Nematodes are tiny parasitic worms that either stick their heads in a plant to suck the sap or actually spend their lives inside the plant. Although they are actually pests and are dealt with in Chapter 4, the problems they cause are included with diseases because those problems have similar symptoms to other diseases and those symptoms continue for the life of the plant as do those of the pathogenic diseases.
Generally, it is difficult to diagnose nematodes by just looking at the plant. Very often a nematode problem will show up with the plant weakening and not looking healthy and vigorous. Put nematodes at the bottom of your list of suspects when you are trying to identify a disease. If no other explanation seems to fit, check the possibility that your plants have nematodes. For detailed information for dealing with this problem go to the file Dealing With Destructive Nematodes