18 DEPARTMENT OF AGRICULTURE 4

difficult. Observations on the host range of this group of nematodes
in different regions and locations, as well as studies on the resistance
of an identical crop variety planted at different locations, have show r
that plants and crops attacked in one location sometimes are not attacked
in another. Evidence is rapidly developing through systematic studied
to prove that there exist many different species or host nematodes, each
of which has its own host range; some regions or localities appear t
have only a single species, whereas in others two or more occur, often
in a mixed state. A similar situation appears to exist in other nematodea
species, for example, the bulb and stem nematode or certain bud ancal
leaf nematodes (Aphelenchidae species). These matters are mentioned
to emphasize the fact that the diagnosis of a nematode infection anal
the evaluation of its capacity for damage must be made with care and
knowledge. i
 In the 1940's a great step forward was made in resolving some o4
the perplexing problems in studies on the root-knot nematodes. First
the experimental work of J. R. Christie and later the taxonomic work ol
B. G. Chitwood proved that the root-knot nematodes constituted many,
rather than a single, species, each having its characteristic behavioil
and plant-food preferences.
 Disease symptoms produced by the various species are rarely specific;l
wilting, discoloration of leaves, swollen and distorted shoots, crinkledI
leaves, leaf spots, gall formation on roots, stems and leaves, galled fruits
and seeds, bunching and dwarfing of the entire plant, blinding of buds
and the phenomena termed dieback, sunburn, winter kill and winter
bronzing may be caused by nematodes. General wilting of plants in the
field over a hot period of the day may be indicative of the presence of1
large numbers of root nematodes, as may also bare spots or areas ex-
hibiting retarded or reduced growth. However, all of these symptoms
likewise may be caused by various other organisms and agents. Again,
therefore, external symptoms alone can not serve as a reliable basis fol
the diagnosis of a nemic plant disease. The identification of the organ-
ism is absolutely necessary. I
 Nematodes, or eelworms, so far as plant-parasitic forms are con1
cerned, are very small animals, less than 2 mm long, usually only from
0.4 to 1 mm (or 1/64 to 1/25 of an inch). Therefore they can not be
seen with the naked eye and must be studied with the microscope.
Although of such small size, their organization is highly complex and
embraces all organs and organ-systems found in higher animals except-
a circulatory system. It is difficult for the untrained worker to differ-
entiate the various genera and species and some of the thousands o01
soil-inhabiting forms may easily be mistaken for those which are defi-
nitely parasitic to plants. The grower and entomologist as well as theq
plant pathologist should keep this in mind and if there is any doubt
regarding the symptoms, an identification should be requested froni