| Specific Diseases of Trees, Shrubs and Ground Covers | ||
|---|---|---|
| Host Plant and Disease | Description | Management Options |
| Ajuga (Ajuga) | ||
| Crown and root rot (fungus) | Wilting and discoloration of lower leaves, soon death of plant. Roots are rotted. Strands or mats of white fungus at base of plant. Very small, round, seed-like bodies form on and around the plant. These bodies are light colored at first, changing to buff and then brown. | Usually develops under warm and wet conditions. Remove and destroy affected plants. A fungicide drench may be necessary before replanting. |
| Almond, Flowering (Prunus) | See "Purple leaf plum." | |
| Alpine currant (Ribes) | ||
| Anthracnose leaf spot (fungus) | Small, brown spots develop first on the lower leaves. Leaves yellow and fall. | Destroy fallen diseased leaves when possible. Fungicides may be necessary. Worse during wet weather or when overhead watering is used. |
| Arborvitae (Thuja) | ||
| Fall needle browning
(physiological) (Figure 27) | Foliage on inner branchlets turn yellow or brown. | A normal process. No control necessary. |
| Ash (Fraxinus) | ||
| Anthracnose
(fungus) (Figure 28) | Irregular tan or brown areas are found on the leaves, especially along the major veins of the leaf blade. The condition usually is noticed in late spring on lower parts of the plant. Deformation and twisting of leaves often result from infections that occ | Control in landscape usually not necessary. Green ash reported to be less susceptible. |
| Heartwood rot (fungus) | On large or mature trees large, woody, perennial, hoof-shaped mushrooms or conks occur on the trunks. This fungus decays the non-living heartwood. | This fungus invades trees through wounds, often those caused by storm damage or improper pruning. Fungicides are not effective. |
| Leaf spot (fungus) | Most ash leaf spots are circular and light to dark brown. White, powdery areas appear on leaves, stems, flowers, or buds. If the infection is widespread, the entire leaf blade may be covered with a thin, white fungal, powdery growth. Some mildew fungi affect older leaves first, such as on oaks, lilacs, zinnias, | Seldom serious on the landscape; control not necessary. |
| Yellows (mycoplasma) | A general yellowing of the tree is associated with production of shoots from buds that form along the stem. Leaves on these shoots are often small and misshapen. Growth reduced. | No specific controls are available or feasible. Green ash reported to be tolerant. |
| Azalea (Rhododendron) (See Rhododendron) | ||
| Leaf spot (fungus) | Angular to irregular brown spots. Lower leaves usually affected first. Some defoliation may occur. | Some cultivars are very susceptible. This disease is most frequent during wet seasons or when plants were watered overhead. More often seen in greenhouses. Fungicides may be used. |
| Leaf gall (fungus) (Figure 29) | Conspicuous white galls on leaves and stem. | An oddity; does little damage. Pick off the galls. |
| Powdery mildew (fungus) (Figure 30) | White, powdery areas appear on leaves, stems, flowers, or buds. If the infection is widespread, the entire leaf blade may be covered with a thin, white fungal, powdery growth. Some mildew fungi affect older leaves first, such as on oaks, lilacs, zinnias, | Deciduous azaleas vary from susceptible to highly resistant. |
| Root and crown rot (fungi) | See Rhododendron for a description of this disease. | See Rhododendron for control or prevention. Maintain mulch. Use organic mulches. |
| Barberry (Berberis) | ||
| Verticillium wilt (fungus) | Wilting and yellowing of foliage is followed by premature defoliation. One limb of the tree may be affected first. Gradual dieback may be apparent. Yellow-brown, brown, black, or greenish-black streaks may be found in outer rings of wood of infected branches. In herbaceous plants, the vascular system of stems and roots are discolored. | |
| Beech (Fagus) | ||
| Heartwood rots (fungi) | On older or mature trees, decay of the heartwood of trunk and limbs is common. | Avoid injury or wounds to the bark. Follow accepted pruning practices. Repair wounds due to storm damage. Fungicides are not effective. |
| Birch (Betula) | ||
