Agriculture Reference
In-Depth Information
Root parasites
Apart from the stem and bulb eelworm, the pest species attack root systems.
These root parasites can be categorized as either ecto- or endoparasites and
also as either sedentary or migratory. The ectoparasites feed on the outside of
the root systems, whereas endoparasites feed internally. The migratory species
destroy plant tissue as they feed and continually move to invade undamaged
roots. The sedentary species induce localized growth modifications in roots,
and these function as long-term nutrient sources for the parasite.
The damage caused by all these root parasites is similar. Basically, the
attenuation of root growth impairs the potential for water and nutrient uptake,
and this predisposes plants to water or nutrient stress and, as a result, growth
rates are reduced and the attainment of a given plant weight is delayed, usually
leading to a lower yield at the end of the growing season (Schomaker and Been,
2006). The degree of damage varies with a number of factors, including the
population density of the pest in the soil and temperature. The size of the plant
is important; as with most disorders, small seedlings are more vulnerable than
larger plants with more extensive root systems.
It is common to protect seedlings by applying a granular nematicide, like
aldicarb, along the drilling line at sowing, thereby protecting the young
seedlings and allowing plants to become established. Soil sterilization using
methyl bromide, or an overall application of nematicide, frequently increases
yields. However, these substances are highly toxic, treatment is expensive, and
re-invasion by the pests can be rapid and populations may then rise above their
original level. In warm, sunny countries nematodes, along with other soil-
borne pests and pathogens, can be controlled by soil 'solarization'. This is the
sterilization of the upper layer of soil as a result of solar heating, following
wetting and sealing under a tightly stretched layer of clear polyethylene. Again
though, pests are likely to reinvade from deeper, cooler layers of soil and the
control will need to be repeated. Some of the pest species have a narrow host
range, and their numbers can be reduced by including non-susceptible crops in
the rotation. However, many of the pest nematodes have a wide host range and
control by crop rotation is impractical.
Stem and bulb eelworm
The stem and bulb eelworm,
Ditylenchus dipsaci
, is an extremely serious pest in
temperate climates and at high elevations in tropical regions. It is difficult to
control once established. There are many strains of
D. dipsaci
indistinguishable by
appearance, some specific to a particular host plant species and some with a
wider host range (Duncan and Moens, 2006). It has so far been difficult to
distinguish races, even using molecular techniques, and this complicates
management using crop rotation or resistance. Unlike the other pest nematodes,
