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Chrysanthemum Diseases
Chrysanthemums and their close relatives Dendranthema and Leucanthemum are important floriculture crops. Although the list of diseases affecting Chrysanthemums or garden mums is long, they are relatively trouble-free given full sun, well-drained soil, adequate fertility, and adequate watering. Regardless of the crop, maximizing plant vigor by appropriate cultural practices is always an important disease management strategy.
Diseases of the Foliage
Leaf spots: Chrysanthemums are subject to several leaf spot fungi including Septoria chrysanthemi, S. chrysanthemella, Alternaria species, and Cercospora chrysanthemi. Symptoms first appear as yellow spots which turn brown to black. Spots often occur on lower leaves first and can coalesce into large necrotic areas and finally death of the entire leaf. Small black fruiting bodies may be seen in the lesions of some leaf spot fungi. Leaf spot diseases are encouraged by prolonged periods of leaf wetness and high relative humidity.
Practice good greenhouse sanitation. Regularly clean up and destroy infected plant debris and hand pick symptomatic leaves from lightly infested plants. Workers should be wash their hands frequently. Avoid splashing water onto plant foliage if possible. If overhead irrigation must be used, water early in the day to allow foliage to dry quickly. In severe cases, applications of fungicides containing azoxystrobin, chlorothalonil, fludioxonil, mancozeb, myclobutanil, propiconazole, or thiophanate methyl may be applied according to label instructions. Septoria leaf spot was once a more common problem in chrysanthemum production than it is now, largely due to the use of fungicides and plant sanitation programs that ensure clean stock.
Powdery mildew (Golovinomyces cichoracearum) is characterized by a white to ash-gray powdery growth on leaves and occasionally stems. Leaves may become distorted; severely infected leaves will shrivel and die. The disease is most serious during hot, humid weather. Unlike most fungal diseases, free water is not required for powdery mildew infection, but high humidity encourages disease development. Management is similar to that of leaf spots. Powdery mildew can be minimized by proper plant spacing, good air circulation, low relative humidity, and adequate light levels. Apply preventive fungicides at the first sign of disease with the active ingredients copper, azoxystrobin, pyraclostrobin, triflumizole, myclobutanil, triadimefon, propiconazole, sulfur, potassium bicarbonate, horticultural oil, or thiophanate methyl according to label instructions.
Gray mold (Botrytis cinerea ) may occur on petals, leaves, or stem cankers as brown, water-soaked spots. Infected plant parts may be covered with gray to brown, powdery masses of spores. Infected buds fail to open. Tender new growth and senescing tissues are most susceptible. Gray mold is favored by extended periods of cloudy, humid, wet weather. The pathogen gains a foothold in small wounds, then progresses to cause disease in healthy tissues.
Practice good sanitation including removing senescing flowers and leaves. Avoid wetting flowers when watering and don't overcrowd the plants. Provide good air circulation and keep humidity down to <80% (See Fact Sheet on Reducing Humidity in the Greenhouse). Apply preventive fungicides as soon as disease is detected. Fungicides with the active ingredients chlorothalonil, fludioxonil, benzovindiflupyr, cyprodinil, mancozeb, copper, fenhexamid, and azoxystrobin are among those registered for Botrytis control. Products containing two active ingredients are increasingly employed in management programs. Biorational products containing Ulocladium and Bacillus species are also available. Be sure to rotate applications among chemical classes as Botrytis can rapidly develop resistance when repeatedly exposed to fungicides within the same FRAC group. Iprodione and thiophanate-methyl are no longer recommended for Botrytis control because resistant strains have become common.
Rusts: Two species of Puccinia causes rust on chrysanthemums. Brown rust is caused by P. chrysanthemi. It is most common in late summer and is characterized by yellowish-green spots on upper surfaces of leaves and dark brown pustules on the undersides of leaves. P. chrysanthemi causes minor damage in the field and is uncommon on greenhouse plants. Severe infestation may damage large areas of leaves and lead to defoliation and reduced flower production. Infection occurs at 60-81°F. Rust management involves the same cultural practices described above. Cultivars resistant to rust include 'Achievement', 'Copper Bowl', 'Escapade', 'Helen Castle', ' Mandalay ', 'Matador', 'Miss Atlanta', 'Orange Bowl', and 'Powder Puff'.
