| Common Diseases of Landscape Plants in Ohio | ||
|---|---|---|
| General Diseases Found on Many Ornamental Plants | ||
| Disease | Description | Management Options |
| Leaf and Flower Diseases | ||
| Anthracnose blight (fungi) (Figure 13) | 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 occurred during bud break and leaf expansion. The blight is commonly found on ash, white oak, maple, and sycamore, particularly when such plants are growing in damp, cool places. Frost injury, herbicide damage, and leaf scorch are often mistaken as anthracnose infection. Examine affected leaves closely for fungal fruiting bodies. | Rake and burn fallen leaves and plant debris. Fertilize and root-irrigate to maintain vigor and promote refoliation of affected areas of the plant. Avoid overhead irrigation on or near affected plants, especially at night or late in the day. Prune away surrounding shade- producing vegetation to promote dry air movement over the affected plants. Thin plants if needed to improve air movement and speed drying time. If necessary, spray with a properly labeled fungicide during bud break in the spring. See materials for "other leaf spotting and blighting fungi". |
| Botrytis blight (fungus) (Figure 14) | Leaves develop brown, rotted areas. Stems and flower stalks may suddenly wilt and fall over. Flower buds may turn dark and fail to open. Opened flowers appear rotted and discolored and fall prematurely. During wet weather, affected areas may be covered with brown, fuzzy sporulation of the Botrytis fungus. | Cut off, rake, and burn plant debris in the fall. Avoid overhead irrigation late in the day or at night. Prune away shade-producing vegetation. Space plants well when planting. Thin plants if needed to improve air movement and speed drying time. If necessary, spray with a properly labeled material during cool, wet weather. See Table 8. |
| Powdery mildew (fungi) (Figure 15) | 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, or chrysanthemums. Others affect newer shoots, such as on roses, flowering dogwood, London plane tree, or crabapples. When new shoots are affected, leaf curling and shoot stunting and twisting may be severe. | Plant susceptible plants in a sunny location and do not crowd them. Prune away surrounding shade-producing vegetation. Thin plants if needed to improve air movement and speed drying time. Avoid overhead irrigation late in the day or at night. At the first sign of disease, spray regularly with a properly labeled fungicide. See Table 8.
Note: Many powdery mildews are not serious enough to warrant the use of fungicide. |
| Sooty mold on leaves (fungi) (Figure 16) | Black sooty growth that partly or completely covers needles or leaves of a tree is caused by various fungi growing on honeydew secretion of sucking insects such as scales, aphids, and mealy bugs. | Determine the insect problem, then use approved insecticides to control them. As the food supply is depleted, sooty mold growth will disappear. Wash plants with a weak soap solution if desired. |
| Rust (fungi) (Figure 17) | Orange to reddish-brown, often powdery pustules or spots will form on leaves, twigs, or fruit. Sometimes galling and disfigurement will be associated with the infections. In midsummer, infections on some plants will produce cup-like structures or tubular growths that will bear abundant orange spores. | Resistant cultivars are available in some cases. Avoid overhead irrigation late in the day or night. Prune away shade-producing vegetation. Thin plants if needed to improve air movement and speed drying time. Rake and burn fallen leaves and plant debris. If necessary, spray regularly during known infection periods with a properly labeled material chosen from Table 8. Rust infection may occur several days or weeks before symptoms are seen. |
| Dodder (Cuscuta) (parasitic seed plant) (Figure 18) | Yellowish/orange vining plant strands grow, twist around, and quickly engulf infected host plants. Tiny flower and seed clusters are borne irregularly along the dodder vines. Affected plants eventually wither and die--mostly a problem on herbaceous ornamentals. | Dodder is a parasitic flowering plant. It is an extremely infectious pest that is hard to eradicate once it becomes established. It has a wide host range. Remove and destroy all infected plant material at the first sign of an infestation. Apply a properly labeled pre-emergent herbicide regularly to affected areas for the next three or four growing seasons to destroy the persistent dodder seeds as they germinate. |
| Leaf spot (fungi and bacteria) (Figure 19) | Most leaf spots start on lower leaves. Many gradually increase in size to form large, dark brown to black areas. Others remain small and distinct but increase in numbers and begin to develop on younger leaves as the season progresses. Small, black, spore-forming pustules can often be seen in the spots when fungi are the cause. For more specific descriptions, see the various host plant sections that follow. | Avoid overhead irrigation, especially late in the day. Prune away surrounding shade-producing vegetation. Do not crowd newly planted material. Renovate older landscapes to reduce shade and plant crowding. For fungal leaf spots, spray plants at first sign of disease with a properly labeled fungicide. More than one spray may be needed. Leaf spot diseases of trees in the landscape are generally not damaging enough to warrant chemical control. |
| Twig and Branch Diseases | ||
