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As with almost every plant you will be growing, the Chrysanthemum is susceptable to disease and almost insects.
It has a really strong constitution and this usually enables it to survive most of them.
If you will take proper care of them they'll be strong and vigorous, but close planting and crowded plants will be sure victims of diseases and insects, especially in the highly inbred varieties that possess beautiful flowers with delicate colors but have very weak constitutions.
Now you will need to know the differences in disease to be able to try to eliminate them. One is a fungus disease that includes mildews, root rot, rustsand leaf spot (Septoria). You will need to spray the plant with copper or sulphur fungicides. You can look in your local store or at your local nursery and find many brands: carefully apply this fungicide following all the directions to have the maximum benefit.
The disease that is caused by foliar or leaf Nematode infestation is never round or circular as in the case with Leaf Spot or rust, and most will agree that a double strength spray of nicotine sulphate plus Bordeaux Mixture is effective for leaf Nematodes. Some others will prefer to propagate the new plants from top cuttings and then destroy the old stools in which the Nematodes winter over, I do prefer this method.
Verticillium Wilt will cause the plant to wilt and wither, and is a bacterial disease for which there is no known remedy: but always look around as there might be something new on the market.
Another thought is that the plants are more susceptable to infection when you grow the plants in the same soil for several years: move them to new soil about every three years.
On the subject of insects, remember on these plants there are two classes of insects, the chewing and the sucking. The chewing types include Caterpillars, Leaf Rollers, Asiatic Beetles, and Cut Worms. Rotenone, which is not
poisonous to humans, is good also, but again please check with your local nursery for their recommendations as these products change all the time.
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