P. horiana causes Chrysanthemum white rust. It was first observed in North America in 1977. Symptoms are white, pinkish or light brown pustules produced on leaf undersides with white, yellow, to pale-green spots on upper leaf surfaces. Chrysanthemum white rust results in leaf distortion, discoloration, defoliation, and plant death. Leucanthemum and Argyranthemum are not susceptible to this disease. White rust is primarily a disease of chrysanthemums in the greenhouse. Disease is most active in cool, humid conditions. Keeping foliage dry and lowering humidity in the greenhouse are critical steps in white rust management. It is subject to a federal quarantine and an eradication program in the United States. For more information, see the Chrysanthemum White Rust Fact Sheet.
Protect healthy plants from both brown and white rusts with fungicides containing azoxystrobin, chlorothalonil, fludioxonil, iprodione, mancozeb, myclobutanil, propiconazole, pyraclostrobin, or thiophanate methyl according to label instructions.
Bacterial leaf spot/ bud blight (Pseudomonas cichorii): Symptoms of bacterial leaf spot begin as water-soaked spots that turn tan to dark brown, sometimes with a yellow border. Discoloration may be prominent along leaf veins or lesions may become angular as bacteria growth is limited by major veins. Dark cankers may also appear on stems. Leaf wilting and death often follow.
Bacteria persist in or on infected plants, crop debris, infected seed, contaminated soil, and infested pots and tools. Key management practices include planting pathogen-free seed and cultivars, resistant varieties, good sanitation, and avoiding overhead irrigation or handling plants when they are wet. Once plants become infected with bacteria, it is best to rogue infected plants and those near them before the disease spreads. Bactericides such as copper and antibiotics are of limited effectiveness and plants cannot be cured.
Bacterial blight/ soft rot (Dickeya chrysanthemi and Pectobacterium carotovorum): Symptoms of bacterial blight extend beyond plant leaves to include water-soaked lesions on stems, darkening and death of buds and stems, and wilt and collapse of upper portions of the plants. Infected cuttings may have brown to black decay at their base. Vascular discoloration and pith rot may also be observed.
Bacterial blight survives in crop debris and is favored by surface moisture, temperatures >80°F, and high humidity. It is easily spread on infested tools, hands, or plants. Start with pasteurized growing media, use pathogen-free stock plants, reduce humidity and increase air circulation, avoid wetting foliage, and practice good sanitation. Regularly inspect crops and dispose of infected plants.
Foliar nematodes (Aphelenchoides ritzemabosi and A. fragariae): Nematodes are microscopic roundworms that live in the soil or in infested plant material. They swim in a film of water on wet plant surfaces and enter leaves through stomata. The development of yellow to brown, V-shaped lesions on lower leaves which advance up the plant is a good indication of nematode infection. Lesions are delimited by leaf veins. Lesions on the leaves eventually coalesce to cover the entire leaf which dies, withers, and falls. Carefully inspect cuttings and plants received from propagators. Remove infested plants and crop debris. Avoid wetting the foliage and overhead irrigation. Both A. ritzemabosi and A. fragariae have wide host ranges and can infect many common ornamentals.
Viruses and other infectious agents: Chrysanthemums are susceptible to a large number of virus diseases including Chrysanthemum Mosaic Virus, Cucumber Mosaic Virus, Impatiens Necrotic Spot Virus, and Tomato Spotted Wilt Virus. Viroid diseases include Chrysanthemum chlorotic mottle viroid and Chrysanthemum stunt viroid. Symptoms of virus and viroid infected plants can be similar and include stunting, spindly growth, and formation of dense rosettes. Flowers may be small, distorted or exhibit streaking and color break. Leaf symptoms are diverse and may appear as leaf yellowing, ring spots, lines, mottling, mosaics, vein clearing, distortion, crinkling, wilt and leaf drop.
Aster Yellows is a serious disease caused by organisms called phytoplasmas. It results in chlorotic foliage, plant stunting, profusion of spindly upright yellow shoots (witches' brooms), few or no flowers, flower distortion, transformation of flowers into leaves and shoots (phyllody), and yellow-green discoloration of flowers (virescence). Aster yellows is transmitted by the feeding activity of the Aster leafhopper (Macrosteles quadrilineatus, AKA M. fascifrons).
