Foliage yellows and then turns brownish-black. In the early stages of infestation, the affected parts of the leaves appear in defined areas bounded by the larger veins. In time the entire leaf is invaded and becomes discoloured. Older leaves are attacked first and then symptoms spread up the stems. On Buddleja there may be no obvious leaf discoloration but the flower spikes are small and contain many aborted flowers as a result of eelworms feeding within the developing flower buds.
Eelworms are microscopic and not visible to the naked eye. To ensure the problem is eelworms, break affected leaves into small bits, and leave in a glass of water for 30 minutes. Tiny, moving masses of eelworms would be seen at the bottom.