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© 2004 Plant Management Network.
Accepted for publication 30 June 2004. Published 7 July 2004.


First Report of Powdery Mildew of Hypericum perforatum (St. Johnís-wort) Caused by an Anamorphic Microsphaera Species in the Pacific Northwest


Dean A. Glawe, Plant Pathologist, Puyallup Research and Extension Center, Washington State University, 7612 Pioneer Way East, Puyallup 98371-4998


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


Glawe, D. A. 2004. First report of powdery mildew of Hypericum perforatum (St. Johnís-wort) caused by an anamorphic Microsphaera species in the Pacific Northwest. Online. Plant Health Progress doi:10.1094/PHP-2004-0707-01-HN.


Hypericum perforatum L. (St. Johnís-wort) has been valued as a medicinal herb for centuries. During the 2002 growing season a specimen plant of H. perforatum with powdery mildew symptoms was observed in a demonstration garden in Kenmore, King County, WA. This report provides information on symptoms of the disease, morphological features useful in characterizing the causal agent, and taxonomic aspects of the fungus.

Signs of the disease included effuse-to-dense white patches of mycelium, primarily on adaxial leaf surfaces (Fig. 1). The fungus formed hyphae with lobed appressoria (Fig. 2). Conidiophores bore single conidia and included cylindrical foot cells (Fig. 3) measuring 13.5-16.5 (-18.5) ◊ 4.5-6.5 Ķm. Fibrosin bodies were absent. Conidia were cylindrical, hyaline, with cell walls exhibiting faint reticulations becoming more pronounced when conidia dried and collapsed (Fig. 4), and measured (29-) 31.5-36 (-38) ◊ (10.5-) 12-17 (-17.5) Ķm. The teleomorph was lacking. A voucher specimen collected October 2, 2002 was deposited with the Mycological Herbarium of the Plant Pathology Department at Washington State University.


     
 

Fig. 1. Symptoms and signs of powdery mildew on H. perforatum.

 

Fig. 2. Lobed appressorium produced on H. perforatum.

 

     
 

Fig. 3. Foot cell produced on H. perforatum.

 

Fig. 4. Conidium produced on H. perforatum.

 

Production of lobed appressoria, conidiophores bearing single cylindrical conidia lacking fibrosin bodies, and straight conidiophore foot cells indicates a strong affinity to Braun and Takamatsuís (2) broadened concept of the genus Erysiphe, which includes the genus Microsphaera (1,2). Morphological features of the fungus resembled strongly those of Microsphaera hypericacearum U. Braun which occurs widely throughout the range of the host including North America (1). However, conidiophore foot cells were smaller than described for M. hypericacearum (1). Microsphaera hyperici Yu & Lai was reported from Hypericum patulum Thunb. in Asia but Braunís monograph does not include information on the anamorph (1). Microsphaera hypericacearum was distinguished from M. hyperici on the basis of ascocarp features (1) which were not seen in this study. Farr et al. (3) included a single powdery mildew occurring on the genus Hypericum in the U.S., listing Erysiphe cichoracearum on an unidentified Hypericum species from an unknown location. The present fungus is clearly distinguishable from E. cichoracearum, which exhibits conidiophores with chains of conidia as well as appressoria which generally are nipple-shaped (1) unlike the lobed appressoria described here. Based on the geographical range and anamorph morphology reported (1) for M. hypericacearum, the present fungus could be assigned to it, although the size of foot cells might exclude it. Unfortunately, although ascocarp features help distinguish M. hypericacearum, teleomorphs often are lacking in powdery mildew fungi in western Washington (4) and so may never be found in this fungus in this region. The available morphological evidence does suggest that, if a teleomorph is someday found, it also will exhibit characteristics of Microsphaera.

Braun and Takamatsu (2) proposed relegating Microsphaera to synonymy with Erysiphe. If taxonomists accept this proposal, the name for M. hypericacearum would become Erysiphe hyperici (Wallr.) S. Blumer, and the name for M. hyperici Yu & Lai would be Erysiphe hypericicola U. Braun & S. Takamatsu (2).

By the end of the growing season, the diseased plant exhibited extensive mycelial growth and sporulation on nearly all of its leaves. Further work would be useful in determining whether such extensive fungal activity could affect production of pharmacologically active compounds in commercially-grown plants.


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 sequencesósome taxonomic consequences. Schlechtendalia 4:1-33.

3. Farr, D. F., Bills, G. F., Chamuris, G. P., and Rossman, A. Y. 1989. Fungi on plants and plant products in the United States. American Phytopathological Society, St. Paul, MN.

4. Glawe, D. A. 2003. First report of powdery mildew of Nandina domestica caused by Microsphaera berberidis (Erysiphe berberidis) in the Pacific Northwest. Online. Plant Health Progress doi:10.1094/PHP-2003-1023-01-HN.