Aster yellow is caused by a virus that is carried to chrysanthemums by leafhoppers. Diseases plants develop distorted flowers and many small, weak shoots. Leaf nematodes are microscopic, parasitic worms that feed on many garden plants. Nematode infected chrysanthemums have dark spots on the under surfaces of the leaves. They often have brown areas between leaf veins.
Leaves wither and dry, but they hang on the stems after they dies.
Plants are stunted and buds do not develop. Nematodes can live 3 or more years in dead chrysanthemum leaves. They also can live a long time in the soil. Do not plant healthy mums in soil where infected plants have grown. Wait at least 3 years or use a soil fumigant.