Cornell University's Disease Factsheets of Trees and Shrubs introduce a disease, the disease's symptoms and strategies for control.
Christmas Tree Diseases and Pests.
Diseases and Pests
Anthracnose of Trees and Shrubs Necrotic spots or scorch-like blight on leaves. During, or shortly after wet weather from late spring until late summer, irregular lesions   emerge and kill areas of the leaf. On sugar maple lesions are reddish brown. Very young leaves attacked may become blackened and shriveled, but infected mature leaves retain normal form. |
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Spruce Spider Mites - Damage occurs to oldest (interior) leaves appearing as yellowing or stippling of foliage. Tap branches over a white piece of paper, mites will appear as dust-like specks moving about. White fast moving mites are predacious, feeding on the pest mites. If these are seen, do not spray. They will control the pest. |
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Apple Scab |
| Bacterial Blight of Lilac Causes new growth to turn dark brown and die back quickly. Cut out and destroy infected shoots as soon as symptoms appear disinfecting tools between cuts. When severe, spray with copper fungicides once in mid-September. |
| Birch leaf Minor Causes dark greenish spots expanding to blotches on leaves of birch. Mostly cosmetic injury only. Control first generation is most important. Highly visible specimens can be treated with acephate, dimethoate, and Imidacloprid (soil drench in November or March). |
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 Black Knot on CherryHard black growth on branches of cherry or plum trees. The black knot fungus gall (the hard black growth) takes two years to develop. In the first year it just looks like a swelling or split in the branch. If you do not cut these out, more galls will develop. You can still cut them out this year. Try to get to them before the leaf and flower buds open. You can also apply a protective fungicide spray in spring when the flowers are opening. Check the pesticide label for directions. |
 Bronze Birch BorerThe larvae is a grub that tunnels under the bark disrupting the flow of water up the tree. This is likely the cause of the wilting branches. Sapsuckers and wood peckers dig into the bark to get at the grubs. Keeping European birch well watered and fertilized can sustain trees, however, this tree looks to be in poor condition and may be too far gone. Take care of the other trees by mulching around the base, and watering during periods of drought. |
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  Cedar Apple RustOn Junipers and red cedars, prune out the red-brown, round or spindle shaped swellings (picture at right) of cedar apple rust and quince apple rust just before swellings begin. The orange jelly-like masses release spores that infect the alternate host (apple, hawthorn) in the second phase of this disease. |
 Diplodia Tip BlightInfection begins on buds, succulent stems, elongating shoots and sometimes on immature needles of pine in spring when shoots begin to grow. Drops of resin exude from small lesions on twigs near tip of branch. Lesions enlarge and shoots stop growing before or during needle elongation. Dying shoots turn yellow-green to straw color causing characteristic tip blight. Lesions become resin soaked and dark reddish brown in summer. In autumn, presence of black fruiting bodies may be found on needles and cones. |
 Dogwood AnthracnoseShortly after leaves have expand (mid-late May and June), spots and blotches appear on infected leaves. These spots have a tan center and a purple or reddish margin. On the bottom of the leaf, tiny brown or black spots may appear beneath these lesions. Blotches can also occur at the tip or along the margin of the leaves – these too have a tan center and purplish margin. |
 Dutch Elm DiseaseFirst seen in the US in the '30's. It still kills elm trees. Early symptoms consist of yellow and flagging of leaves. Isolated trees may live for many years before becoming infected with the disease. Trees infected with the disease can not be cured. The only control is preventing the disease from becoming established. The elm bark beetle spreads the disease so control should be targeted at them. Dead elm trees should be cut down and all bark removed to kill over wintering beetles if the wood is to be stored for firewood. |
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| European Chafer At dusk you may notice thousands of beetles flying around trees. The bugs are light brown/gold in color and are about 1/2 inch long by 1/4 inch wide.The beetle does not harm trees but the grub, which will be hatching from eggs in August (the adults are mating now), can cause damage to the roots of grass. You can check for the grubs in late August or early September by peeling back a layer of sod. If you see 8-10 grubs per square foot, then you may want to treat the lawn with an insecticide for grubs. Contact your local Cooperative Extension or nursery for recommended chemicals. |
 FireblightLeaves die back quickly and blackening affecting trees and shrubs in the rose family (rosacea), particularly ornamental apples, pears and MT Ash. Carefully remove and dispose of infected branches as soon as they appear. Cut at least 6-12" below signs of infection. Prune out any remaining cankered branches during dormant season. Avoid high rates of nitrogen. |
 Gypsy Moth CaterpillarGypsy moth caterpillars can cause early defoliation of many trees. If the tees are healthy and haven't been stressed the previous year, no control is necessary. Look for caterpillars that looked shriveled, and hang down in an inverted "V" (picture right). This indicates that parasites are killing the caterpillars. |
 Honeylocust Plant BugDamage to Honeylocust trees in spring can be recognized by stippling, deformity and defoliation. Nothing can be done once damage is seen, but plan to spray with horticultural oil next spring from late April through early May. |
