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. Alternaria spp .
Symptoms consist of spots on the leaves.
Leaves may wither prematurely. The lower leaves are infected first. With a hand lens white masses of spores may be seen on the leaf spots. Hand pick and destroy the infected leaves. Also, regularly clean up and destroy dead plant debris in the garden to reduce spore populations.