PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
CONDITIONS
FAVOURING
Symptoms of root
knot on tomato
Symptoms of foliar
nematode on
chrysanthemum
Nematodes are well suited to living in the soil because it is well insulated
against sudden or large temperature changes and affords protection from the
direct lethal rays of the sun.
SOIL-INHABITING. NEMATODES
Most plant parasitic nematodes spend all or part of their lives in the soil. Soil structure
determines the distribution of nematode species more than anything else (exceptions
may be those with moderate host ranges).
Soil moisture. Young and adult nematodes require adequate soil moisture,
preferably as a film of moisture around the soil particles. This allows them to move
freely and provides adequate aeration for their survival.
Aeration. Nematodes require an adequate oxygen supply for respiration, so that
soils should have pore spaces with a diameter of about 20 m. Smaller pore spaces
inhibit plant parasitic nematodes.
Temperature. Nematodes dislike extremes of temperature and sudden or large
temperature changes, and generally require temperatures greater than 15 o C to
increase their numbers.
Nematodes prefer a well-buffered soil where there is unlikely to be sudden
changes in acidity or alkalinity.
Soils high in organic matter are thought to be unfavourable for
development of plant parasitic nematodes because they have large populations of
predatory nematodes.
Distribution. Nematodes occur in greatest abundance in the surface layers
(15-30 cm). Some nematodes may be found at much greater depths.
Continuous cropping. Many crops which are relatively tolerant of nematode
damage, eg squash and zucchini, may only suffer losses if the area is replanted
immediately after a susceptible host has been grown.
Stage of crop development. A well grown crop can withstand significant root
infection with nematodes but a 2 nd planting of the same crop in the same ground
will certainly develop a damaging nematode infection while it is young and will not
produce a good crop.
Type of tillage practices. There can be substantial differences in the
nematode fauna under tillage practices and probably in the rest of the soil biota as
well (pages 257, 263).
NEMATODES AFFECTING ABOVE-GROUND. PLANT PARTS
These nematodes spend part of their lives in the soil and so are affected by the
conditions discussed above.
Additionally, leaf and stem and bulb nematodes are spread more rapidly when
plants are wet. They escape from the soil and swim up on the outside of plants
in a thin film of water. Leaf nematodes are favoured by free water on leaf and
stem surfaces.
ENVIRONMENT
Does it favour the crop or root knot?
SUSCEPTIBLE
HOST PLANT
ROOT KNOT
PRESENT
Fig. 135. Nematode-disease triangle.
Nematode diseases 261