PLANT NEMATODES THE GROWER SHOULD KNOW 29

are stored or otherwise kept under conditions favorable to the nematode.
This situation explains the many opportunities for the distribution of
this pest through infected plant material as, for example, nursery stock
and root crops such as potatoes, carrots, etc. The spread of the larvae
by active migration is slow.
 Over 1700 different plant species are known to be attacked by root-
knot nematodes. However, they exhibit a wide variation in the degree
of susceptibility to attack and in the seriousness of the disease that
follows. Thus some suffer severely from a slight attack (e.g. cyclamen)
while other species are very tolerant (e.g. mulberry tree). It should be
mentioned here that germinating seeds and young seedlings are particu-
larly attractive to root-knot nematode larvae (Fig. 8) including even
seedlings of plants otherwise immune or highly resistant. Seeds should
therefore never be planted in soil badly infested by root-knot nema-
todes, or by any other plant-pathogenic nematode form, nor should such
soil be used for seed-testing purposes. Germination will be poor and
most of the emerging seedlings will be sickly. Thus the seedlings of the
tung oil tree (Aleurites fordii Hemsl.) are very susceptible and may die
from an early infection but after the first year may recover and then
develop a high degree of resistance.
 It was mentioned earlier that in some plants the root-knot nematodes
break through the root surface so that the egg-producing females pro-
trude from the root and may then be seen with a hand lens as globular,
whitish bodies with yellowish or brownish egg masses attached. (Some-
times not even swellings are formed.) Under such conditions the para-
site is even more injurious than in those cases where a smooth, un-
cracked gall without necrotic tissues is formed. This is due to the fact
that roots cracked or opened by the action of nematodes are at once
invaded by a whole group of secondary agents including other nema-
todes, fungi, and bacteria and this usually leads to quick decay. Plants
thus attacked naturally suffer much more than those which form
"smooth," uncracked or, if such an expression is permitted, "healthy" galls.
 Frequently growers or scientific workers looking for these nematodes
with the conception that galls or knots are its symptoms, have often
considered the nematode absent because there were no galls. Thus they
have missed the root decay initiated by the nematodes which is usually
much more serious than the galls.

 SEDENTARY TYPES
 This leads us to other sedentary types of plant-parasitic nematodes
 of roots, namely the sugar-beet nematode and related species like the
 golden nematode of potatoes. In these species even the growing larvae
 normally protrude from the root surface and the adult females generally
 hang on the outside of a root. In contrast with root-knot nematodes, the
 members of the sugar-beet nematode group have a characteristic which