8 DEPARTMENT OF AGRICULTURE

every instance the nematode itself should be located and determined
before a final diagnosis is made. Let us support this statement by an
example. It is widely assumed that the root-knot nematodes may be
correctly recognized and properly diagnosed through the symptoms they
cause on the roots of host plants, namely conspicuous swellings, knots
or galls. (Figs. 1-8) But even in this instance it is fallacious; such
symptoms are only indicative but certainly not proof of an infection by
root-knot nematodes because: (1) there are knot-forming nematodes
found on roots of plants other than the root-knot nematodes (e.g. Dity-
lenchus radicicola [Greeff] Filipjev, or Nacobbus dorsalis Thorne &q
Allen), (2) root-knot nematodes may be present without causing the
formation of knots (this is often seen, e.g., in cotton or corn where theq
nematode breaks through the surface of a root without forming knots)
and roots are then mistakenly judged free, (3) other organisms mayq
form similar knots, e..g% the slime molds (Plasmodiophora species) caus-
ing clubroots in members of the cabbage family, the crown gall organism|
(Bacterium tumefaciens Sm. & Town) and certain viruses; also so-called



 e.-
























 Figure 2.-Roots and pods of a peanut plant (Arachis hypogaea L.) infected
with root-knot nematodes which formed only small galls on the roots but blackish I
lesions on the pods. This attack at the ends of pods causes interruption of growth
and misshapen and often dwarfed pods.