Mums are susceptible to fungal disease and insect infestations.
Chrysanthemums, also called mums, come in hundreds of different varieties, each offering flowers in colors of crimson, yellow, gold and pink. Chrysanthemums cannot tolerate excessive moisture as this makes them susceptible to fungal diseases that may kill them. If you notice your mums wilting, turning brown or simply not growing at all, check for the presence of a fungus or virus.
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Fungal infections, such as bud rot, verticillium wilt and mildew, commonly afflict chrysanthemums. Symptoms show up on the plant's leaves. Rusted leaves develop brown spots under the leaves, and later the leaves die. To control rust, apply sulfur to the entire plant. If the mums suffer from mildew, white and gray fuzzy spots will appear on leaves. Sulfur also helps control mildew. If the mum's leaves turn brown or red, the plant may suffer from Septoria leaf spot. Infected leaves also bear black spots. Bud rot infects the mum's unopened buds, causing them to stay closed. Treat both problems with a Bordeaux mixture fungicide.
Chrysanthemums that wilt, stop growing or producing flowers may suffer from verticillium wilt. Verticillium also turns leaf veins yellow. This soil-dwelling fungus is difficult to remove completely. Fungicide has little effect once verticillium has infected the mum. Pull up any infected mums and burn them to prevent spreading the disease. Do not put any new mums in the infected soil. The new mums will become infected, as well.
Several diseases cause mums to look stunted. One viral disease, aster yellows, transmits to mums via an insect called the leafhopper. Infected mums grow weak shoots and small flowers. Another viral disease, chrysanthemum stunt, causes leaves to fade. If you touch an infected part of the plant, it is possible to transmit the disease to other plants. Leaf nematodes also cause stunted mums. These microscopic worms slowly eat away at the mum's leaves. Look for brown patches between leaf veins and undeveloped flower buds. For all these diseases, it is best to remove and burn the infected plant as quickly as possible.
Diseases are hard to stop once they start, but proper care all season long should keep your mums healthy. Fungicides and disease-prevention sprays ward off many problems. Apply them every week when it rains heavily to ward off developing fungus. Give mums room to breathe by planting them in open areas. Keep the leaves as dry as possible when you water. It is best to water mums in the early morning.