Shrub Disease Treatment

Common Diseases on Shrubs

Boxwood blight

Boxwood blight is caused by the fungus, Cylindricladium pseduonaviculatum. This disease causes lesions on the leaves, with lesions on stems following soon after. If not treated, the infection will cause the plant to drop its leaves, causing stress and eventual death. Click HERE to learn how SavATree can help treat Boxwood blight!

Fire blight

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 leaves. Affected twigs develop a “shepherd’s crook”.

Needle, tip, or twig blights

Needle, tip, or twig blights occur on many species of shrubs including arborvitae, cypress, and juniper. They start by turning the 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 Phompsis, Diplodia or Coryneum.

Leaf Spot

Leaf Spot causes small, irregular brown to black spots to appear on leaves, which will eventually drop prematurely. 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.

Powdery Mildew

Powdery Mildew is a common fungal disease that creates a white, powdery appearance on leaves, stems, and buds. Infection often leads to leaf yellowing, premature leaf-drop, and slow growth.

Pachysandra volutella blight

Pachysandra volutella blight targets Japanese pachysandra. The stems of an infected plant will darken and die. If conditions are particularly wet, orange/pink fungal spores will colonize the surface of dead stems. Brown and tan spots will develop on the leaves and eventually spread over the entire leaf. Concentric line patterns may be visible.

Root Rots

Phytophthora is a root rot commonly found on oaks and dogwoods, as well as new plantings, that causes a slow decline of the tree. Infected leaves will turn light green or yellow and may even fall off of the tree. This species can survive in the soil for years as long as moist conditions persist.

Thielaviopsis or more commonly known as Black Root Rot is a difficult disease to control once it is started. It can be identified by dark brown or black spores within the root, followed by black lesions on the root. This is an opportunistic pathogen that attacks plants under stress.

Cankers

Volutella is a common disease of boxwoods, as all species and cultivars are susceptible. This pathogen produces sunken lesions on stems, which then causes girdle stems and results in dieback. Leaves of affected branches will first turn light green-yellow, change to red-bronze, and eventually become straw. Proper pruning and destruction of diseased branches and fallen leaves will prevent spreading.

Botryosphaeria is a common fungal disease but only attacks trees and woody shrubs that are already stressed or weakened by other pathogens. It can be detected by little black bumps on the surface of the bark, and some trees will weep gummy sap. If caught early, the canker can be removed and the entire plant can be saved.

Cytospora is a common disease found on spruce trees and large, mature trees in general. This type of canker kills random branches throughout the tree canopy, badly deforming them and damaging the trees overall aesthetic. Cankers can be found close to the trunk of the tree on dead or dying branches. The needles on the dying branches will turn brown and fall off.

Common Pests in Shrubs

Boxwood psyllid

Boxwood psyllid is capable of infesting all species of ornamental boxwood. These insects overwinter by laying orangish eggs in between bud scales. Nymphs hatch early in the season and a white, waxy substance they secrete may cover the plant and be detrimental to its health. Psyllids suck sap from buds and young leaves, resulting in a cupping of new foliage.

Scale insects

Scale insects come in many varieties and can attack a wide range of shrubs, feeding on plant juices. They typically appear as immobile bumps. In most cases, you won’t be able to see legs or other body parts. Some species excrete a sticky substance called honeydew, which attracts other insects and mold.

Leafminers

Leafminers cause damage by tunneling into foliage and feeding on plant material. Over time, infestations can cause premature leaf-drop or dieback. Species that may affect your shrubs include, boxwood leafminers, holly leafminers, and arborvitae leafminers.

Spider mites

Spider mites are common pests with a wide range of targets, including fruit trees, vines, berries, vegetables and ornamental shrubs. Spider mites hatch out early in the growing season but remain active for a long time. Damage from spider mites appears as yellowed stippling on foliage. You may also notice fine webbing on plants.

Lace bug insects

Lace bug insects affect susceptible evergreen species such as rhododendrons, azaleas and Andromeda. Damage from previous seasons will appear as yellow stippling on leaves and dark fecal spots on the undersides of leaves. If lace bugs were present the preceding season, they may become an issue again.

Borers

Borers are a class of insect that bore into the woody parts of the plant, pushing out sawdust (or “frass”) and weakening your shrubs and ornamentals. Over time, they cause dieback and decline. Varieties that can affect your shrubs and ornamentals include dogwood borers, peach tree borers, rhododendron borers, and lilac borers.

Aphids

Aphids are little, green, soft-bodied insects that are among the most destructive insect pests on cultivated plants. They feed by sucking the nutrients out of plants, which weakens plants significantly, in turn harming flowers and fruit. These insects multiply quickly, so it’s important to get them under control before the reproduction process begins.

Caterpillars (Bagworms & Webworms)

Bagworms feed on many trees and shrubs but prefer evergreens, specifically junipers, arborvitae and Leyland cypress. As they grow, so do their silk bags, which help disguise them. Heavy infestations can result in a tree becoming completely defoliated and will kill hosts such as red cedar or juniper.

Webworms can be found on over 600 different species of deciduous trees including oak, elm and maple, but they prefer fruit and nut producing trees. They tend to be yellow, green or orange with two rows of black spots on their back with long white hairs covering their body. Tree health is typically not compromised because they only feed on leaves that will eventually die off in autumn.

Root Weevils

Root Weevils are destructive little insects that will invade the root system of a healthy plant. They come out to feed in the night and eat the plant from the roots up. The leaves of the plant will look irregular, as bite marks around the edges will become apparent. Young insects look more like worms and will be found in the soil, while adult weevils are beetle-like and can be black, brown or gray.

Sawflies

Sawflies look similar to hornets and get their name from the saw-like extension at the top of their body. Their damage includes leaf holes, skeletonizing leaves and leaf roll up. A light infestation may cause little damage to the plant’s appearance which is easily removed through pruning. A large number of sawflies can seriously damage or even kill a tree.

Adelgids

Adelgids are insect pests that can damage the growth of trees by sucking out the plant’s contents. Adelgids solely feed on conifers, including Douglas fir, hemlock, larch, pine and spruce. They can produce white, cottony tufts on branches, twigs, bark, needles and cones. When active in large quantities, they may cause yellowing, premature needle droppage and dieback.

Leaf Beetles

Japanese Beetle is a leaf beetle that is a noted pest of about 300 species of plants. They have iridescent copper-colored bodies, green heads and white spots under their wing covers on each side of the body. They cause damage to plants by feeding on the foliage and leaving behind the veins.

Viburnum Leaf Beetle is a leaf beetle that causes similar damage to the Japanese Beetle. They chew holes in leaves creating a lace-like pattern. Severe infestations can cause complete defoliation of a shrub, which weakens the plant over time and can be fatal. The larvae are tiny and range in color from yellowish-green to light brown with black spots and dashes on their bodies, while adults are a little bigger and yellowish-brown in color.

Willow Leaf Beetle is a leaf beetle that causes similar damage to the Japanese Beetle and Viburnum Leaf Beetle. Adults chew holes in the leaves, while larvae completely skeletonize the leaves. With a heavy infestation, all of the leaves may turn brown, causing the tree to appear dead or scorched. The adult beetles are stout, oval, and metallic bluish-green, while the larvae are bluish-black in color and are slug-like.