| Cankers (fungi) | Young twigs and branches die from the terminals and appear burned. Branches may be bent over at the tip, resembling a shepherd's crook. Dead leaves generally remain on the branch. Affects members of the rose family. | Not usually a problem on river birch, unless under extreme drought or low fertility. |
| Iron chlorosis (physiological) | On high pH soils interveinal chlorosis may develop due to unavailability of iron. | See chlorosis under oak. |
| Leaf spot (fungus) | Anthracnose leaf spot on some species, especially river birch. Small, round spots with a dark border. | Control usually not necessary |
| Rust (fungus) | White, powdery areas appear on leaves, stems, flowers, or buds. If the infection is widespread, the entire leaf blade may be covered with a thin, white fungal, powdery growth. Some mildew fungi affect older leaves first, such as on oaks, lilacs, zinnias, | Not a problem on river birch, the most commonly planted species. Occasionally reported on less frequently planted species. |
| Black locust (Robinia) | ||
| Trunk and limb decay or rot (fungi) | At first there may be no outward signs of decay or rot. However, if cavities or areas of exposed wood occur on the trunk, or if branch stubs are deteriorating, wood rot is likely. Decay is certain if mushrooms in the form of bracket or conks are seen on the limbs or trunk. This is a problem of older or mature trees. | The most effective procedure against this problem is prevention. Decay fungi enter only through wounds. If wounds occur they need to be treated immediately. If pruning is necessary follow accepted methods--no stubs. Fungicides are not effective |
| Boxwood (Buxus) | ||
| Cankers and leaf spot (fungi) | Poor growth occurs on some branches. Leaves die. Small, rose-colored, waxy pustules are commonly found on leaves and branches. These pustules are generally caused by secondary fungi colonizing stressed tissues. Another fungus is found on yellow leaves forming small, black fruiting bodies. | Remove and destroy infected parts. Remove leaves from branch crotches. Found mostly on winter-injured cultivars or plants stressed for other reasons. Plant only those cultivars known to be winter hardy. |
| Buckeye (Aesculus) | See "Horsechestnut". | |
| Buckthorn (Rhamnus) | ||
| Decline (nematodes) | Stunting and yellowing is caused by root-feeding nematodes. This is the principal cause of decline of buckthorn. | Do not replant in affected areas. |
| Callery pear (Pyrus) | ||
| Bacterial fire blight (bacteria) | Young twigs and branches die from the terminals and appear burned. Branches may be bent over at the tip, resembling a shepherd's crook. Dead leaves generally remain on the branch. Affects members of the rose family. | Some cultivars are reported resistant. Bradford cultivar not recommended. Fire blight is not as severe on Callery pear as on certain highly susceptible apples, pears, crabapples, and firethorn. Infection is generally limited to short shoots. |
| Catalpa (Catalpa) | ||
| Leaf spots (fungi) | Brown spots scattered over the leaf. Worse on trees injured by midges, according to some reports. | Control midges with appropriate insecticides. Leaf spots are generally not serious enough to warrant specific control measures. |
| Powdery mildew (fungus) | White, powdery areas appear on leaves, stems, flowers, or buds. If the infection is widespread, the entire leaf blade may be covered with a thin, white fungal, powdery growth. Some mildew fungi affect older leaves first, such as on oaks, lilacs, zinnias, | Seldom serious enough to warrant control. |
| Verticillium wilt (fungus) | Wilting and yellowing of foliage is followed by premature defoliation. One limb of the tree may be affected first. Gradual dieback may be apparent. Yellow-brown, brown, black, or greenish-black streaks may be found in outer rings of wood of infected branches. In herbaceous plants, the vascular system of stems and roots are discolored. | |
| Cherry, Ornamental (Prunus) | See "Purple leaf plum". | |
| Cinquefoil, Shrub or Shrubby (Potentilla) | ||
| Powdery mildew (fungus) | White, powdery areas appear on leaves, stems, flowers, or buds. If the infection is widespread, the entire leaf blade may be covered with a thin, white fungal, powdery growth. Some mildew fungi affect older leaves first, such as on oaks, lilacs, zinnias, | Neither rust nor powdery mildew are common in Ohio. Seldom require control measures. |
