is the cause of the “beet sickness” of the soil. The course of the
disease is as follows. At the end of July light-coloured patches are
found here and there among the normally developed beet. The leaves are
weak and limp, and the outer ones especially get yellow, spotted, and
die off. Later on the inner leaves die as well, after which the top of
the beet becomes black and the whole root gradually decays. In less
severe cases the beet may recover towards autumn and develop new heart
leaves, but the roots remain small and the crop is poor, often being
only one-third of its normal amount. Kühn has proved by infection
experiments that the sole cause of beet sickness is a nematode, of which
the life history is as follows. The female is found fixed to the
branches of the root; it is citron-shaped, about one twenty-fifth of an
inch long (Fig. 134, 1 and 10), and contains on an average three hundred
and fifty eggs. Some few of these, together with a jelly-like substance
making up an “egg-sac,” may pass out to the exterior, but the large
majority develop in the body of the female, which ultimately becomes a
mere sac enclosing the eel-like larvæ. The female is killed by the
process. The liberated larva (Fig. 134, 4) seeks out a root (about one
twenty-fifth of inch thick), and bores into it. Here it lives as a
parasite, causing the disease of the attacked beet plant. The larva
quickly sheds its old skin, assumes a thicker form (Fig. 134, 5), ceases
to move, and gradually causes the outer skin of the root to bulge out
externally (Fig. 134, 3, _a_). The distinction between the sexes now
rapidly makes its appearance. A thick motionless larva, destined to
become a male, temporarily ceases to feed, shrinks within its old skin,
develops a thin new one, and ultimately becomes a long eel-like worm
(Fig. 134, 6, 7, 8), which grows into an adult male (8). In the stage
represented in Fig. 134, 8, the animal still lies under the outer skin
of the root, which never bursts during its development, but the mature
male bores out of its larval skin and out of the root, passing into the
soil, where it finds and fertilizes the female, which in the meantime
has developed but remains attached to the root. The female develops in a
simpler way, by the gradual distension and growth of a larva (not by a
process of re-formation) and gradual development of the female sexual
organs. As the larva passes from the stage of Fig. 134, 5, into that of
Fig. 134, 9, and later on into the adult condition, 10, the outer skin
of the rootlet is ruptured, and the female comes out from its tissues,
remaining, however, attached to its outside (Fig. 134, 1). The entire
development from egg to sexual adult takes four or five weeks, and there
may be six or seven successive generations, the reproduction is
consequently very rapid.
It also obviously follows that “beet sickness” of the soil is especially
prevalent in fields where there has been an excessive amount of beet
culture. The disease, however, may suddenly appear in fields which have
been hitherto “safe” for beet, and in many such cases it has been proved
to result from manuring with artificial compost rich in refuse from
affected fields. The disease frequently appears, too, in fields where
beet have never been cultivated, but where cabbage has been grown for a
long time. It has been shown, especially by Kühn’s investigations, that
the beet eelworm can live in many plants both cultivated and wild,
_e.g._ of the former, cabbage, rape, mustard, garden cress, chickling
peas, mangold, oats; of the latter, charlock, spurrey, couch grass.
These researches are of the greatest importance both for understanding
the way in which beet sickness spreads and in combating it.
_Preventive Measures_: Manufactured compost must not be used as a manure
on beet-fields. The refuse from infested beets, if used on other fields,
must be mixed with one-sixth its bulk of quicklime. The boots of
labourers employed in beet-sick fields, the hoofs of horses working in
them, and also the implements used, must be carefully cleansed lest
infected earth should be carried to other fields. _Remedies_: Kühn has
recommended the use of plants which attract the eelworms
(“lure-plants”). He sows on beet-sick land rapidly germinating plants,
of kinds which the worms readily attack, and weeds them out again when
they have become infested by the parasites, but before these have had
time to mature and re-enter the soil. The eelworms are thus allured into
the plants grown, and destroyed with them. Such lure-plants must be sown
as thickly as practicable, so that the soil may be penetrated by as many
slender rootlets as possible. After these plants have been dug up, a
second lot should be grown, since all the eelworms will not have
attacked the first lot; and it is even advisable to grow a third batch.
Kühn used as lure-plants the various kinds of cabbage, also summer rape
(_Brassica rapa_), since this plant has a great attractive power for the
beet eelworms, and can hold a large number of them in its numerous, much
branched rootlets. I cannot go into all Kühn’s researches here, and will
only mention the following. In the course of the year 1880 part of a
beet-sick field had three successive crops of lure-plants grown upon it,
each being dug up from thirty to forty days after sowing. The field was
ploughed in autumn, suitably manured the next spring, and sown with beet
in the middle of April. The other part of the piece of land was treated
in exactly the same way, except that no lure-plants were grown upon it.
A difference was very soon seen between the two plots, and there was a
very great difference at the time the crop was ready to be gathered in.
The plants on all parts of the first plot were in a flourishing
condition, but those on the second plot were in many places either
killed outright or else small and misshapen. The crop succeeding the
lure-plants was three times as great as it had been before, and almost
equal to that of a healthy field. Later on, Kühn made an important
discovery; he found that larvæ which have reached the thickened
motionless stage, depicted in Fig. 134, 5, require a considerable amount
of food to keep them alive, and enable them to develop further. If the
plants are disturbed in such a way as to kill the rootlets containing
the larvæ in this stage, these are unable to develop any further. Kühn
caused a kind of horse machine to be made, adapted for rapidly
destroying the lure-plants in the fields. For further details, his
original memoirs may be consulted.
The =Root-knot Eelworm= (_Heterodera radicicola_).
This second species of the genus _Heterodera_ develops much like the
beet eelworm, living like it on the roots of plants, but more deeply
within them, so that the body of the female (which swells till it
becomes pear- or flask-shaped) does not cause the outer part of the
infested root to project, and does not reach the exterior. Where the
eelworms collect, galls are formed which decay later on, when the
eel-shaped larvæ escape into the soil, very soon, however, to enter the
roots again at their slender tips. The galls are thickenings in the
course of the root, and are never lateral appendages, like, for
instance, the well-known “tubercles” in the roots of leguminous plants
(_e.g._ clover, pea). The root eelworm lives in the roots of more than
fifty plant species in the most various parts of the world, and infests
both weeds and cultivated plants. It is especially damaging to clover
and lucerne, in which not only the galls die, but also all parts of the
root below them. In this country the roots of cucumber and tomato are
sometimes attacked (E. A. Ormerod).