Begonia: Leaf Nematode

Problem
Angular brown leaf blotches develop on the lower leaves first, then on the upper leaves. The blotches enlarge and eventually the leaves curl up, wither, and drop off. The plant is stunted, and new leaf buds may not develop.

Analysis
(Aphelenchoides olesistus)

This plant condition is caused by nematodes--microscopic worms that live and feed inside the leaf tissue. Infestation occurs when the foliage is wet. Nematodes migrate in the thin film of water on the outside of the leaf to infect healthy tissue, and are spread from plant to plant by splashing water. They are most severe in warm, humid areas. Leaf nematodes can survive for 3 or more years in plant debris and in the soil.

Solution
Remove and destroy severely infested plants. Pick off and destroy all the infested leaves and the next two leaves directly above them. As much as possible, avoid wetting the foliage. Inspect new plants carefully to be sure they are not diseased, and do not plant in infested soil. Spray weekly with an insecticide containing malathion or dimethoate using a spreader-sticker, until the symptoms stop spreading. (For information on spreader-stickers, see Applying Pesticides.)