Large numbers of microscopic nematodes (Aphelenchoides) can infest the foliage of chrysanthemums, begonias and other annuals and herbaceous perennials. Once infested, the leaves begin to yellow and then turn brownish-black. In the early stages, the affected parts of the leaves appear in defined blocks bounded by the larger veins. However, eventually the entire leaf is invaded and becomes discoloured.

Control

There is no chemical control available for this infestation; all affected leaves (and possibly the whole plant) should be burned as soon as symptoms are noticed.