Common Shrub Diseases

Fire blight is caused by the bacteria Erwinia amylovora, over 130 members of the Rosaceae family are susceptible, including; apple and crabapple, cotoneaster, firethorn, hawthorn, mountain ash, and pear. Infected leaves, stems, twigs and fruits will turn brown to black. Fire blight will blacken the smooth-barked, green branches and affected twigs develop a “shepherd’s crook.”

Needle, tip or twig blights occur on arborvitae, cypress and juniper species turning tips of twigs and ends of needles brown or grey. Black, pimple-like fungal fruiting structures may develop on needle surfaces. These blights can be caused by fungi in the genera PhompsisDiplodia or Coryneum.

Cercospora species and Colletotrichum gloeosporioides are two of several fungi which cause leaf spot diseases on Rhododendron species and azalea cultivars. Small, irregular brown to black spots will appear on leaves, which will eventually drop prematurely, when infected by Cercospora spp. Lesions caused by Colletotrichum gloeosporioides are round, brown spots visible on both upper and lower surfaces of leaves. These fungal diseases may first appear as symptoms of winter desiccation.