Chrysanthemum sp. (Shasta daisy) can get Alternaria leaf spot that causes concentric rings of light and dark. Septoria leaf spot usually causes brown along the margins and tips of leaves. Botrytis may cause flowers to brown and die early as well as cause irregular brown spots on leaves and shoots. Fungicides and sanitation are helpful.
Chrysanthemum sp. (mums) may get Botrytis on buds, flowers, leaves, and stems. Infected areas are brown to dark brown in color (the gray fuzzy appearance requires proper moisture conditions). Infections start as small spots and rapidly expand to encompass the entire bud or flower. Sometimes mums get rust diseases that can be whitish or brownish in appearance. Fungicides and sanitation are helpful.
Fusarium wilt is also a possibility. Fusarium may enter through roots or at the soil line (collar rot). Plants wilt, then turn brown and die. Plants may go "piece meal" or all at one time. Rhizoctonia and Pythium can cause root rots and collar rots. Pythium occurs when soils drain poorly or are kept wet by over watering. Rhizoctonia may occur when soils are wet followed by hot dry conditions. Control is to fix drainage problem, reduce watering and then treat the soil with the correct fungicide.
Written by James Schuster, Extension Educator, Horticulture, and reviewed by Bruce Paulsrud, Extension Specialist, Plant Pathology and Phil Nixon, Extension Entomologist, University of Illinois at Urbana-Champaign.