PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
METHOD OF
FEEDING
NEMATODE HEAD
The digestive system is
developed for handling
a liquid diet.
Most plant parasitic
nematodes have a hollow
stylet or spear which can be
thrust forward (like a tongue)
from its mouth to puncture
holes in plant cells. It then
withdraws or “sucks out”
the contents, including the
nutrients, from the plant cell.
Nematode head. A hollow digestive
tube extends from the mouth to the anus
(adapted from Agrios, 1997).
SYMPTOMS
Above
ground
symptoms
Below
ground
symptoms
Disease
complexes
Nematodes also feed
on algae, lichens and
are often found on
healthy trees
DIRECT FEEDING DAMAGE.
The mechanical injury caused by nematodes feeding causes only slight injury to
plants. Most plant damage is caused by the nematodes secreting saliva which
they inject into plants during feeding. This may result in:
Tissue breakdown, eg rotting
Abnormal cell enlargement and cell multiplication, eg galls
Abnormal cell division, eg large number of lateral roots
General stunting of tomatoes, turf, etc
5-10% of crop production is lost to nematodes in developed countries
LEAVES
ROOTS
Chlorosis (non-specific water stress/deficiency type symptoms
due to nematodes feeding on or in the root), eg root knot and root
lesion nematodes
Dead areas, scorches, blotches, eg foliar nematodes
Leaf distortion, eg stem and bulb nematode
Spicules (tiny lumps), eg stem and bulb nematode
Excessive root branching, eg beet nematode
Galls, eg root knot nematodes
Injured root tips, eg root lesion nematodes
Rotting, eg stem and bulb nematode in bulbs
INDIRECT DAMAGE.
Transmission of virus diseases. In Australia, only a few species of nematodes
can transmit virus diseases of plants, eg the dagger nematode (Xiphinema sp.) can
transmit the grapevine fanleaf virus, stubby root nematodes (Paratrichodorus spp.)
can transmit at least 6 plant viruses. The nepoviruses (nematode-transmitted,
polyhedral particles) are a group of about 46 viruses that infect many plant families
that cause probably the most serious viral diseases of horticultural crops, particularly
perennial woody and bulb crops. Many have not been recorded in Australia.
Nematode-bacterial disease complexes. Annual ryegrass toxicity
(ARGT) is the poisoning of livestock by toxins contained in bacterially-infected
annual ryegrass (Lolium rigidum). The toxins are produced by bacteria
(Rathayibacter toxicus, formerly Clavibacter toxicus) which are carried into the
ryegrass by a seed-gall nematode (Anguina funesta).
Nematode-fungal disease complexes. The fungus is not transmitted by the
nematode. Plant varieties susceptible to a particular soil fungus are damaged even
more when the plants are infected with nematodes, the damage being considerably
more than the sum of the damage caused by the nematode, eg root knot, or the fungus,
alone, eg Fusarium and Verticillium wilts, Phytophthora and Rhizoctonia root rots.
BENEFICIAL.
Breakdown organic matter. Bacterial-feeding nematodes in the soil increase
the turnover of plant nutrients (specifically nitrogen); fungi also feed on nematodes
and nematodes can feed on fungi and organic matter, etc.
Some species are used as biological control agents.
Numbers and species of nematodes in soil can act as indicators of biodiversity.
Nematode diseases 253