| Rust (fungus) | White, powdery areas appear on leaves, stems, flowers, or buds. If the infection is widespread, the entire leaf blade may be covered with a thin, white fungal, powdery growth. Some mildew fungi affect older leaves first, such as on oaks, lilacs, zinnias, | |
| Cotoneaster (Cotoneaster) | ||
| Canker (fungus) | Young twigs and branches die from the terminals and appear burned. Branches may be bent over at the tip, resembling a shepherd's crook. Dead leaves generally remain on the branch. Affects members of the rose family. Dieback of branches on some species, especially following severe winters. | Infection usually occurs after plants are predisposed by winter injury. Prune out affected branches. |
| Fireblight (bacterium) | Young twigs and branches die from the terminals and appear burned. Branches may be bent over at the tip, resembling a shepherd's crook. Dead leaves generally remain on the branch. Affects members of the rose family. | Some species are resistant. |
| Crabapple (Malus) | ||
| Fireblight
(bacterium) (Figure 31) | Sudden wilting, dying, and browning or blackening of new shoots. Leaves on these shoots die; ends of branches appear burned. Initial infections often occur on blossoms. | Remove infected branches by pruning them off 8 to 12 inches below the diseased tissue. Avoid excess nitrogen fertilizer. Plant resistant varieties. See list on page 18. If pruning shears are used, wipe them well with 70 percent alcohol between each cut. |
| Frogeye leaf spot
(fungus) (Figure 32) | Tan spots are small but can run together to cause large dead areas. White, powdery areas appear on leaves, stems, flowers, or buds. If the infection is widespread, the entire leaf blade may be covered with a thin, white fungal, powdery growth. Some mildew fungi affect older leaves first, such as on oaks, lilacs, zinnias, | Not common or serious on most recommended varieties; See Some Suggested Crabapples for Ohio under Chapter 5. |
| Powdery mildew (fungus) | White, powdery areas appear on leaves, stems, flowers, or buds. If the infection is widespread, the entire leaf blade may be covered with a thin, white fungal, powdery growth. Some mildew fungi affect older leaves first, such as on oaks, lilacs, zinnias, | Not common or serious on recommended varieties in Ohio. See Some Suggested Crabapples for Ohio under Chapter 5. |
| Rust (fungus) | White, powdery areas appear on leaves, stems, flowers, or buds. If the infection is widespread, the entire leaf blade may be covered with a thin, white fungal, powdery growth. Some mildew fungi affect older leaves first, such as on oaks, lilacs, zinnias, The fungus that causes this disease also produces rust galls on some species of junipers. | Not common or serious on most recommended varieties in Ohio. See Some Suggested Crabapples for Ohio under Chapter 5. Avoid the few known rust-susceptible cultivars or do not plant near susceptible junipers. |
| Scab
(fungus) (Figure 33) | Olive green to black spots with radiating edges on leaves. Leaves turn yellow and fall prematurely beginning in June or July. Small, round dark areas that become corky are found on the fruits. Trees may be 75 percent defoliated 6 to 8 weeks before normal time. | Rake and destroy or haul away fallen leaves in the fall. Plant resistant varieties; See Some Suggested Crabapples for Ohio under Chapter 5. Apply fungicide sprays several times at 10 to 14 day intervals starting at early leaf development. Be sure to obtain good coverage if planting susceptible cultivars. |
| Scorch and dieback | These diseases commonly result from winter damage as well as from poor root function coupled with high temperatures and moisture shortage. In some cases, bacterial infection of the plant is involved. Scorched leaves are brown around the edges and sometimes between the veins. Leaf yellowing and wilting may occur in late summer. Dieback and decline is often mistaken for Verticillium wilt. No wood streaking is present, however. Girdling roots may also cause these symptoms. See Chapter 7 for identification of Verticillium. | |
| Dogwood (Cornus) | ||
| Anthracnose blight
