© 2005 Plant Management Network.
First Report of Powdery Mildew of Scabiosa columbaria (Dove Pincushions) Caused by Erysiphe knautiae in North America
Dean A. Glawe, Plant Pathologist, Puyallup Research and Extension Center, Washington State University, 7612 Pioneer Way East, Puyallup 98371-4998; and Gary G. Grove, Associate Plant Pathologist and Extension Plant Pathologist, Irrigated Agriculture Research and Extension Center, Washington State University, Prosser 99350-8694
Corresponding author: Dean A. Glawe. firstname.lastname@example.org
Glawe, D. A., and Grove, G. G. 2005. First report of powdery mildew of Scabiosa columbaria (dove pincushions) caused by Erysiphe knautiae in North America. Online. Plant Health Progress doi:10.1094/PHP-2005-1024-01-BR.
Scabiosa columbaria L. (dove pincushions) (Dipsacaceae) is a perennial species introduced into North America, where it is grown as an ornamental landscape plant. Little information is available on pathogens attacking it in North America. The only fungi listed on S. columbaria in the U.S. Systematic Botany and Mycology Laboratory Fungus-Host Database (3) are Botrytis cinerea Pers.: Fr. and Peronospora dipsaci Tul.: de Bary, based on reports in California. Erysiphe polygoni DC is listed on S. atropurpurea L. and on Scabiosa sp. from an unknown U.S. location or locations (3). Dreistadt (2) also listed Erysiphe polygoni as the causal agent of powdery mildew on Scabiosa spp. However, we have been unable to find a description of powdery mildew or its causal agent on S. columbaria in North America. Recently we encountered a powdery mildew fungus on this host species in Washington and determined it to be Erysiphe knautiae Duby. This report describes the disease as well as diagnostic features and taxonomy of the causal agent.
Powdery mildew of S. columbaria ‘Butterfly Blue’ was observed on overwintered, field-grown plants at a commercial propagation facility located near Mabton, Yakima Co., WA in April 2005. Signs of the disease included white to grayish patches of dense, sporulating mycelium on adaxial and abaxial leaf surfaces (Fig. 1). The fungus formed superficial hyphae with lobed appressoria (Fig. 2). Conidiophores (Fig. 3) formed cylindric foot cells measuring (17-) 22.5-33.5 (-37) × 7.5-10 (-12) μm, and single conidia (Fig. 4) that lacked fibrosin bodies, were hyaline, ellipsoid-ovoid in shape, and (18-) 29.5-41 (-46.5) × (15-) 16-21.5 (-22.5) μm. The teleomorph was not observed.
Braun (1) included three species of Erysiphales occurring on Scabiosa spp. One species, Leveillula taurica (Lév.) Arnaud, can be excluded readily because it forms internal mycelia and distinctive dimorphic conidia that are either lanceolate or cylindrical (1). Another species, Sphaerotheca dipsacearum (Tul. & Tul.) Junell, can be excluded because Sphaerotheca species form chains of conidia with fibrosin bodies (1), unlike the conidia described herein (Fig. 3) that formed singly and lacked fibrosin bodies. The fungus described herein matched the features of E. knautiae described by Braun (1) including morphology of appressoria and foot cells, and formation of single, ellipsoid-ovoid conidia without fibrosin bodies. Based on morphological features and host the fungus in question was determined to be E. knautiae.
It is probable that previous reports of E. polygoni on Scabiosa spp. in North America (2,3) also refer to E. knautiae. North American mycologists have tended to view Erysiphe polygoni as attacking a wide range of host plants, including a number of fungi now distributed among a variety of other species (1,4). Braun (1) regarded E. knautiae to be "very rare" in North America although occurring throughout Europe and Asia. It is possible that the fungus was introduced into North America on live host material.
The fungus can cause significant losses. Of the S. columbria plants sampled in the propagation facility, more than 20% were infected by E. knautiae in late April and more than 63% were infected in late August and therefore were unsuitable for use in propagation. Further research is needed to assess more fully the geographic distribution and possible economic impact of this pathogen in North America.
1. Braun, U. 1987. A monograph of the Erysiphales (powdery mildews). Beih. Nova Hedwigia 89:1-700.
2. Dreistadt, S. H. 2001. Integrated pest management for floriculture and nurseries. Univ. Calif. Statewide Integ. Pest Manag. Proj. Publ. 3402.
4. Parmelee, J. A. 1977. The fungi of Ontario: II, Erysiphaceae (mildews). Can. J. Bot. 55:1940-1983.