Chrysanthemums, or garden mums, like full sun and fertile, well-drained soil. They need regular watering because their roots are very shallow. Drought will cause woody, stunted growth. Overwatering, on the other hand, causes yellowing leaves that blacken and drop. Although the list of diseases that may attack chrysanthemums is long, mums are relatively trouble-free.

This disease more likely to cause upper stem rot, with brown or gray coloured decay.  Be careful to use sterilized soil for planting your new cuttings, or treat cuttings with an approved antibiotic such as streptomycin

This is a bacterial infection, causing large swellings on the crown and adjacent roots.  Typically, you need to pull and destroy affected plants. 

If your chrysanthemums develop yellow-brown spots (starting on the lower leaves) and gradually moving up the stems your cultivars may be under attack from Nematodes, which  are thin roundworms which are hard to see without a magnifying glass.  These pests can last through the winter in trash in the soil, and when you plants are growing, can make their way up the stems and  enter the leaves.  Leaves turn yellowish and die.  Other garden plants may be affected as well. To control nematodes, you may wish to remove some soil near the plant in addition to the affected plant material.  Mulching and insecticidal soap can also help.

GRAY MOLD

This disease is caused by a Botrytis fungus, something which can affect many other plants as well.  Leaves show brown  spots.  Trouble shows up as a grayish patch, often in areas with minimal ventilation.  Ensure adequate space between you plants by either spacing or pruning.  Chemical control is available if needed.

Several different kinds of fungi can cause this problem. Uually, these spots are at first yellowish, and then become dark brown and can become large - even up to an inch in diameter. Leaves, starting usually with lower leaves, may wilt and wither prematurely.  Remove and destroy any infected leaves, being careful to pick up any which may have fallen to the ground.  A host of chemical solutions are available at your gardening center, should such become necessary, and be familiar with the directions.

This problem is caused by a type of fungus, which appears as reddish-orange spots on your plants.  The first solution is to remove and destroy affected areas.  If your plants are closely planted, move them further apart, or transplant every second one to get better ventilation around your greenery.  If this doesn't result in success, chemical controls are available.  As with most plants, some varieties are more rust-resistant than others, so you may wish to look at that option for subsequent years.

This is a fungus-based disease, causing yellowing and browning of the leaves, which die from the base of the plant upwards. The fungus starts in the ground, so is a bit tough to control.   Suffice it to say that you need to destroy all plants which are infected, pulling them by the roots, since that is the entry point of this pest.  You may need to go looking for plant cultivars which offer resistance to wilt, if increasing the soil pH doesn't give the desired result.

Chrysanthemums are subject to lots of virus diseases, which can make the plants spindly, or stunted.  Since none of these plants will amount to anything, the typical response is to pull them by the roots and destroy them.  Do not put them into the compost pile.  While there isn't much in the way of cures, the best you can do is to recognize that viruses are brought to the plant by some means (called 'vectors'), usually insects, so you need to control leaf-hopping or sucking insects.  The secondary control is to remove nearby weeds, some of which may harbour these viruses, and giving the aforementioned insects a readily-available virus source.