PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
EXAMPLE OF A NEMATODE DISEASE
Scientific name
Soil-inhabiting nematodes (Meloidogyne spp.).
Root knot occurs widely in Australia especially in
warmer climates causing serious damage to many
plants. It is the world’s most damaging
nematode genus and can be serious in
glasshouses and is common even in virgin land.
Host range
More than 2000 species of plants, including:
Vegetables, eg bean, carrot, parsnip, potato,
tomato (major nematode pest of vegetable crops).
Ornamentals, eg cut flowers, carnation, roses,
chrysanthemum, dahlia, gerbera, nursery stock.
Fruit, eg Chinese gooseberries, papaw, stone fruits,
grapevines. Field crops, eg clover, lucerne, lupin,
peanut. Weeds, eg many species, eg fat hen.
Different strains have different host ranges.
Symptoms
Above ground symptoms. Affected plants
often grow slowly, are stunted, paler green or more
yellow than normal and wilt readily during hot
weather. Plants may die prematurely, reducing yield.
Symptoms are similar to those of nutrient deficiencies.
Affected roots are unable to supply the aboveground
parts of the plant with sufficient water and nutrients.
Confirmation of root knot is only possible by
removing the plant and examining the roots.
Below ground. Nematodes about 0.5 mm
long, enter roots stimulating the surrounding tissue
to enlarge and produce swellings or galls on the
roots. These galls vary in size from small to large
knots up to 25 mm in diameter. Galls caused by
Meloidogyne hapla are much smaller than those
caused by other species. In plants with fleshy
underground parts such as potatoes, galls look like
pimple-like outgrowths, the surface of the tuber
may become warty, roughened and discoloured. If
one of the outgrowths is cut across, nematodes
may be seen as small glistening bodies embedded
in the tissue of the tuber.
Root knot
Root gall, eelworm
General. In severe infestations, seedlings and
older plants may be killed.
Affected roots commonly become infected by a
range of secondary bacteria and fungi which enter
through the roots, eg Fusarium wilt, Pythium,
Rhizoctinia, hastening root breakdown.
Infection of older crops may or may not reduce yield
significantly. Affected plants are not usually killed.
There is an association between carrot defects
and nematodes. The proportions of forked,
galled, constricted and split carrots and the
weight of unmarketable carrots were correlated
with population densities of Meloidogyne.
javanica in the soil.
Diagnostics.
Do not confuse root knot:
– In leguminous plants, eg peas, beans, clover and
lucerne, with galls caused by beneficial nitrogenfixing
bacteria. Root galls caused by nematodes
are not easily detachable, galls resulting
from nitrogen-fixing bacteria are.
– In Brassicas, with galls caused by club root
which are spindle-shaped, larger and less evenly
distributed on the lateral feeding roots.
– In pome fruit, with galls caused by woolly aphid.
– Generally, galls caused by crown gall (a bacterial
disease) are larger and may be up to the size of a
large football.
– Misshapen roots in carrots may also be caused by
soil structure, number of passes with a rotary
hoe, and other root diseases.
Aids to diagnosis
– Some can be seen with a hand lens or dissecting
microscope (x10 magnification). Specialized
knowledge is needed to tell one type of nematode
from another. Many nematodes are beneficial.
– You can detect root knot nematode infestation of
soil by growing a susceptible host, eg certain
tomato varieties, for several weeks and then
washing the soil from roots and examining them
for evidence of galling. Cut up galls in water and
worm-like nematodes should be easy to see with
a dissecting microscope. Mature forms of root
knot are shaped like a sac.
– Confirm diagnosis as above, identification to
genus requires professional expertise.
Fig. 136. Root knot (Meloidogyne spp.) galls on:
Left: Tomato roots. Right: Potatoes and carrots.
Photo NSW Dept of Industry and Investment.
268 Nematode diseases