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© 2007 Plant Management Network.
Accepted for publication 4 September 2007. Published 12 November 2007.


First Report of Powdery Mildew of Fringed Willowherb (Epilobium ciliatum) Caused by Podosphaera epilobii in North America


Steven T. Koike, Plant Pathology Farm Advisor, University of California Cooperative Extension, Salinas 93901; and Dean A. Glawe, Plant Pathologist, Department of Plant Pathology, Washington State University, and Professor, College of Forest Resources, Box 352100, University of Washington, Seattle 98195


Corresponding author: Steven T. Koike.  stkoike@ucdavis.edu


Koike, S. T., and Glawe, D. A. 2007. First report of powdery mildew of fringed willowherb (Epilobium ciliatum) caused by Podosphaera epilobii in North America. Online. Plant Health Progress doi:10.1094/PHP-2007-1112-01-BR.


Fringed willowherb (Epilobium ciliatum Raf.), also known as American willowherb, American willowweed, and northern willowherb, is a summer annual, ruderal plant that is found in diverse habitats (3). Willowherb grows in natural vegetation areas, pastures, roadsides, and disturbed soils. The plant is also a weed in landscapes and agricultural fields. Because the tufted seeds are readily wind dispersed, the weed can be widely distributed. In 2006 and 2007, a powdery mildew was found on E. ciliatum growing in commercial vegetable fields and greenhouses in the Salinas Valley (Monterey Co.), CA. The fungus was identified as Podosphaera epilobii (Wallr.) U. Braun & S. Takam. This report is the first documentation of P. epilobii on E. ciliatum in North America.

Mycelium was amphigenous, primarily on adaxial leaf surfaces, and also developed extensively on stems. Powdery mildew did not appear to inhibit growth of the plant, and the main symptom on diseased willowherb was slight buckling and twisting of leaves. Compared to diseased willowherb plants found in outdoor fields, powdery mildew growth was more extensive on the willowherb collected from greenhouse environments. Mycelium produced appressoria (Fig. 1) that were indistinct to somewhat papillate. Conidiophores (Fig. 2) produced chains of conidia and formed cylindrical, straight foot cells (Fig. 3) that measured (37-)47.5-84(-86) × 9.5-11.5 µm. Conidia (Fig. 4) were ellipsoid-ovoid, hyaline, contained fibrosin bodies, and measured (25-)26-31(-32) × (14-)15-18.5(-19) µm. The teleomorph was not observed.


     
 

Fig. 1. Appressorium (arrow) formed by Podosphaera epilobii.

 

Fig. 2. Conidiophores of Podosphaera epilobii forming chains of conidia.

 

     
 

Fig. 3. Conidiophore of Podosphaera epilobii with cylindrical, straight foot cell and chain of conidia.

 

Fig. 4. Conidia of Podosphaera epilobii containing fibrosin bodies, one of which is designated with an arrow.

 

On the basis of morphological features and host, the fungus was identified as P. epilobii [previously designated Sphaerotheca epilobii (Wallr.) Sacc. (1,2)]. The only other powdery mildew species recognized by Braun (1) on Epilobium spp. is Leveillula taurica (Lév.) G. Arnaud. The anamorph of L. taurica produces dimorphic conidia, typically from conidiophores that emerge through host stromata, and the conidia lack fibrosin bodies (1,5). In contrast, the anamorphic features of the fungus we observed matched those for P. epilobii. Previous reports of powdery mildews on E. ciliatum include Sphaerotheca humuli (DC.) Burr. from California (4) and S. macularis (Wallr.: Fr.) Lind from Idaho and Washington. Braun (1) relegated S. humuli to synonomy with S. macularis [now designated Podosphaera macularis (Wallr.) U. Braun & S. Takam.] which, he noted, occurred only on Humulus spp. Podosphaera epilobii previously was reported on several other Epilobium species in the region, including E. lactiflorum Hausskn. in California and several additional species in Alaska (4).

Braun’s monograph (1) included only species of Epilobium as hosts for P. epilobii, although Farr et al. (4) also cite single reports of this fungus on species of Oenothera and Fuchsia. It therefore appears unlikely that E. ciliatum serves as a source of inoculum for the powdery mildews that infect the numerous and important field grown vegetable crops and greenhouse ornamental plantings in the Salinas Valley. However, this host could possibly play a role as a source of inoculum for greenhouse-grown Onagraceae.


Literature Cited

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

2. Braun, U. 1995. The Powdery Mildews (Erysiphales) of Europe. Gustav Fischer Verlag, Stuttgart.

3. DiTomaso, J. M. 2007. Weeds of California and Other Western States. Agric. and Natural Resourc., Univ. of Calif., Oakland, CA.

4. Farr, D. F., Rossman, A. Y., Palm, M. E., and McCray, E. B. Fungal Databases. Online. Systematic Botany & Mycology Laboratory, USDA-ARS, Washinton, DC.

5. Glawe, D. A., Grove, G. G., and Nelson, M. 2006. First report of powdery mildew of Gaillardia × grandiflora (blanket flower) caused by Leveillula taurica in North America. Online. Plant Health Progress doi:10.1094/PHP-2006-0112-01-BR.