'Awabuki' viburnum is a favored ornamental hedge or shrub noted for its large shiny green leaves useful for screens and formal hedges. The last few years have seen a new disease organism move into our area and infect these beautiful landscape plants. The disease known as downy mildew on viburnum, a species specific water mold disease, can make a disturbing mess of these plants. There are some management techniques for the home landscape that can help lessen the damage.
The 'Awabuki' viburnum is really different than any other viburnum with very large glistening, almost mirror-like leaves. Homeowners often use 'Awabuki' as a screen around pools or just as an impressive tall hedge. Downy mildew is a water mold organism that develops when nighttime temperatures range from fifty to seventy-two degrees F. Mostly occurring from November to March, this downy mildew needs a cool, foggy and humid environment, typical of some nights in our area, to develop. Most of the damage appears on the newest leaves with yellow specks and reddish brown blotches – almost a bronze appearance on the leaf surface. On the underside of the leaf, you will notice whitish-grey downy growths. Soon after, the leaves will often drop with some portions of the plant becoming defoliated – near eighty percent defoliation in severe cases. The disease can spread rapidly and can be moved about with rain, wind and irrigation.
While you have no control on environmental conditions like high humidity and cool temperatures, you can reduce overcrowding of plants so that there is good air movement. Remove the fallen leaves which can re-infest plants next year. Do not provide overhead watering if at all possible – micro-irrigation at the soil level is more efficient and does not wet the leaves. Do not water at night. Also, do not over-fertilize as this makes the leaves much more succulent and open to infection.
Chemical treatments can be made with the use of fungicides, but work best when used as a preventative. Rotating chemical families will help reduce the possibility of fungicide resistance – a real problem if one type of fungicide is used all the time. Fungicides to use would include copper octanoate, chlorithalonil, and extracts of neem oil, as examples. Always read the pesticide label as the label is the law.
The 'Awabuki' viburnum is worth protecting from downy mildew. A bit of cultural practices, possibly augmented with some properly timed fungicides, can help reduce this disfiguring disease. For more information on the suppression of all types of fungal disease in our area, please call our Master Gardener volunteers on the Plant Lifeline on Mondays, Wednesdays and Fridays from 1 to 4 pm at 764-4340 for gardening help and insight into their role as an Extension volunteer. Don't forget to visit our other County Plant Clinics in the area. Please check this link for a complete list of site locations, dates and times - http://charlotte.ifas.ufl.edu/horticulture/Plant%20Clinics%20Schedule.pdf.
Palmateer, A. J. (2016) Viburnum Downy Mildew. The University of Florida Extension Service, IFAS.
Caldwell, D. (2011) Viburnum Downy Mildew Disease on Awabuki (Mirror leaf) Viburnum. The University of Florida Extension Service, IFAS – Collier County