| Specific Diseases of Herbaceous Annuals and Perennials | ||
|---|---|---|
| Disease and Host Plant | Description | Management Options |
| Aster (Aster) Hardy Aster | ||
| Botrytis stem rot | 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 | |
| Fusarium wilt (fungus) | Look for dark streaks up one side of the stem on older plants. Death of the entire plant may soon follow initial symptoms. Base of the stem is dark with a brown vascular ring just beneath the surface. Roots will be rotted. Pink fungus spores may be seen at the base of the stem. | Grow plants in a new location each year. Maintain plant vigor with appropriate watering, fertilization, etc. Not serious on the hardy asters. China asters 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, | Control seldom 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, Yellow-orange pustules on underside of leaf. | Destroy goldenrod and wild asters that are weed hosts. |
| Yellows (mycoplasma) | New growth is yellowish; plants are dwarfed. Upper leaves are erect. Flower heads open unevenly or only partly and have a yellowish-green cast. | Pull up and burn diseased plants when noticed. Control weeds, many of which are also hosts. Use insecticides to control leafhoppers that spread the mycoplasma to neighboring asters. |
| Aster (Callisteghus) China Aster | All of the diseases except powdery mildew reported above on hardy asters occur on the annual China aster and may be more serious on the China aster. | |
| Chrysanthemum (Chrysanthemum) | ||
| Botrytis blight
(fungus) (Figure 96) | 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 Only occasional on garden mums. | |
| Foliar nematode (nematode) | Wedge-shaped portions of leaf turn yellowish-brown between the veins. These run together, causing the leaf to die and fall. | Remove infected plants. Do not plant mums for three years in the affected area. Treat soil with a soil fumigant labeled for the particular site. |
| Leaf spot
(fungus) (Figure 97) | Small circular spots, at first on the lower leaves, becoming irregular dark brown to black. Leaves shrivel but commonly hang down on the stem. | Keep the foliage dry when possible. If the disease is serious, fungicides may have to become part of the control program. Clean up and destroy plant debris. |
| 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, Generally not common or serious on garden mums. | |
| 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, Pale yellow flecks develop on upper leaf surface, followed by brown, powdery pustules on lower leaf surface. | Usually of minor importance on chrysanthemums in Midwest gardens. |
| Stunt and virus diseases (viruses and virus-like agents) | Chlorosis, mosaic, and stunting are some of the symptoms. Flowering earlier than normal. | More of a potential problem on greenhouse mums. A virus-indexing program by the specialist propagator is the key to managing these diseases. |
| 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. | Some chrysanthemum cultivars are resistant, but no up-to-date list is available. Not a common problem in recent years. |
| Columbine (Aquilegia) | ||
| Crown and root rot (fungus) | Plants wilt, white mats or cottony patches of fungus often visible at base of plant; small hard, dark bodies are also visible at base of plant and within lower stem when cut lengthwise. 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. | Soil drenches may be helpful. These fungi survive in the soil for long periods of time. Plant in a new location. |
| Coreopsis (Coreopsis) (Tickseed) | ||
| 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, | Control seldom necessary. |
| Stem rot (fungus) | Plants wilt, lower stem discolored. White mats or cottony patches of fungus visible at base of plant. Hard, black bodies visible on or within the stem. | Not very common. The fungus may persist for long periods of time in the soil. If serious, plant in a new location. |
| Dahlia (Dahlia) | ||
| Botrytis blight (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 Flowers, leaves, and stems are affected especially under conditions of high humidity for prolonged periods. | Remove and destroy infected or old flowers. |
| Mosaic stunt or dwarfing (viruses) | A common characteristic is stunt or dwarfing with a tendency to produce many shortened lateral shoots. A bushy plant with mottled leaf color is produced. Scattered areas of yellow or light green tissue occur in the leaves at first. Conspicuous ring spots develop on leaves in some cases. | Dig up and remove or destroy diseased plants. Plant only tubers from healthy plants. Control aphids or other vectors with approved insecticides. |
| 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, | No treatment usually necessary for powdery mildew. |
| Ring spot (virus) | Scattered areas of yellow or light green tissue occur in the leaves at first. A conspicuous ring pattern develops later on the leaves. | Control thrips with approved insecticides. Thrips spread the virus to neighboring plants. Plant only tubers from healthy plants. |
| Delphinium (Delphinium) (See Larkspur) | ||
| Black leaf spot (bacterium) | Black, irregular spots occur on upper surfaces of leaves, petioles, stems, and flowers; the lower leaf surfaces have brown spots. Lower leaves are infected first. Disease gradually progresses upward until the entire stalk is killed. | Cut off and burn old tops. Avoid overhead irrigation. Prune away shade-producing vegetation. If necessary, spray with properly labeled material. |
| Bud and crown rot (bacterium) | Infection results in a rapid wilting of the whole plant and finally death. The decay is characterized by a strong, offensive odor. | Avoid planting in low, poorly drained areas. Avoid overhead irrigation, especially late in the day. Control soil insects in affected areas. |
| 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, | Control not usually necessary. |
| Root, stem or crown rot (fungi) | Two different fungi can cause this disease and kill the plant. In both cases white strands or mats of fungus become visible on and around the base of the plants. Large, hard, black fungus bodies soon form on or in the stems. A second fungus forms small, round, seed-like, tan to dark brown bodies on the white fungal strands and stems of diseased plants. | |
| False dragonhead (Physostegia) | Remove diseased plants and surrounding soil. Do not plant susceptible plants in the area for several years or chemically treat the soil. | |
| Crown and root rots (fungi) | Plants wilt and die. White, fluffy strands of fungus are visible at base of the plant. One fungus produces hard, black bodies on the outer stems or inside. They can be observed when stems are cut lengthwise. Another fungus produces small, round, seed-like bodies. These are cream-colored, becoming tan and brown. | 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. Soil drenches may be helpful. The fungi survive for a long time in the soil. |
| Fuchsia (Fuchsia) | ||
| Botrytis (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 Causes a leaf and stem rot. | Develops under high humidity. Remove badly blighted parts. |
| 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 common. When observed, it usually is on plant material brought to the garden center from another location. |
| Garden Lily (Lilium) | ||
| Botrytis blight (fungus) | Symptoms are numerous circular to oval leaf spots. They are light brown and are surrounded by a reddish-purple margin. Lesions also occur on buds, flowers, and stems. Bulbs may be affected. | Destroy diseased plant parts. Plant good- quality bulbs. |
| Stem rot (fungi) | Two different water mold fungi cause stem rot. One causes rot of the lower stem, and another rots the stem near the top of the plant and is called top rot. | 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. |
| Virus diseases (viruses) | Symptoms are flecking, necrotic streaks, and mottling of the leaves; plants may be stunted, flowers distorted or subnormal Several different viruses can affect a single plant. | Remove and destroy diseased plants. Keep insect vectors (aphids) under control. More of a problem on Easter lily in the greenhouse. |
| Geranium (Pelargonium) | ||
| Bacterial blight (bacterium) | Small, water-soaked spots, later becoming brown. Yellow patches and wedge-shaped brown areas develop. Leaves wilt or yellow. Later, stems develop internal vascular discoloration and finally a brown to black stem rot. | Purchase symptom-free plants. Sanitation and other practices by the greenhouse propagators are important in providing disease-free plants for the garden. |
| Botrytis gray mold
(fungus) (Figure 98, 99) | Leaf and blossom blight are the symptoms. 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 | |
| Rust
(fungus) (Figure 100, 101) | Rust-colored spore pustules in concentric circles form on the lower leaf surface. Yellow spots form on the upper surface. | Not common or persistent on plants in the landscape. |
| Stem and root rots (fungi) | Caused by soil-borne fungi. Roots are invaded and rotted. In some cases, a black rot develops at the base of the stem. Often a problem during propagation when the cuttings are being rooted. | Sanitation and use of pathogen-free rooting materials are important. |
| Gladiolus (Gladiolus) | ||
| Corm rots (fungi, bacteria) | Several speciea of fungi and bacteria result in corm lesions and leaf spots. | Commercially produced corms should be free of decay. If saving corms, avoid injury when digging. Store in cool, dry locations. Discard badly diseased corms. |
| Fusarium yellows and corm rot (fungus) | Symptoms are variable. Leaves may yellow. Base of leaves become diseased and the corms rot. Flowering may be delayed or lacking. | Some cultivars are tolerant. Inspect corms and discard those with any signs of decay or rot. |
| Mosaic (viruses) | Mottling of leaves, whitish streaks, and color abnormality in flowers are characteristics of mosaic. | Destroy diseased plants. Control insects, especially aphids, which transmit the virus to healthy plants. |
| Hollyhock (Alcea) | ||
| Anthracnose blight (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 | Anthracnose leaf blight and other fungal leaf spots are reported but are seldom serious. |
| Root, stem or crown rot (fungus) | 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. Sclerotinia (a fungus) causes this disease. | |
| Rust
(fungus) (Figure 102, 103, 104) | Abundant orange to dark brown pustules on leaf blades, petioles, and stems. 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, | This rust is systemic. It survives inside the crown of the plant. Destroy the plant. Purchase rust-free plants. |
| Iris (Iris) | ||
| Crown rot (fungus) | Tips of outer leaves die. The dead tissue develops until the entire leaf is dead. As rot progresses inward at the base of the plant, the leaves collapse. White fan-like fungus growth may be seen between leaves near the ground line. Rhizomes are not destroyed but are weakened. Light tan to brown fungal structures, the size of mustard seeds, are present on rhizomes and on the base of the plant. | Do not permit crowding of plants. Dig out rhizomes and replant only healthy material. Before replanting, treat the area with a properly labeled fungicide or plant in an area not recently planted with iris. |
| Leaf spot (fungus) | Small, oval spots are brown and often have a water-soaked margin. They may enlarge and blight most of the leaf. Older spots have gray centers and are dotted with small, black spore clusters. 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, | In the fall, clean up and destroy the leaves. Fungicides may be necessary in some cases. |
| 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, | Can be a problem on some species, but not reported on the common rhizomatous garden species in Ohio. |
| Soft rot (insect followed by bacterium) | At first leaves wilt slightly, then later become limp and die. The rhizome develops a soft, slimy rot and later turns dry and granular, finally decaying entirely. The problem is always associated with wounds or injury, usually those caused by the iris borer. | Do not permit crowding of plants. Dig out infected rhizomes and cut out rotted areas. Treat with insecticide for borer control. |
| Larkspur, Annual (Delphinium) | ||
| Root, stem and crown rots (fungi) | See Delphinium. The diseases listed under Delphinium may occur on Larkspur, but most are not common on the Annual Larkspur. | |
| Marigold (Tagetes) | ||
| Basal stem rot
(fungus) (Figure 105) | Plant wilts, sunken and discolored area on lower stem. Roots often rot as well. | Caused by a water mold fungus, or sometimes by Botrytis. Not common. Do not crowd plants and plant in a well-drained site. |
| Monarda (Monarda) | ||
| 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, | Powdery mildew may be a problem when clumps become too large or crowded. Provide good air circulation. |
| 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, | Usually does not require control. |
| Narcissus (Daffodil) | ||
| Basal rot (fungus) | Primarily affects the bulbs, rotting the interior tissues at first, Usually a disease of bulbs in storage. Lightly infected bulbs, if planted, result in stunted plants, yellow leaves, and premature death. | If storing bulbs, dry them quickly; avoid wounding when digging and cleaning prior to storage. Discard all infected bulbs. Bulbs from commercial sources should be free of disease. |
| Stem nematodes (nematodes) | Flower buds fail to open; mis-shaped stems and leaves; plants decline. Bulbs are also affected. | Destroy diseased plants and affected bulbs. |
| Pansy (Viola) | ||
| Anthracnose and leaf spots (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 Size and color of spots vary depending on fungus involved. | Fungicide sprays may sometimes be necessary. |
| Botrytis blight (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 | |
| Root rot (fungus) | Older leaves turn yellow. Plants are stunted and may die. Roots rot and may become black. | Destroy diseased plants. |
| Peony (Paeonia) | ||
| Bud blast (environmental) | Bud blast is a condition where flower buds abort. It is caused by planting too deep. | |
| Botrytis blight
(fungus) (Figure 106, 107) | 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 All parts of the plant may be affected. | Sanitary measures are helpful; remove and destroy diseased plant parts. |
| Leaf blotch (fungus) | Glossy, dark purple spots appear on top of the leaf; spots are dull chestnut on the lower surface of the leaf. Worse during moist weather on plants in shaded sites. Leaves are usually not killed. | Destroy old infected foliage at the end of season. Avoid overhead irrigation. Prune away surrounding shade-producing vegetation. Spray at first sign of disease with properly labeled fungicide. |