(fungus) (Figure 34) | Small, purple-bordered spots and irregular blotches on leaves. Flower bracts can also be affected. Twigs can be infected through leaf petioles or through small lesions in the bark of shoots. Twig dieback occurs and in time larger branches die. The leaf spot phase can be mistaken for other fungal leaf spot diseases. If you suspect dogwood anthracnose, send samples to the Plant Pest and Diagnostic Laboratory. | Flowering dogwood is very susceptible, although some cultivars may be less susceptible. Other dogwoods are more resistant. Kousa or Chinese dogwood is sometimes used as replacement, although it flowers a month later. Avoid heavily shaded sites. Fungicides applied at bloom and one month later may be helpful. Some Cornus florida X Cornus kousa hybrids are reported resistant. |
| Canker and dieback (fungus) | Dieback of branches and stems. Cankers form and contain small raised black dots which contain the fungus spores. Prevalent on the shrubby species of dogwood. | Water during dry periods. Fertilize properly. Prune out diseased branches. |
| Leaf spot (fungi) | Angular, grayish spots with dark purple margins found first on lower leaves, especially during wet weather. Septoria fungi are the most common causes. | Red and yellow twigged dogwoods are particularly susceptible. Avoid overhead watering. Fungicides may be used if necessary. |
| Powdery mildew
(fungus) (Figure 35) | White, powdery areas appear on leaves, stems, flowers, or buds. If the infection is widespread, the entire leaf blade may be covered with a thin, white fungal, powdery growth. Some mildew fungi affect older leaves first, such as on oaks, lilacs, zinnias, Becoming a more common problem. New growth is usually affected with leaves twisted and rolled. Reddening of foliage is sometimes apparent. On some species found on older leaves. | Plant in lightly shaded area. Avoid dense shade. Promote conditions that improve air circulation. |
| Scorch and dieback (physiological) | These diseases commonly result from winter damage as well as from poor root function coupled with high temperatures and moisture shortage. In some cases, bacterial infection of the plant is involved. Scorched leaves are brown around the edges and sometimes between the veins. Leaf yellowing and wilting may occur in late summer. Dieback and decline is often mistaken for Verticillium wilt. No wood streaking is present, however. Girdling roots may also cause these symptoms. See Chapter 7 for identification of Verticillium. | Do not plant in full sun or other unfavorable sites. Control borers. |
| Elms (Ulmus) | ||
| Black leaf spot (fungus) | The most common leaf spot is small, black, slightly raised and becomes shiny. Spots may be numerous. Leaves turn yellow and fall. | Most frequent on Chinese and American elm. Control not usually necessary. Destroy fallen leaves, if feasible. |
| Elm yellows or Elm phloem necrosis (mycoplasma) | This resembles Dutch elm disease in its early stages. Wilting and curling of still- green leaves is also a common early symptom. Leaves eventually turn yellow, then brown. These early symptoms are more apt to be throughout the tree, unlike the early symptoms of Dutch elm disease. Discoloration of cambium and phloem tissue (inner bark) at the base of the tree is perhaps the most distinctive symptom. The tissue turns yellow or caramel brown. It often has a wintergreen odor. Death of the tree is fairly rapid. | Control of the elm leafhopper, which carries the infectious agent, should be carried out as a protective measure if the disease is present on other trees in the area. This may be difficult. European and Asiatic elms and some hybrids are resistant. Diseased trees should be removed. |
| Wilt and dieback or
Dutch elm disease
(fungus) (Figure 36, Figure 37) | Yellowing, wilting, and then browning will appear on leaves of individual branches. The sapwood in these branches will be streaked dark brown. The streaks will show up as circles or rings of brown in cross section. The fungus spreads rapidly in most elms, eventually killing the tree. | Integrated control programs can be effective in slowing the progress of the disease in a community. These programs involve controlling the insect vector (European elm bark beetle) of the fungus with proper insecticide treatment, prompt and complete pruning out of infections as soon as identified, removal and destruction of dead trees in the area, root graft isolation of healthy trees, use of resistant elm hybrids or varieties, and injection treatment with fungicides. Trained arborists should be consulted regarding the implementation of these integrated control programs. A community-wide effort generally is necessary. Plant hybrids and selections tolerant or resistant; be sure they are resistant to the elm leaf beetle, a serious insect pest. |