| Cankers and twig or tip blights (fungi) (Figure 20) | Canker infections show up as definite areas that vary in color from the surrounding healthy bark. When cankers girdle the twig or branch, the end of the branch dies. Then the result is twig blight. Small bumps (spore-forming pustules of the fungus) are often seen in the canker. Under some conditions, these may produce an ooze of orange, white, or brownish spore masses during rainy weather. Close examination of blighted twigs will generally reveal areas of infection at the base of the dead tissue. | Prune out cankers or blighted branches. Do not prune during warm, rainy weather. Reshaping of the plant may be necessary if many branches are removed. Fertilize to encourage new growth and quick healing of pruning wounds. Water during dry weather. Fungicide treatments are not effective preventives or curatives for cankers but may aid in prevention of twig or tip blights. |
| Fire blight (bacterium) (Figure 21) | 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. | Prune out branches well below signs of damage. Dip pruning tool in 70 percent alcohol between each cut. Avoid heavy nitrogen fertilization, which produces more susceptible soft growth. Avoid splashing water. Plant resistant varieties or species when available. |
| Root and Crown Diseases | ||
| Chlorosis (or yellowing) (Figure 22) | This condition can occur in response to many types of environmental stresses, including those that result in root decline. Leaves will be smaller than normal. The yellowing may be general or only between the major veins of the leaf. One branch or the entire plant may be affected. In later stages or during droughts, leaf scorch, wilt, and leaf drop may occur. | If the condition has resulted from a nutritional imbalance, fertilization with "trace" minerals and/or addition of materials to increase soil acidity (lower the pH) may help. Vertically mulch to relieve soil compaction if needed. Water deeply once a week during dry spells, but only if the soil has dried to moderate levels within the fibrous root-growth areas. Control root-damaging insects with appropriate soil insecticides. Check for spider mites and treat appropriately. Improve drainage if wet soils are suspected to be the cause of the problem. Recently planted material can be lifted and replanted if planted too deeply. |
| Leaf scorch and dieback (decline) (Figure 23) | 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. | Investigate and attempt to correct the root stresses if present. Vertically mulch to open the soil, increase aeration, improve water penetration, and improve drainage. Water deeply once a week during dry spells. Pruning may be helpful to improve the root-shoot balance. Add plant fertilizers only if soil tests indicate shortages, imbalances, or pH problems. Protect cold-sensitive plants from winter wind. Be sure soil is moist in the fall. Plants, especially evergreens, need moisture from the soil to counter the damaging effects of winter wind and sun. If girdling roots are the cause, it may be possible to cut them out. |
| Dieback | Rapid dieback may be due to damage from rabbits, mice or moles feeding on the lower stems or trunks. | |
| Root, stem or crown rot (fungi) (Figure 24) | Leaves turn yellow, wilt, and droop. Leaf petioles and the base of the leaf blade may turn brown or black. Later, brown, rotted tissue may be evident at the base of the plant. Plants become loose and wobbly, pulling easily from the soil. These diseases are most severe during high soil moisture and high temperature periods. | Once plants become infected, they rarely recover. Remove diseased plants. Improve drainage of the planting site. Do not plant sensitive plants in wet areas or cover roots with heavy mulches. Never use plastic sheeting as a mulch or weed preventer. Porous landscape cloth or organic mulches are preferable to plastic. Buy disease-free plants with vigorous root systems. Before replanting affected areas, treat soil with a fungicide for water mold root rots and one for other root and stem-rotting fungi. See Table 8 for appropriate materials. |
| Crown gall (bacterium) (Figure 25) | Infected plants may be stunted and do not flower well. Galls or larger masses of tissue form at the crown or at the point of grafting. Galls on stems and roots may be found on some plants. | Purchase gall-free plants. Remove and burn severely infected plants. Keep plants vigorous with proper amounts of fertilizer and water. Control insect and mite pests that further weaken infected plants. Consider using other plants that are not susceptible. |
| Vascular System Diseases | ||
| Wetwood or slime flux (bacterium and yeast) | Sap seeps out of the infected areas through cracks, wounds, and pruning cuts and flows down the trunk. The foul-smelling substance often kills the bark. Insects are commonly attracted to the fermenting sap. This disease is often found on maple, birch, elm, oak, poplar, sycamore, and willow. | There is no cure or preventive measure. Avoid damaging bark in any way. Drill into the infected wood to relieve internal sap pressure. Holes should be 6 to 14 inches below the fluxing area, drilled on an upward slant. Insert a piece of pipe to carry the flux away from the tree and base of the trunk. Water infected trees deeply once a week during dry spells. |
| Verticillium wilt and
dieback (fungus) (Figure 26) | 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. | If herbaceous ornamentals are involved, resistant cultivars may be available. If resistant cultivars are not available, do not plant susceptible plants in infested beds. Severely infected trees will have to be removed. Trees with one infected branch should have the branch removed and the tree fertilized and watered during dry spells. Apply mulch after planting. Use resistant plant types if replanting is needed. See the list on Table 2 for resistant plant suggestions. |
| Bacterial leaf scorch (bacteria) | See "leaf scorch and dieback" above. | If bacterial leaf scorch is suspected, consult the OSU plant and pest diagnostic clinic or a professional arborist for diagnosis. Use the general stress- management practices discussed above. |