Fungi are often responsible for leaf spot diseases in trees, shrubs and herbaceous plants, but they are not the only pathogen that can cause this ailment. Viburnums are susceptible to several genera of fungi that can cause this disease, including Cercospora and Phyllosticta. Symptoms of this condition emerge as discolored patches or spots on the plant's foliage. The spots become larger and eventually merge together as the disease progresses. Viburnum plants are also vulnerable to a type of algal leaf spot caused by Cephaleuros virescens, which causes distinct red, fuzzy spots on infected leaves.
Doublefile viburnum (Viburnum plicatum var. tomentosum) is a cultivar of the Japanese snowball viburnum (V. plicatum). This cultivar is one of the most beautiful variations of viburnum currently in circulation, according to the Clemson University Extension. Doublefile viburnum is vulnerable to several groups of diseases that can mar its aesthetic value and decrease its vibrancy. Some of these conditions are serious enough to cause permanent or fatal damage.
Leaf Spot Diseases
Much like leaf spot diseases, mildews and molds are generally caused by fungal pathogens but rarely pose a serious threat to their host's health. Powdery mildew, caused by the Erysiphe sparsa fungus, and downy mildew, caused by Plasmopara viburni, are both common fungal parasites of doublefile viburnum in North America. These diseases attack emerging green growth, usually leaves, buds and flowers, of their host. The damaged areas often turn to a brown or black color as they decay. Thee colonies of fungus are usually white or light gray, which makes them easy to spot and identify.
Dieback and Canker
Fungi from the genus Botryosphaeria are responsible for a serious viburnum disease called Botryosphaeria dieback and canker. According to the Clemson University Extension, this disease is more common on stressed or weak plants that can't fight back against an internal infection of the fungus. This fungus penetrates the bark of its host and infiltrates the plant's softer wood and circulatory system. It then spreads internally, rotting away at the vital tissues at the core of the plant's trunk and branches. Cankers appear along damaged branches as the disease progresses.
Armillaria Root Rot
The Armillaria mellea fungus, also known as shoestring fungus or oak root rot, is one of the most insidious pathogens of doublefile viburnum plants. It is particularly dangerous because it can attack many species, so it can spread between trees and shrubs with ease. The fungus spends most of its life cycle in the upper layers of fertile soil, where it seeks potential host plant tissues to prey on. It penetrates the root system of its target and begins to rot away the structure. This damage disrupts the plant's circulation and inhibits the uptake of vital nutrients. Even mature plants may succumb to this disease and die in just a few years.
- University of Illinois Extension: Doublefile Viburnum
- Clemson Cooperative Extension; Viburnum Diseases and Insect Pests; Joey Williamson; December 2007
- University of Minnesota Extension; Verticillium Wilt and Trees and Shrubs; Cynthia L. Ash; 1994
- Clemson Cooperative Extension; Viburnum; Marjan Kluepfel et al.; May 1999