| Scorch and dieback (physiological) | These diseases commonly result from winter damage as well as from poor root function coupled with high temperatures and moisture shortage. In some cases, bacterial infection of the plant is involved. Scorched leaves are brown around the edges and sometimes between the veins. Leaf yellowing and wilting may occur in late summer. Dieback and decline is often mistaken for Verticillium wilt. No wood streaking is present, however. Girdling roots may also cause these symptoms. See Chapter 7 for identification of Verticillium. | |
| Wetwood or slime flux (fungus) | Wilting and yellowing of foliage is followed by premature defoliation. One limb of the tree may be affected first. Gradual dieback may be apparent. Yellow-brown, brown, black, or greenish-black streaks may be found in outer rings of wood of infected branches. In herbaceous plants, the vascular system of stems and roots are discolored. | |
| Euonymus (Euonymus) | ||
| Anthracnose (fungus) | Irregular tan or brown areas are found on the leaves, especially along the major veins of the leaf blade. The condition usually is noticed in late spring on lower parts of the plant. Deformation and twisting of leaves often result from infections that occ One of several leaf spot diseases, none of which are routinely common or serious. | Fungicides can be used, but usually not necessary in landscape situations. |
| Crown gall (bacterium) (Figure 38) | Roots and stems are infected with large galls. Generally a problem on evergreen types. | Destroy heavily infected plants. Prune out galls if few are present. Disinfest pruning shears between cuts in 70 percent alcohol. Plant other shrub types if problem persists. E. alatus (burning bush) is not susceptible. |
| Powdery mildew (fungus) | White, powdery areas appear on leaves, stems, flowers, or buds. If the infection is widespread, the entire leaf blade may be covered with a thin, white fungal, powdery growth. Some mildew fungi affect older leaves first, such as on oaks, lilacs, zinnias, | A problem on some evergreen types, not a problem on burning bush (E. alatus). Chemical sprays not usually necessary. |
| Firethorn (Pyracantha) | ||
| Fire blight (bacterium) | Young twigs and branches die from the terminals and appear burned. Branches may be bent over at the tip, resembling a shepherd's crook. Dead leaves generally remain on the branch. Affects members of the rose family. Blossoms may be blackened. Shoots dieback. Leaves do not drop. | Do not purchase infected plants. Some varieties are resistant. |
| Scab (fungus) (Figure 39) | Fruits are covered with scabby lesions and turn black. Leaves are covered with dark sooty areas, turn yellow and drop. | Plant resistant varieties. If susceptible, spray before and after flowering with a fungicide. |
| Forsythia (Forsythia) | ||
| Failure to bloom (abiotic) | Flowers fail to appear on otherwise healthy plants. | The plant forms flower buds for the following year in the late summer. Because forsythia is such a vigorous grower, many gardeners prune the plant back during the winter. This removes the flower buds. In addition, low winter temperatures (below 10oF) often kill the flower buds. |
| Twig and branch gall (fungus) | Nodular, abnormal growths of varying sizes appear along stems. In severe cases, twigs and branches will die. | This disease was once thought to be due to the crown gall bacterium. Recent evidence points to a fungal pathogen in some cases and a genetic disorder in other cases. Prune out and destroy infected branches. No chemical sprays are known to be effective. |
| Twig blight, dieback (fungus) | Caused by Sclerotinia and recognized by black, hard, conspicuous bodies in the pith when stems are split length-wise. These fungal structures are white at first. | Prune and destroy affected branches. Not a common problem. |