© 2009 Plant Management Network.
First Report of Powdery Mildew of Poison-Hemlock Caused by Erysiphe heraclei in North America
Steven T. Koike, Plant Pathology Farm Advisor, University of California Cooperative Extension, Salinas, CA 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, WA 98195
Corresponding author: Steven T. Koike. firstname.lastname@example.org
Koike, S. T., and Glawe D. A. 2009. First report of powdery mildew of poison-hemlock caused by Erysiphe heraclei in North America. Online. Plant Health Progress doi:10.1094/PHP-2009-0312-01-BR.
Poison-hemlock (Conium maculatum L., Apiaceae) is a widely distributed biennial especially known for its toxic piperidine alkaloids that can poison livestock that eat this plant. In 2008, a powdery mildew was observed on poison-hemlock growing in coastal (Monterey and Santa Clara counties) California. The causal agent was determined to be Erysiphe heraclei DC., previously unreported on this host in North America.
Signs of the pathogen included effuse patches of white mycelia on adaxial leaf surfaces of older foliage (Fig. 1), leaf petioles, flower pedicels, and immature seed capsules of wayside plants. Disease was widespread on poison-hemlock plants that were surveyed in both counties, with disease incidence averaging 33%. Powdery mildew did not appear to cause obvious damage to this host. Hyphae were ectophytic with lobed appressoria (Fig. 2). Conidiophore foot cells were cylindrical, straight, and measured (L × W) (24-)25.5-39(-47) × (6-)7-10.5 µm (Fig. 3). Conidiophores bore single conidia. Conidia (Fig. 4) were short-cylindrical to cylindrical, lacked fibrosin bodies, and measured (28-)33.5-44(-47) × (12.5-)13-17.5(-18.5) µm. The teleomorph was not observed. The fungus was determined to be E. heraclei on the basis of host and morphology of the anamorph.
Erysiphe heraclei has been reported from C. maculatum throughout much of Europe and countries in the eastern Mediterranean (2). Other powdery mildew species reported from this host include Leveillula taurica (Lév.) G. Arnaud in France and Italy, and an unidentified Oidium in Argentina and Portugal (2). Leveillula taurica is easily distinguished from E. heraclei on the basis of its dimorphic conidia and internal mycelia (1). Erysiphe heraclei occurs on a wide range of Apiaceae, including economically important species such as celery (Apium graveolens L.) (4), parsley [Petroselinum crispum (Mill.) Nym. ex. A. W. Hill] and carrot [Daucus carota L. subsp. sativus (Hoffm.) Arcang.] (3).
Because poison-hemlock often grows near commercial vegetable fields in Monterey and Santa Clara counties, a host range experiment was conducted using powdery mildew collected from this weed. Twelve plants each of celery (cv. Conquistador) and parsley (cv. Evergreen) were inoculated by gently pressing poison-hemlock leaves with sporulating colonies of the fungus against leaves of potted plants; inoculated plants were maintained and grown in a greenhouse. After two weeks, inoculated celery developed powdery mildew but inoculated parsley plants did not. Uninoculated celery and parsley plants did not develop the disease.
Results of this study suggest that poison-hemlock may play a role in the epidemiology of powdery mildew disease of celery. This possible link is supported by observations from 2007 and 2008 in which both celery crops grown in this Santa Clara Co. location, as well as poison-hemlock surrounding the fields, were infected with powdery mildew. Interestingly, adjacent parsley plantings did not exhibit the disease. These observations, and the negative results of our pathogenicity test for poison-hemlock isolates inoculated onto parsley, suggest that the local E. heraclei population may exhibit host specificity within the Apiaceae. Further research on the host range of E. heraclei may provide useful information regarding the epidemiology and control of powdery mildew diseases caused by this fungus. Presently, powdery mildew is not an economic concern for celery production in California; however, because poison-hemlock is host to both this and other celery pathogens, growers may be advised to control this weed.
1. Braun, U. 1987. A monograph of the Erysiphales (powdery mildews). Beih. Nova Hedwigia 89:1-700.
3. Glawe, D. A., Pelter, G. Q., and du Toit, L. J. 2005. First report of powdery mildew of carrot and parsley caused by Erysiphe heraclei in Washington State. Online. Plant Health Prog. doi:10.1094/PHP-2005-0114-01-HN.
4. Koike, S. T., and Saenz, G. S. 1997. First report of powdery mildew caused by Erysiphe heraclei on celery in North America. Plant Dis. 81:231.