Hardy chrysanthemums that develop yellow-brown spots starting on the lower leaves and gradually moving up the stems may be infested with foliar nematodes. Nematodes are slender, unsegmented roundworms that are barely visible to the unaided eye. Foliar nematodes overwinter in the soil, in infested plant material. They swim up the film of water on the plants, created by spring rains, and enter leaves through the stomata. Yellow-brown spots on the leaves eventually run together and cover the entire leaf, which dies, turns brittle, and falls.
Severe infestations can kill entire plants.
Mulch plants in the spring to discourage nematodes from climbing up from the soil, and avoid spraying water on the leaves when watering. This disease is caused by the fungus Puccinina chrysanthemi . Rust infection causes pale areas to appear on upper leaf surfaces, with powdery orange pustules or spots directly beneath on the undersides of the leaves. Severely infected plants are much weakened and fail to bloom properly. Set new plants farther apart and provide better ventilation.