© 2004 Plant Management Network.
First Report of Powdery Mildew of Myosotis sylvatica (Wood Forget-me-not) Caused by Golovinomyces cynoglossi (Erysiphe cynoglossi) in Washington State
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. firstname.lastname@example.org
Glawe, D. A. 2004. First report of powdery mildew of Myosotis sylvatica (wood forget-me-not) caused by Golovinomyces cynoglossi (Erysiphe cynoglossi) in Washington State. Online. Plant Health Progress doi:10.1094/PHP-2004-1124-01-HN.
Wood forget-me-not (Myosotis sylvatica Ehrh.: Hoffm., Boraginaceae) cultivars are popular ornamentals in the Pacific Northwest. In western Washington, this species frequently displays symptoms and signs of powdery mildew after anthesis. There are no published records of this disease in Washington. This report records the disease from King, Pierce, Snohomish, and Skagit counties and describes and illustrates symptoms of the disease as well as taxonomically important features of the causal organism that was determined to be Golovinomyces cynoglossi (Wallr.) V. P. Gelyuta.
Signs of the disease (Fig. 1) included effuse to dense patches of white- to grayish-brown mycelia on stems and leaf surfaces that frequently exhibited chlorotic mottling. The fungus formed superficial hyphae with nipple-shaped appressoria (Fig. 2); conidiophore foot cells that were cylindrical and measured (41-) 44.5-54.5 (-67.5) × 9.5-12.5 µm; and conidia (Fig. 3) that formed chains and were ovoid to cylindrical, lacked fibrosin bodies, and measured (21.5-) 22.5-31 × (10.5-) 12-17 (-20.5) µm. In one of 12 specimens, mature ascocarps were found on plant stems. Chasmothecia were dark brown to black, subspheroidal, 91-120 µm in diameter, with mycelioid appendages, and contained multiple asci that were saccate, short-stipitate, and measured 56.5-65.5 × 33.5-38 µm. Asci typically matured to contain two ovoid, pale yellowish ascospores measuring (19.5-) 20-24.5 × (13-) 14-16 (-17) µm; frequently, additional abortive ascospores were observed (Fig. 4). A voucher specimen was deposited with the Mycological Herbarium of the Department of Plant Pathology at Washington State University.
Fungal morphological characteristics and host matched the features described by Braun (1,2) for E. cynoglossi (Wallr.) U. Braun, now designated as Golovinomyces cynoglossi. Braun (1,2) distinguished this species from E. cichoracearum DC, currently known as Golovinomyces cichoracearum (DC.) V. P. Gelyuta, which he regarded as restricted to members of the Asteraceae. He (1,2) summarized evidence suggesting that E. cichoracearum in the broad sense is a complex of diverse species that should be differentiated on the basis of host specialization and, in some cases, morphological differences.
This is the first report of this powdery mildew pathogen on M. sylvatica in Washington but there are several reports of E. cichoracearum from Myosotis spp. in North America. Farr et al. (3) listed E. cichoracearum on M. sylvatica in CA. They (3) also listed an unidentified Oidium species on M. sylvatica in OH. Erysiphe cichoracearum was listed (4) on Myosotis spp. in the Pacific Northwest, but no details of its distribution within the region were included. Braun (1) listed North America in the geographical range for E. cynoglossi.
Although unsightly, it is uncertain whether powdery mildew otherwise damages significantly M. sylvatica. In a self-seeding population of the plant observed in a private garden in Seattle during 2000-2004, each year the disease was difficult to find until plants began senescing after anthesis, after which time 100% of several dozen plants were infected. Discovery of the teleomorph in one specimen suggests that it may be significant in over-wintering in western Washington. Further work would be useful in determining whether young plants are resistant, or if the timing of disease development results from low levels of primary inoculum or some other factor. It also would be interesting to ascertain the effect of powdery mildew on seed production and viability.
1. Braun, U. 1987. A monograph of the Erysiphales (powdery mildews). Beih. Nova Hedwigia 89:1-700.
2. Braun, U. 1995. The Powdery Mildews of Europe. Gustav Fischer Verlag, Jena.
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. Pscheidt, J. W., and Ocamb, C. M., eds. 2002. Pacific Northwest Plant Disease Management Handbook. Oregon State Univ., Corvallis.