EPIDEMIOLOGY AND MANAGEMENT OF DOWNY MILDEW, A NEW PATHOGEN OF COLEUS IN THE UNITED STATES

B.R. Harlan , M.K. Hausbeck
Coleus (Solenostemon scutellarioides), a herbaceous bedding plant, has been prized by gardeners for its bright colorful foliage since Victorian times, and in recent years has seen a resurgence in popularity in the United States. Coleus plants exhibiting disease symptoms were observed in New York and Louisiana in 2005 and throughout much of the United States by 2006. This pathogen was determined to be Peronospora sp. Epidemiological studies were conducted at Michigan State University to determine the optimal environmental conditions for sporangial release in the greenhouse. Additionally, host resistance and reduced-risk fungicides were investigated as management tools. Concentrations of airborne sporangia were monitored by placing a 7-day volumetric spore sampler in a greenhouse with coleus infected with Peronospora sp. Coincident hourly leaf wetness, temperature, and relative humidity data were collected. Two years of spore trapping in a greenhouse with downy mildew-infected coleus showed high relative humidity (>95%) followed by lower relative humidity prompted the release of especially high numbers of sporangia into the atmosphere. Two experiments screening coleus for susceptibility to downy mildew included 28 cultivars which were rated for percentage of leaves sporulating and density of pathogen sporulation. ‘Volcano’ and ‘Black Dragon’ consistently showed susceptibility to downy mildew. None of the cultivars included in the study showed disease resistance. ‘Freckles’ had limited downy mildew symptoms and pathogen sporulation. Reduced-risk fungicides that controlled downy mildew on inoculated plants when compared with untreated inoculated plants included fenamidone, mandipropamid, and azoxystrobin. Selected, new and experimental fungicides that are not yet labeled were also effective. The results of an experimental approach that defines the environmental parameters that favor downy mildew sporulation, identifies coleus cultivars that offer some disease resistance, and tests reduced-risk fungicides will offer growers an effective management program.
Harlan , B.R. and Hausbeck, M.K. (2012). EPIDEMIOLOGY AND MANAGEMENT OF DOWNY MILDEW, A NEW PATHOGEN OF COLEUS IN THE UNITED STATES. Acta Hortic. 952, 813-818
DOI: 10.17660/ActaHortic.2012.952.103
https://doi.org/10.17660/ActaHortic.2012.952.103
Peronospora sp., cultivars, reduced-risk
English

Acta Horticulturae