OTHER SOIL ANIMALS - The Soil - page 109

flask-shaped cyst after it dies. Such cysts can lie dormant in the soil for several years
until the eggs are stimulated to hatch out by moisture and root secretions. The cysts
are 0.3-1.0mm across and so are clearly visible to the naked eye looking like tiny
earthworm cocoons.
Local infestations of these eelworms can build up in the soil until they manifest
themselves as 'sick' patches in a crop, sometimes resembling nitrogen deficiency. In
order to estimate their populations, detailed procedures of sampling and extracting
the cysts have been devised. Because eelworms are never uniformly distributed in the
soil, this involves taking a large number (500) of small soil samples across a field,
mixing them thoroughly, washing a subsample over a series of sieves, and finally
picking out the cysts under a microscope. Each cyst found per 500 gram sample is
equivalent to a million per acre and each may contain several hundred eggs. Ten eggs
per gram can cause noticeable crop symptoms, and 100 eggs per gram could produce
total crop failure.
The main agricultural pests in Britain include the beet eelworm and the potato
root, cereal root, pea root and carrot root eelworms. Each of these will attack several
related crops, and often wild plants as well. For example, the beet eelworm is known
to attack more than 30 species including mangold and spinach in the beet fam-
ily (Chenopodiaceae), turnip and cress in the cabbage family (Cruciferae) and also
docks (Polygonaceae) and chickweed (Caryophyllaceae). The potato root eelworm
Globodera rostochiensis is now placed in a different genus from the other British spe-
cies because its cysts are spherical rather than pear-shaped ( Fig. 37 ) .
Feeding by parasitic nematodes often produces a reaction in plant tissues which
is not matched by any other invertebrate group. Some of the dividing cell walls dis-
solve away creating 'giant' cells with multiple nuclei enclosed within thickened outer
cell walls. These giant cells may extend for several millimetres along a root. A heavy
infestation interferes with normal root system development and disrupts the transport
of water and nutrients, so producing the symptoms of wilting and mineral deficiency
mentioned earlier. Less severe lesions can nevertheless be harmful by opening the
way for secondary infections by fungi and bacteria. Some eelworms are known to be
carriers of plant viruses. Species of Longidorus ( Fig. 36 ), for example, transmit rasp-
berry ringspot virus to various soft fruit, and tomato black ring virus to onion, potato
and sugar-beet. In this respect they are similar to the aphids that transmit yellow dwarf
virus of barley, and to the 'big bud' mites which transmit reversion disease of black
currants.
The elimination of parasitic eelworms by chemical methods is difficult and in-
volves the use of powerful soil sterilants. However, a large measure of control has
been achieved since the 1960s through a combination of cultural methods, especially