There is no cure for virus, viroid, or phytoplasma infected plants. Start with pathogen-free plants from a culture indexing program. Remove and destroy infected plants. Remove weeds that may also be hosts for pathogens and/or their vectors. Control the insects that transmit these diseases. Disinfect tools and equipment frequently.
Vascular Wilts
Chrysanthemums are subject to two vascular wilt diseases caused by Fusarium oxysporum f.sp. chrysanthemi and Verticillium dahliae. Both pathogens persist in the soil for many years.
Fusarium Wilt (Fusarium oxysporum f.sp. chrysanthemi and f. sp. tracheiphilium): The first signs of this disease are yellowing of foliage, stunting, and wilting, often along one side of plant. Plants may appear water stressed and foliage may turn brown and die. Stems show a reddish brown discoloration of the vascular system. Cool weather can delay the onset of symptoms. Fusarium is spread in contaminated soil and infected cuttings and is favored by warm temperatures, high relative humidity, overwatering, and poor drainage. Start with pathogen free cuttings or plants and pasteurized growing media. Adjust pH to 6.5 to 7.0 and use nitrate nitrogen fertilization. Avoid highly susceptible cultivars such as 'Bravo', 'Cirbronze', 'Illini Trophy', 'Orange Bowl', 'Royal Trophy', and 'Yellow Delaware'.
Verticillium Wilt: Symptoms of Verticillium wilt often appear only after blossom buds have formed; young vigorous plants may be symptomless. Foliage becomes yellow and wilted, sometimes only along leaf margins and on one side of the plant. Leaves begin to die from the base of the plant upward and often remain attached. Stems may exhibit dark streaks in the vascular system. Flowers may be absent.
This disease is favored when cool weather is followed by hot temperatues. Start with pasteurized growing media and pathogen-free cuttings. Many cultivars are at least partially resistant. Avoid susceptible cultivars including 'Bright Golden Ann', 'Echo', 'Glowing Mandalay', Mountain Peak', 'Puritan', and 'Wedgewood'. Verticillium can persist as survival structures called microsclerotia for many years in soil. Greenhouse sanitaton and clean stock programs have made Verticillium wilt less common in chrysanthemum production than it once was.
Diseases of the Flowers
Ray Blight: Caused by Ascochyta chrysanthemi (Mycosphaerella ligulicola), this disease affects the ray florets and may extend into floral stalks. Symptoms include a brown rot of ray florets the can extend into the receptacle. Flowers may be deformed and one-sided. Bud blast can occur in severe cases. Lower leaves and stems can also be affected. Leaf lesions are brown to black and can vary in shape and size. Leaves and stems may rot, and foliage may distort or die on one side of stem. Brown stem lesions may be seen near leaf nodes and are slightly sunken.
A. chrysanthemi persists in plant debris and spores are spread by wind and water. The disease is favored by overhead irrigation or rain. Start with pathogen-free cuttings. Avoid wetting foliage and flowers and keep humidity low. Good sanitation is essential. Protect foliage with chlorothalonil, myclobutanil, mancozeb, or iprodione.
Petal Blight (Itersonilia perplexans): This fungus also infects flowers of China aster (Callistephus) and some weeds in the Asteraceae. Small reddish-brown specks form on petals. On older flowers, the specks enlarge until the entire blossom is affected. Petal blight is most severe when temperatures are in the 60s.
Rogue and dispose of severrely infected plants. Individual flowers may also be removed. Provide good air circulation and don't overcrowd plants. Avoid overhead irrigation and keep flowers dry. Control weeds, especially those in the Asteraceae. Protect plants with propiconazole, myclobutanil, or potassium bicarbonate.
Diseases of Roots and Crowns
Like all floricultural crops, Chrysanthemums are subject to root and crown rots caused by species of Pythium, Phytophthora, and Rhizoctonia. See Root Diseases of Greenhouse Crops Fact Sheet.
For a full listing of products labeled for managment of these diseases, consult the New England Greenhouse Floriculture Guide.
References
- Dreistadt, S.H. 2001. Integrated Pest Management for Floriculture and Nurseries. University of California Division of Agriculture and Natural Resources. Publication 3402.
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