North America. Because of the distinctive, highly-branched chasmothecial appendages (Fig. 3) the fungus is easily distinguished from other Microsphaera species. It therefore seems unlikely that past researchers in AK confused it with another fungus. Present evidence suggests that, similar to the case in Europe, the fungus may have been brought into AK along with one or more of its introduced hosts. However, the route and means by which the fungus entered AK is unclear. Occurrence of the teleomorph in AK, and its putative origin in a region with similarly severe winter conditions (3), suggests that perennation of the fungus can occur in AK conditions. Perennation in infected buds occurs in this species (3) and might offer an alternative to ascocarps as a means of perennation in AK.

Given the restricted host range, known history of dissemination across Europe, and distinctive morphology, this species may have potential as a model organism to assess dissemination of exotic powdery mildews in North America through human activities or other means.


Literature Cited

1. Braun, U. 1987. A monograph of the Erysiphales (powdery mildews). Beih. Nova Hedwigia 89:1-700.

2. Braun, U., and Takamatsu, S. 2000. Phylogeny of Erysiphe, Microsphaera, Uncinula (Erysipheae) and Cystotheca, Podosphaera, Sphaerotheca (Cystotheceae) inferred from rDNA ITS sequencessome taxonomic consequences. Schlechtendalia 4:1-33.

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.