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Honeylocust Pod Gall Midge |
| Japanese Beetles Feeding on leaves does not substantially hurt the tree. You can try Japanese beetle traps to lure the beetles away from your plum. Using the traps actually increases the number of beetles in an area, so place them 100 yards or more away from the plants your protecting. Controlling grubs in the lawn may help if you have a large lawn. But check for grubs in late August by pealing back sod and counting grubs. 5-8 grubs/sq ft indicate a high population however they may not be Japanese beetle grubs so you may need to have them identified at your local Cooperative Extension. |
 Leaf Blister of OakLeaves become infected just as buds open. Bulges are seen on the leaf surface, or depressions as viewed from the reverse side. Numerous blisters sometimes coalesce and involve the entire leaf. |
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Leaf galls |
 LichenA blue-green fungus that grows on bark of trees. Lichen are non-aggressive to plants and colonize after the plant is under stress in humid shaded locations. The slow growth of stressed plants give lichen a foothold to start growing. Lichen are formed by a symbiotic relationship between a fungus and an algae. The algae produces sugar through photosynthesis which feeds the fungus and the fungus collects moisture and nutrients for the algae. Removing lichen from a plant causes more harm than leaving it. |
 Magnolia ScaleThe insect appears as a brown "bump" on the twig. The young black crawlers of Magnolia scale will be present in early October signaling the next opportunity to control this insect. Apply horticultural oil, insecticidal soap, acephate, carbaryl, or Malathion, as crawlers appear. Imidacloprid (soil drench), or dormant oil could be used in early spring (April). |
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  Maple Trumpet SkeletonizerThe leaves of maple curl and turning brown. Open up the rolled leaves to find the "trumpet" that this leaf feeding caterpillar hides in. Late season leaf loss causes no harm to trees so control is unnecessary. |
 Oak AnthracnoseTrees in the "white oak" family are susceptible to oak caused by the fungus Apiognomonia sp. Newly expanding leaves develop brown spots and shrivel up. Severe cases can cause branch dieback but t is too late for control now anyway (June). The best thing that you could do would be to remove the fallen leaves in autumn to reduce the spread of the disease to next years leaves. In severe cases fungicides before bud break will prevent a second occurrence. |
  Pear thripsMaple leaves damaged by the pear thrips are dwarfed, mottled yellow to green-brown, and distorted. This causes the tree to have a thin crown, and the effect resembles late frost damage. Blister-like scars develop along the veins and petioles of the foliage. Moderately damaged foliage can place the trees under some stress and possibly cause premature leaf drop in early fall. |
 Pine Gall RustGlobose or pear shaped woody galls form on branches of pine trees, on main stem in some species. Galls enlarge to 1-10 cm before dying. In May and early June powdery yellow-orange spores appear. In second year and third year galls develop bark collars at one or both ends. |
 Powdery MildewA white powdery fungus that appears on leaves. It will not effect the long term health of the shrubs. To reduce the occurrence in the future, be sure the shrubs are in full sun and get good air circulation - thinning pruning of over head trees and thinning the shrubs themselves can accomplish this. |
 Quince Rust
Fruits of hawthorn or crabapple become uniformly covered with tiny horn-like projections and then die and dry out. |
 Rhabdocline needlecastAppears as purplish-brown bands, or spots, on the needles of douglas fir. First look for discolored needles near the base of the tree. Apply a registered fungicide when 50% of the buds have opened. Severely infected trees should be cut and removed. |
 Sapsucker InjurySap suckers create a series of hole like this - 00000. They eat the insects that are attracted to the sap and will reopen the holes as the sap stops flowing. This damage can eventually kill areas of the trunk or girdle the tree. |
 Sooty MoldA black fungus that forms on the secretions (honeydew) of aphids and scale insects. |
 Spruce GallCooley spruce gall adelgid causes small, pineapple shaped galls on the ends of branches. Treat mid-to-late April, before galls appear, with horticultural oil (do not use on blue spruce). |
 Tar SpotAffects Norway maple (including Crimson King that is mistakenly called "red" maple) causing large black spots on leaves and may cause early defoliation. In severe cases, the leaf also shows a wilting or scorching. This disease of the foliage does not harm the tree so control is not warranted. Remove leaves or mulch them to reduce occurrence of the disease next year. |
 Viburnum Leaf BeetleThe grubs riddle leaves of Snowball bush and other viburnums with holes, in some cases causing them to shrivel up. In July, the adult beetle emerge and may cause further defoliation of susceptible species. |
| White Pine Weevil Causes the top of white pine or spruce to turn brown and wilt over like a shepherds crook.  Prune out and destroy the brown, crooked leaders before late July before the larvae emerge as an adult beetle. Other pines and firs may also be attacked. |
  Winter InjuryDamage to plants from roadside salt can become evident by various symptoms. Evergreens (spruce, pine) will show brown or yellow needles (see winter damage below). On deciduous plants (trees an shrubs that loose their leaves in winter) salt spray causes buds to die and new sprouts to form right below them resulting in a witches broom effect. |
Arborvitae Leaf Minor - Damage starts in tips of shoots causing leaves to be yellow or brown. Hold up to light to look for larvae within the leaf. If not too severe, prune off brown foliage. 
Carpenter Ants and Wood Decay
