Downy Mildew on Viburnum
Downy Mildew on Awabuki Viburnum
During the winter months when nights are cool and foggy, conditions are right for downy mildew to form on Awabuki Viburnum (Viburnum odoratissimum var. awabuki). The disease, which is difficult to control, tends to attack stressed plants and manifests itself by causing light green progressing to reddish brown spots on the leaves followed by yellowing and leaf drop. It is not unusual to see almost complete defoliation on some plants. It is important to rake up and dispose of fallen leaves as these serve as a source of inoculum for the disease. Also, be sure sprinklers are not hitting the foliage as this tends to disperse the spores and spread the disease.
Once the hot weather returns in April, the disease tends to dissipate, however the damage remains and newly emerging foliage may be attacked by aphids which cause leaf distortion and deformity. Aphids though can be easily killed with insecticidal soaps and oils. Although downy mildew is difficult to control once symptoms are evident, it may be possible to prevent the disease with applications of fungicides before symptoms appear.
More information on Awabuki Viburnum is available at the link below: