| Disease (causal agent) | Symptoms | Survival of pathogen and effect of environment | Comments on control |
|---|---|---|---|
| Bacterial blight * (Dickeya chrysanthemi) |
Water-soaked lesions; pith becomes jellylike; tops turn black and exude drops of liquid. Stem may break or split. | In plant debris. Favored by high temperatures (80° to 90°F), absence of free water, and high humidity. | Use disease-free cuttings. Reduce humidity in growing areas. Dip cuttings in streptomycin. |
| Cottony rot * (Sclerotinia sclerotiorum) |
Stems rotted; flower rot is similar to gray mold. Cottony, white fungal mass may occur on rotted tissues. Black sclerotia may form inside or outside stems. | Sclerotia in soil. Spores produced from sclerotia are airborne but infect only through flowers and dead tissues. Favored by high humidity. | Same as for gray mold. Also, treat soil with PCNB before planting and spray foliage with iprodione or thiophanate-methyl. |
| Crown gall (Agrobacterium tumefaciens) |
Irregular or round galls on stems and sometimes leaves. | Soil and galls. Infection favored by moist conditions. | Destroy infected plants. |
| Foliar nematode (Aphelenchoides ritzemabosi) |
Dark green, angular spots in leaves develop progressively upward from base of plant. Leaves turn yellow. These nematodes are rarely important in California. | Adults can survive up to 3 years in dead leaves. Spread by splashing water. Leaves need to be wet for infection to take place. | Use disease-free plants. Discard infected plants. Avoid overhead irrigation. Control weeds. Submerge infected plants in hot water (115°F) for 10 minutes.** |
| Fusarium wilt * (Fusarium oxysporum f. sp. chrysanthemi and f. sp. tracheiphilum) |
Unilateral chlorosis of one or more leaves near the apex, followed by curvature of the stem towards the affected side. As the disease progresses, there is a general chlorosis and wilt and stunting of leaves. The vascular system becomes a reddish brown. | Soilborne and carried in cuttings. Favored by high soil temperatures (80°F). | Use disease-free cuttings in clean soil. Treat soil (see Verticillium wilt). Adjust pH of soil to 6.5 to 7.0 and use nitrate nitrogen. Avoid planting highly susceptible cultivars (Bravo, Cirbronze, Illini Trophy, Orange Bowl, Royal Trophy, Yellow Delaware) |
| Gray mold * (Botrytis cinerea) |
Brown, water-soaked spots on petals. Fuzzy gray fungal spores form on decayed tissues. Rotting of lower leaves. Fungus may enter and girdle stem. | In plant debris. Favored by high humidity, low temperatures (50° to 60°F), and water on plant. | Keep humidity low; avoid overhead irrigation. Protect foliage with a fungicide, especially lower-dense foliage. Mist blooms with chlorothalonil, iprodione, or fenhexamid. |
| Hollow stem (Pectobacterium (=Erwinia) carotovorum) |
Pith of rooting cuttings deteriorates and collapses. Affected tissues are brown. Surviving plants do not grow satisfactorily and pith collapse may extend upward involving several internodes. Red Torch, Tempo, and Tempter are very susceptible cultivars. | Bacteria may be present in vascular bundles of symptomless chrysanthemums. Bacteria also present in undecomposed debris. Favored by high temperatures and high moisture. | Use disease-free cuttings. Reduce humidity in growing areas. Streptomycin dips may be helpful. |
| Powdery mildew * (Golovinomyces cichoracearum) |
White powdery growth on leaves and stems. Found mainly on older leaves. | Airborne spores produced only on living plants. Favored by high humidity, crowding of plants, and cool weather. | Spray with piperalin to eradicate existing infections. Protect foliage with myclobutanil or triadimefon. |
| Pythium root rot and basal stem rot * (Pythium spp.) |
Girdling black lesions occur near soil line. Plants stunted because of reduced root system caused by root rot. Plants may die. | Soilborne pathogen. Spores spread in water or in soil. Favored by excess soil moisture and poor drainage. | Treat soil as for Verticillium wilt. Drench plants with oomycete (water mold) specific fungicide. |
| Ray blight (Phoma chrysanthemi) |
Basal leaf and stem rot. Below ground stem infection may cause a one-sided distortion and necrosis of foliage. Blackish rot of petals (ray blight) may extend into flower stalk. | In chrysanthemum refuse. Waterborne spores. Favored by rainy weather. | Use disease-free cuttings. Avoid wetting foliage and keep humidity low. Protect foliage with chlorothalonil. |
| Rhizoctonia stem rot (Rhizoctonia solani) |
Stem rotted at soil line. Plants stunted. Dark fungal strands may be visible with hand lens. May kill plants. | Soilborne fungus. Favored by warm, moist conditions. | Avoid deep planting. Spray base of transplants with thiophanate-methyl, chlorothalonil, or iprodione after planting, or treat soil with PCNB before planting. |
| Rust * (Puccinia tanaceti) |
Small pustules of powdery, chocolate-brown spores on undersides of leaves and on stems. | Airborne spores produced only on living plants. Free moisture necessary for infection. Principally a field disease. | Use resistant cultivars. Protect foliage with triadimefon or mancozeb before rust starts to build up. Avoid wetting foliage. Keep humidity low. |
| Septoria leafspot (Septoria obesa, S. chrysanthemi) |
Irregular or circular, brown or black dead spots develop progressively upward from base of plant. Tiny black fungal fruiting bodies in centers of spots. | In plant debris and in soil debris for 2 years. Spores spread in splashing water. Favored by wet weather. | Protect foliage with a fungicide at first sign of disease. Greenhouse: avoid wetting foliage. Keep humidity low. |
| Verticillium wilt * (Verticillium dahliae) |
Yellowing and wilting of foliage, may be one-sided. Leaves die and dry upward from the base of the plant. Vascular tissue may be discolored. | Soilborne for many years. Carried in cuttings and root divisions. Favored by cool weather followed by hot weather during flowering. | Use resistant cultivars and pathogen-free plants. Fumigate with chloropicrin or other chloropicrin combinations. In sunny climates, soil solarization might be considered. |
| White rust (Puccinia horiana) |
Whitish pustules on the lower leaf surface. On the upper surface the infection is evident as pale-green to yellow spots up to 4 mm in diameter. Raised, waxy, pink-colored pustules are formed on lower leaf surface and can coalesce into large areas covering the leaf, Pustules found on stems and sometimes flowers. As they mature and produce spores, they turn whitish in color. | This microcyclic rust survives on living chrysanthemum foliage. Teliospores in the leaf pustules germinate in place producing basidiospores that are airborne and infect by direct penetration. Favored by the same conditions as ordinary rust (cool, damp weather), except that direct sunlight destroys airborne basidiospores. Principally a greenhouse disease. | White rust is widespread throughout many areas of the world but is not confirmed as established in the United States. Any offshore introduction of this pathogen is a significant threat to the United States floriculture industry. It spreads rapidly in greenhouse and nursery environments, resulting in severe losses. As a quarantine pest, detection leads to federal and state regulatory action. See USDA Plant Pest Program for Chrysanthemum White Rust. |
| Virus or viruslike disease | Symptoms | Host range and natural spread | Comments on control |
| Aspermy (Tomato aspermy virus) |
Flower distortion and reduction in flower size. Color break in florets of red, bronze, and pink cultivars. Foliar symptoms not apparent. Some cultivars are symptomless. | Transmitted by handling, cutting tools, and vegetative propagation. Also transmitted by aphids. | Use disease-free cuttings. Control aphids; remove infected plants. |
| Chlorotic mottle (Chrysanthemum chlorotic mottle viroid) |
Mottling followed by complete chlorosis. May be confused with nutritional problems. Symptoms somewhat masked under low light and cool temperature conditions below 70°F. | Spread by handling, cutting tools, and vegetative propagation. | Use disease-free cuttings. Remove infected plants. |
| Spotted wilt (Tomato spotted wilt virus, Impatiens necrotic spot virus) |
Frequently one-sided in plant. Ring patterns on leaves of some cultivars. Leaf distortion and necrosis. Dark necrotic streaks on stems. Flowers may be distorted and with some necrosis. | Thrips-transmitted; not spread by cutting knife, but can be spread by vegetative propagation. Many weeds and perennial ornamental plants act as reservoirs of virus (dahlia, calla, nasturtium, mallow, knotweed, plantain, and others). | Eliminate nearby susceptible ornamental plants and weeds. Control thrips inside and outside growing areas. Use virus-free cuttings. |
| Stunt (Chrysanthemum stunt viroid) |
General stunting of plants. Foliage may be pale with upright, young leaves. Flowers are smaller than normal and some cultivars may flower 7 to 10 days early. | Viroid is easily transmitted by handling, cutting knives, vegetative propagation, etc. Not spread by insects. Viroid has a wide host range. Symptomless in some plants. | Obtain disease-free plants from a propagation specialist using an indexing program. |
| Chrysanthemums are also susceptible to Ascochyta blight (Mycosphaerella ligulicola, Ascochyta chrysanthemi), aster yellows * (aster yellows phytoplasma), charcoal rot (Macrophomina phaseolina), and southern blight * (Sclerotium rolfsii). | |||
| * For additional information, see section on Key Diseases. |
| ** For additional information, see section on Nematodes. |