Monitor herbaceous perennials for signs of foliar nematodes. Infested leaves are off-color, often turning purple or brown with a distinct margin as the nematodes cannot move through the leaf veins. Leaf damage will reflect the vein pattern of the leaf: leaves with parallel veins have stripes or rectangles; those with net-like venation will have spots outlined by the particular pattern of the veins. There may be large numbers of fallen leaves under severely affected plants. Foliar nematodes can be seen using a microscope by placing small pieces of symptomatic leaves in water; foliar nematodes will usually emerge from the edges of lesions and swim actively in the water, where they may be seen using 30X to 45X magnification.