| Root knot (nematode) | Sprouts are weak and spindly. Larger roots are short, stubby, knotted, and gnarled. Smaller roots contain many small swellings. | Discard badly infected plants and plant clean plants in non-infested soil. Treat the soil with a properly labeled nematicide or have soil fumigated. Destroy old plants, and plant new material in a new location. |
| Root, stem or crown rot (fungus) | 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. | Remove and destroy all plant debris. In severe cases the entire plant should be dug and destroyed. |
| 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. | |
| Petunia (Petunia) | ||
| Botrytis blight (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 | |
| Root, stem or crown rot (fungi) | 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. At least three different fungi may be involved. Plants wilt and die. | More likely on plants set too deep. Be sure planting site is well drained. |
| Phlox (Phlox) | ||
| Leaf blight (physiological) | Symptoms first observed on lower leaves that die starting at the base. Found on older plants, especially old stems from which new shoots arise in the spring. | The problem is associated with moisture translocation in the plant. Old stems are unable to supply water to the leaves in dry weather on perennial phlox. Mulching may help. |
| Leaf spot (fungus) | Spots are tan to dark brown and circular with light gray centers. 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 and rarely needs control. |
| 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, | Perennial or garden phlox and annual phlox are susceptible. Resistant cultivars may be available. Fungicides may be necessary. |
| Stem nematodes (nematodes) | Plants decline; stem tips and leaves yellow, then die, and flowering is poor. Winter dieback may be severe. | Remove and destroy infected plants. Do not plant phlox back into severely affected beds. |
| Snapdragon (Antirrhinum) | ||
| Botrytis blight (fungus) | Flowers are invaded and eventually the smaller branches are infected, killing the entire flower spike. 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 | Remove infected plant parts and thin plants to maintain good air circulation (faster drying of plant parts). |
| Leaf spot and blight (fungi) | Grayish-white to brown leaf spots depending on the fungus involved. In some cases stem lesions may develop, causing a more extensive blight. | Remove badly diseased plants and general sanitation measures should be practiced. Fungicides may be helpful in some situations. |
| 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 on garden snapdragons. |
| 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, | Rust-resistant varieties are available. |
| Stem, crown and root rot (fungi) | Stems near the ground become gray or discolored; entire plant then wilts. A visible white cottony fungus growth develops on the outside of the stem near the ground. Large, hard, black bodies form on the outside and within the stems. Botrytis (see above) also can cause a grayish stem rot near the base of the stem. Also a microscopic water mold fungus causes stem and root rot of snapdragons. See page 30. | |
| Tulip (Tulipa) | ||
| Botrytis blight
(fungus) (Figure 108 and 109) | Young shoots, leaves, stems, flowers, and bulbs may all be affected. Grayish spore masses develop on diseased areas. | High humidities and frequent rain periods are favorable conditions for disease development. Badly affected plants should be destroyed. Do not plant bulbs with abnormal spots. |
| Basal stem and bulb rots (fungus) | Plants may not emerge; bulbs rot. Leaves that do develop soon die and rot at soil level. | Select clean bulbs for planting. If disease occurs, remove and destroy plants. If the disease has been common, avoid planting in that area. |
| Zinnia (Zinnia) | ||
| Alternaria leaf blight
(fungus) (Figure 110) | Circular to irregular leaf spots develop that are purple to dark brown and up to 1Ž2 inch in diameter. Centers often become grayish-white. Leaves become brown and dry. Much smaller spots develop on the ray flowers. Girdling stem lesions may occur. | Avoid overhead watering. Destroy diseased plants before overwintering. |
| Bacterial leaf spot
(bacterium) (Figure 111) | Reddish-brown to dark brown, angular spots up to 1/8 inch in diameter, with a prominent yellow halo. Spots may coalesce. | Avoid overhead irrigation. The disease is seed-borne; do not plant diseased seedlings. Destroy diseased plants, especially if infected when young. |
| Powdery mildew
(fungus) (Figure 112) | Grayish-white, powdery growth on the leaves, in patches at first. Usually develops in late summer and is more severe in shaded locations. | Plant in full sun. On problem sites, fungicides may be useful. |