Chrysanthemum: Leaf Nematode

Problem
Fan-shaped or angular yellow-brown to gray leaf blotches develop progressively upward from the lower leaves. The blotches join together and the leaf turns brown or black. The leaf then withers, dies, and hangs down along the stem. The plant is stunted and new leaf buds do not develop. In the spring, young, succulent, leafy growth becomes thickened, distorted, and brittle.

Analysis
(Aphelenchoides ritzema-bosi)

The cause of this damage is a microscopic worm (nematode) that lives and feeds inside the leaf tissue. The nematode is restricted in its movement by larger leaf veins. This confined feeding range creates the angular shape of the blotch. When the foliage is wet, the nematode migrates in the thin film of water on the outside of the leaf to infect healthy tissue. This pest is spread from plant to plant by splashing water. It penetrates the plant tissue by entering through small breathing pores on the underside of the leaf. Leaf nematodes are most damaging in the wet-summer, warmer regions of the country. They can survive for 3 years or more in plant debris and in the soil.

Solution
Remove and destroy severely infested plants. Pick off and destroy all of the infested leaves and the next 2 leaves directly above them. Avoid wetting the foliage as much as possible. Check new plants carefully to be sure they are not diseased, and do not replant them in infected soil. Spray with an insecticide containing malathion using a spreader-sticker (see Applying Pesticides). Spray weekly until the symptoms stop spreading.