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Photo by Joan Allen, UConn

 

Chrysanthemum season is upon us. This traditional and beautiful fall flower adds a splash of color when most other garden plants are fading for the season. Chrysanthemums have a long list of potential pest and disease problems but, most often, they are free of problems during their short reign of glory in the fall. This article will cover some of the most common problems, their symptoms and what to do to prevent or minimize them.

Several fungi can cause damage to leaves, flowers, and stems. They can cause spotting of leaves or petals and sometimes dieback of plant parts. The fungal disease ray blight results in spotted or killed leaves and stems along with flowers that may be blackened and deformed on one side. Fungal spore production, spread and new infections are all favored by moist conditions. Practices that minimize humidity and leaf wetness will in turn reduce these diseases. If possible, avoid overhead irrigation by watering at the base of the plant.  If this is impractical, water early in the day to promote rapid drying.  Space plants to allow for good airflow between them. Remove diseased plant parts or severely diseased leaves to protect those remaining. While fungicides are not typically necessary, they may be applied as directed on the label in severe cases to protect other plants.

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Symptoms of ray blight including aborted and damage flowers. Photo credit: Central Science Laboratory, Harpenden , British Crown, Bugwood.org

There are two important rust diseases of chrysanthemum. Symptoms on the upper surface of the leaves are quite similar for the two of them and consist of pale yellow leaf spots. To check for rust, flip the leaves over and look for sporulation.  Brown rust, the most common, will have small mounds of dark brown spores on the underside and white rust would have pale beige to peach colored spore masses.  White rust is important to report to state plant pathologists because it is an introduced and regulated disease.  The objective of these regulations and responses (plant destruction/quarantines) is to prevent this disease from becoming established in the United States. While this disease is primarily found in greenhouse and production facilities, it has been found in the landscape in Connecticut before. To inquire about a possible case, contact the UConn Plant Diagnostic Lab (email: joan.allen@uconn.edu, phone: 860-486-6740).

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Chrysanthemum brown rust spores (left) by Penn State Department of Plant Pathology & Environmental Microbiology Archives, Penn State University, Bugwood.org and white rust pustules (right) by Joan Allen, UConn.

Other problems that can affect the leaves and sometimes other parts include a bacterial leaf spot, foliar nematodes and a number of virus and viroid diseases. Symptoms of bacterial leaf spot of chrysanthemum are tan to dark brown areas that may be bordered by yellowing. Brown areas may be delimited by major leaf veins, giving the spots an angular appearance. Spotting may be associated with wilt or dieback. Spread is via splashing water, infected plant debris, or contaminated tools, hands, etc. Avoid working among wet plants and overhead irrigation. Remove and discard infected plants.

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Bacterial spot of chrysanthemum. Photo credit: R.K. Jones, North Carolina State University, Bugwood.org

Foliar nematodes are microscopic worm-like organisms that move in a film of water on plant surfaces. They enter leaf tissue through the stomates (pores) and feed and reproduce within the leaves. Their movement within the leaf is restricted by the veins so that damage appears as a patchwork of angular leaf spots. Minimizing leaf wetness will reduce spread. Remove symptomatic leaves if there are only a few; if there are many, it’s best to discard the plant.

Foliar nematode injury on Chrysanthemum. Photo credit: Penn State University, Bugwood.org

Virus and viroid symptoms can include yellowing, stunting, rings or mottled patterns on the leaves, or plant deformity. Many of these pathogens are spread by insects that feed by piercing and sucking sap from the plant. Infected plants should be discarded.

In addition to this already somewhat long list, chrysanthemums may succumb to vascular wilt diseases caused by the fungi Fusarium and Verticillium.  These soil-borne fungi infect via the roots and grow within the plants xylem (conductive tissue) resulting in impaired movement of water and nutrients from the roots to the upper parts of the plant. Symptoms include leaf dieback, often on one side of the plant and sometimes beginning with the lower or older leaves first, wilt, and brown discoloration of the vascular tissue within the lower stem. Because both of these can survive without a suitable host plant in the soil for several years or more, alternative and non-susceptible plants should be planted in affected areas. Many cultivars are resistant to both diseases.

Root rot can affect chrysanthemum and can be caused by both fungi and water molds. Healthy roots should be creamy white and crisp or firm. If the plants are wilting or dying back, a check for brown, soft roots can be done by pulling the plant and inspecting them. Excess soil moisture due to poorly drained soil or overwatering promotes root rot. Avoid planting in poorly drained sites and avoid overwatering.

Quite a few insect and mite pests can occur occasionally on mums, too. Several aphid species are attracted to them and high populations can cause yellowing, stunting or deformed new growth. Many aphids can be removed with a strong spray of water. Other alternatives include insecticidal soap and horticultural oil.

If chewing injury is observed, that could be caused by beetles or caterpillars. For beetles, neem products may repel them from feeding. Caterpillars can be killed using products containing the biocontrol agent Bacillus thuringiensis (Bt).

Four-lined plant bug nymph and typical feeding injury on a leaf.  Photo credit: http://www.extension.umn.edu/garden/insects

 

Several true bugs including the four-lined plant bug and tarnished plant bug feed on mums and many other plants by inserting piercing and sucking, straw-like mouthparts into leaves or stems and withdrawing sap. Tarnished plant bugs sometimes feed just below the flower buds, resulting in wilt of the stem.

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Chrysanthemum leafminer tunneling. Photo credit: Gyorgy Csoka, Hungary Forest Research Institute, Bugwood.org

 

Winding, irregular tunnels or blotches in the leaves are caused by the chrysanthemum leafminer. This pest is the larval stage of a fly. Insecticides are not generally recommended. Remove affected leaves or squash the miner within the leaf to kill it.

While this may seem like a daunting list of potential problems, I should restate that chrysanthemums in the landscape are usually pretty free from problems. The best ways to minimize the likelihood of trouble are to purchase healthy, vigorous plants free of any evidence of problems and to provide adequate water and an ideal site for the new plants. Once they’re in place, check on them regularly. Spotting signs of a problem early will give you the best chance of stopping it before it does serious damage.