PLANT NEMATODES THE GROWER SHOULD KNOW 39

make them migratory, tissue parasites, par excellence. From an economic
point of view they are major pests. Unfortunately, their significance,
common occurrence, and widespread distribution have been almost
completely ignored until very recently. The various species are diffi-
cult to distinguish and their classification is still greatly confused. A
form attacking the Irish potato was recorded as one of the first plant
nematode pests in this country. It is Pratylenchus scribneri Steiner
'which was observed in 1889 by F. Lamson Scribner on potatoes in
Tennessee. It was not given a name then, but recently has been estab-
lished as a true species. Observed in different states east of the Rocky
Mountains, it appears to occur mostly in potatoes. A second species,
P. leiocephalus Steiner, 1949, well characterized by the angular contour
of its head, has been found on potatoes in Florida as well as on a va-
riety of other crops. (Fig. 27). While these two species are monosexual
forms, there are other species which are bisexual. In entering and
migrating through root tissues they destroy them by breaking through
cell walls and feeding on their contents with organisms of decay follow-
ing in their wake. Root tissues penetrated by meadow nematodes usually
,exhibit necrotic lesions which frequently are reddish in color at first,
turning dark brown to black later on. The eggs are deposited within the
roots, where, not uncommonly, accumulations of specimens, including
larval stages, adults and eggs, may be seen occurring as "nests." Obser-
vations show these forms to be extremely destructive (Fig. 28 A & B)
since by attacking and destroying a root at a point close to the stem
they render useless the entire distal portion of the organ. Certain plants
try to repair the damage by forming new side roots above the attacked
points. Then as new lesions are developed on these side roots still
more side roots are formed with the result that root systems attacked
by these nematodes frequently exhibit a bearded or matted appearance.
Sometimes the cortex of roots so attacked begins to decay and then
easily slips off. The axial cylinder is rarely entered. On potatoes surface
pimples are formed which later change to blackish depressions. When
very numerous, these pimples and depressions disfigure the tubers to
such a degree that the crop is rendered unmarketable (Fig. 29). Under
certain conditions, particularly in orchards and on perennials where
they remain undisturbed for years, they may multiply to enormous num-
bers. Cases have been observed where large trees lose significant por-
tions of their root systems by the attacks of great numbers of these
meadow nematodes, with the result that branches, limbs, parts of the
crown, and even entire trees show the effect by yellowing foliage, de-
foliation, death of limbs and branches or even of whole trees. It is dif-
ficult to estimate the damage caused by this type of nematode on plants.
They cannot live on dead, decaying tissues and are not generally found
in roots in the process of decay. Such roots abound with a variety of
fungi and bacteria and so-called saprophytic nematodes, while the
primary culprit has escaped to the surrounding soil or to still healthy
tissue. The study of the meadow nematode is still in its infancy. Ex-