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. Nematodes can become dormant and survive for over a year
in fallen leaves. 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. Foliar nematodes are easily confused with
leaf spot ,but fungal leaf spots are most often black, not brown. They additionally
infest hosta and ferns.
Remove infested plant material, along with the surrounding soil. Mulch
plants in the spring to discourage nematodes from climbing up from the soil,
and avoid spraying water on the leaves when watering. Foliar sprays with
insecticidal soap may help reduce nematode populations.