PLANT NEMATODES THE GROWER SHOULD KNOW 5

Growers as "alfalfa sick soil," "clover sick soil," "soil tired of beets," "tired
of potatoes," etc.
I It is agreed that with our monoculture, that is, a single identical
crop planted over wide areas not infrequently year after year or at
least in close succession, pests and diseases of such crops are provided
with favorable conditions for development. They therefore increase
Tremendously, particularly in the absence of any natural checks. But
here again in our attempts to look for and control pests and diseases,
phe above-ground parts of our crops have attracted most of our atten-
tion and consumed most of our efforts. This has led to modern pest
control of these aerial parts by spraying and dusting practices and by
fumigation. A great deal of research work has been and still is being
(lone in this connection. However, the root systems and other under-
ground parts of plants have not gotten their share of attention although
Ioil-borne pests and diseases, particularly our plant nematodes, are
equally favored by our monocultures.
) Fortunately plant nematodes are slow in their spread and are usually
not of such a lethal virulence as many of the fungous, bacterial, and
firous diseases. An outright killing may be effected on seedlings but
this is rather an exception on fully grown plants. Nematode diseases
pause a reduction in growth and vitality, but generally are not com-
pletely lethal. Thus it would appear that, in general, the host-parasite
relationship between plants and nematodes is a highly balanced one,
that is, the parasite does not at once kill the host and so deprive itself
 mf a means of existence. This situation has made these pests appear less
impressive and thus has been another reason for the under-estimation
pf their importance.
 It is also a fact that many of these pathogenic nematodes, more or
less unconsciously, have been kept under control to some degree by cer-
tain agricultural practices. Plowing, disking, harrowing, and cultivation
pre some of the operations that help and are not to be under-estimated
as a means of reducing noxious nematodes in the soil. This is accom-
plished by exposing the nematodes to the sun, to drying by wind, to
starvation by depriving them of a living host, to mechanical injury, etc.
FCrop rotation also has long been recognized as an extremely helpful
agricultural practice through the tendency to reduce the nematode popu-
tation by starvation providing, of course, that one or more crops used
in the rotations are not a host of the particular nematode or nematodes
(present in a given soil. Thus various agricultural practices, to some ex-
tent, have been effecting a certain amount of control over soil-borne
nematode pests of plants, but also have assisted in delaying a proper
appraisal of their importance.
 I This situation began to change with the coming of soil sterilization
 by steam and other means including the use of efficacious soil fumigants.
 The efficient control of root-parasitic nematodes and other biotic factors