44 DEPARTMENT OF AGRICULTURE

in the South. Many additional types of root-attacking nematodes could
be mentioned. Usually most of them occur only in smaller numbers
and are not known to cause serious diseases. Future research, however,
may uncover many surprises.

 HOST PLANTS OF ROOT-KNOT NEMATODES

 Many plants are more or less immune to root-knot, i. e., the worms
do not find their roots suitable medium in which to grow and reproduce.
Probably there are substances in the sap which poison the worms. As
a whole plants of the true grass family, including corn, oats, rye, canes,
sorghums and millets, as well as Crab and Bermuda grasses, etc., are
quite resistant. Still some varieties of corn, oats and cane are sometimes
seriously attacked. Velvet beans and beggarweeds are practically im-
mune. Generally speaking, trees and shrubs are not as apt to be at-
tacked as herbs. There are numerous exceptions to this rule.
 Other plants seem to be more or less tolerant, i.e., they are able to
make satisfactory growth and produce a good crop of fruit although

















 Figure 29.-Potatoes attacked by Scribner's meadow nematode (Pratylenchus
scribneri Steiner). The Figure at left shows a potato with a lighter infection, while
that of the one at right is very heavy. The arrow in the figure at right points to a
single small lesion from which alone 335 nematodes were isolated; thus the nema-
todes in this potato were conservatively estimated at from 12,000 to 15,000.

they are heavily attacked and their roots are badly knotted. Mulberry
trees and sunflowers are such plants.

 Unfortunately, most truck and garden crops are severely injured by
the worms. There are, of course, all graduations between highly suscep-
tible and practically immune plants. The following list includes most
of the susceptible plants commonly grown in Florida, given in the