'"`UNIQ--btaxobox-00000000-QINU`"' Author: Tom Creswell, Purdue University; Mary Ann Hansen, Virginia Tech; Margery Daughtrey, Cornell University

Pathogen

The Peronospora species causing downy mildew on coleus is an oomycete in the order Peronosporales. Palmateer, et al. describe the pathogen as having coenocytic mycelium and producing conidia which are slightly brown, ovoid to ellipsoid, mostly nonpapillate, and ranging from 17.0 to 24.5 by 16.5 to 25.5 µm. Conidia are borne on monopodially branched hyaline sporangiophores (343.5 to 561 by 9.5 to 15 µm) with slightly swollen bases, which emerge from stomata. The sporangia upon their sporangiophores form a sparse, white to gray layer on the lower leaf surface of infected plants. Oospores have not been reported for this coleus pathogen.

This pathogen is also reported on agastache and perilla. It is different in host range and ITS sequence from the closely related Peronospora lamii, which has a host range that includes certain other members of the mint family. It also differs from the downy mildew of basil, P. belbahrii. These species would be difficult to separate on morphology alone.

Symptoms and Signs

Symptoms and signs of coleus downy mildew may include:

leaf curl and distortion
leaf yellowing or stippling
necrotic leaf spots
white sporulation of the pathogen on lower leaf surface
defoliation

Symptoms of coleus downy mildew may vary depending on the variety of coleus and time of infection. Prior to defoliation, leaves may curl downward or appear light green, have necrotic spots, or be distorted. Initial symptoms may be overlooked because the leaf symptoms are not uniform throughout the plant and overlapping foliage may mask symptoms. During cool, moist weather the pathogen produces white to gray sporangia on the lower leaf surface that are diagnostic for the disease. Sporangia are not always present, however.

Ecology and Spread

Coleus downy mildew spreads in two main ways:

Vegetative propagation and/or sale of infected plants
Wind-blown or water-splashed sporangia

Peronspora sp. grows systemically throughout the plant and reproduces by means of sporangia, which form on the lower leaf surface under cool, moist conditions. Sporangia are easily spread short-distance by air currents or splashing water and they may be wind-blown long distances to new locations. Because sporangia may not be produced until weeks after the plant is initially infected, asymptomatic plants may mistakenly be assumed to be disease-free.

Geographic Distribution

Downy mildew on coleus has been reported from both Germany and several states of the United States, with the pathogen identified as either Peronospora sp. or P. belbahrii. Definitive studies are needed to formally describe and name the downy mildew that affects coleus, agastache and perilla, which differs in important respects from P. belbahrii, a pathogen of basil.

Management

Downy mildew diseases spread more easily in high humidity so reducing humidity in greenhouses and choosing planting sites in the landscape with good air movement can help reduce disease severity and spread. While all cultivars, both seed and cutting propagated, are susceptible, there is a range of reactions in various cultivars

The results of susceptibility trials at Michigan State University and Cornell University for 102 cultivars allowed grouping the cultivars into high, medium or low susceptibility. Management tools for commercial growers include systemic and protective fungicides as recommended by Hausbeck, Harlan, and Enzenbacher.

Homeowners may find fungicide application impractical but products containing copper, mancozeb and phosphorous acid are available.

Consult your local extension specialist or agent for recommendations relevant to your particular state. Remember: the label is the law.

Diagnostic procedures

Plants showing characteristic leaf curl, distortion, yellowing, stippling or necrotic leaf spots should be examined by dissecting microscope for the presence of the pathogen on the lower leaf surface. Compound microscope examination of typical downy mildew type sporangia and conidia provides further confirmation of the disease. No other downy mildew pathogens are currently known to exist on coleus.

Resources and References

Coleus Downy Mildew - Michigan State University Extension

British Society of Plant Pathology: New Disease Reports (2007) 16, 11. Downy mildew of coleus (Solenostemon scutellarioides) caused by Peronospora sp. in Florida. A.J. Palmateer, P.F. Harmon and T.S. Schubert. http://www.ndrs.org.uk/article.php?file=2007-72.asp

Belbahri L, Calmin G, Pawlowski J, Lefort F, 2005. Phylogenic analysis and real time PCR detection of a presumably undescribed Peronospora species on sweet basil and sage. Mycological Research 109, 1276-1287.

Daughtrey, Margery. 2013. Coleus Downy Mildew (video). Greenhouse Grower. http://www.greenhousegrower.com/video/crop-inputs/disease-control/v-coleus-downy-mildew/

Daughtrey ML, Holcomb GE, Eshenaur B, Palm ME, Belbahri L, Lefort F, 2006. First report of downy mildew on greenhouse and landscape coleus caused by a Peronospora sp. in Louisiana and New York. Plant Disease 90, 1111.

Francis SM, 1981. Peronospora lamii. Commonwealth Mycological Institute. Descriptions of Pathogenic Fungi and Bacteria. No. 688. Kew, Surrey, UK.

Acknowledgments

An undescribed species of Peronospora (downy mildew of coleus)

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