PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
‘Overwintering’
In infected host plants, eg weeds, perennial
ornamentals and field crops. It is not seed-borne
and does not survive in soil.
Spread
By the common brown leafhopper
(Orosius argentatus) which is brown
speckled and about 3 mm long. It
breeds on weeds which can be infected
with tomato big bud. Overseas other leafhopper
species may also transmit it. Tomato big bud is
transmitted in a persistent manner. Leafhoppers
acquire tomato big bud after feeding on infected
hosts for several hours or days but cannot transmit
big bud immediately. During this latent period the
phytoplasma multiplies and circulates within the
vector finally accumulating in the salivary glands.
Leafhoppers are infective for the rest of their
lives, through several moults but tomato big bud is
not passed from adults to eggs.
By vegetative propagation from infected
plants.
Not seedborne.
Conditions favoring
Crops surrounded by weeds where leafhoppers
breed.
Plants which are vegetatively propagated.
Leafhoppers build up rapidly at temperatures
> 16 o C.
At certain times of the year, particularly after hot
and dry weather, leafhoppers migrate from
drying weeds where they breed, to ornamental
plants, vegetables and other herbaceous plants.
Migration most commonly occurs in Oct/Nov.
Management (IDM)
Are you a commercial grower or home gardener?
1.Prepare a plan that suits your situation.
2.Crop, region. Know the variations, wide host range
and vector.
3.Identification. This must be accurate, so consult a
diagnostic service (page xiv) to ensure correct
diagnosis and correct control measures are used, ie
that the problem really is caused by tomato big bud
and not herbicides, etc.
4.Monitor crops regularly for diseased plants and
vectors and record your findings. Know when,
where, what and how to monitor.
5.Threshold. Nil thresholds for some commercial
crops. Home gardeners tolerate some diseased plants.
6.Action/control involves roguing infected plants,
not propagating from infected plants. Home gardeners
should not attempt to control the vector by spraying.
7.Evaluation. Review the success of your plan.
Recommend any necessary improvements. Continue
regular crop inspections.
Control methods
Control is difficult. To minimize losses:
Sanitation.
There is no cure for infected plants so they
should be removed and destroyed.
Weeds known to harbour the leafhopper vector
should be destroyed.
Resistant varieties.
No tomato varieties are resistant to big bud, the
resistance of different varieties of ornamental
plants is not known.
Disease-tested planting material.
Seed can be saved from infected plants (disease
is not seed-borne).
Do not propagate vegetatively from infected
plants.
Insecticides.
There are currently no registered pesticides
which will cure a plant of phytoplasma
infection in the field.
Where tomato big bud is a problem in commercial
seedbeds, surrounding vegetation which may
harbour leafhoppers, may be sprayed with an
appropriate insecticide to control the leafhoppers.
Regular insecticide applications to field crops in
spring and early summer will, at the most, only
reduce the number of infected plants.
Home gardeners should not attempt to control
the insect vector.
Fig. 147. Tomato big bud (greening).
Above: Gazania. Left: Greening of floral parts. Right: Healthy plant.
Right: Parsnip. Left: Healthy plant. Right: Greening of floral parts.
PhotoCIT, Canberra (P.W.Unger).
290 Virus and virus-like diseases