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Hydrangeas are vulnerable to both stem nematodes and root nematodes. When infested with stem nematodes (Ditylenchus dipsaci) the stems become swollen and split, and the leaves drop off. Root knot nematodes (Meloidogyne incognita and M. hapla) attack the roots, of course. The entire shrub becomes sickly, wilted, and stunted, with yellow or bronze foliage. It declines slowly and dies. If you look at the roots, you'll find that they're pooly developed and may be even partially decayed.
Nematodes are not insects, but slender, unsegmented roundworms. Most live in the soil and are 1/20 to 1/10 inch long. Add lots of compost, especially leaf mold, to the soil around infested hydrangeas to encourage beneficial fungi. Fertilize with liquid fish emulsion, pouring it into the soil as a drench; it repels nematodes.
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