Promote • Educate
28 September/October 2019
Gardening
Damage to plants
There are three problems following TPP
attacks on plants.- Disease is the most important one.
Psyllids are the primary carriers
and vectors of the bacterial disease
‘psyllid yellows’, caused by Canditatus
Liberibacter solanacearum. Adults and
nymphs acquire it from infected plants
when they feed, then spread it to
healthy plants. Currently, the disease is
thought to be caused by a toxin that is
transmitted during the insect’s feeding
activities, especially by the nymphs.
The disease has a devastating effect
on the plant’s development and the
quality of the fruit or other edible parts
cropped for human consumption. - Direct damage. Plants can be
overwhelmed by very high numbers of
psyllids feeding on them, weakening
them and reducing their vigour. - Indirect damage. The honeydew
produced by psyllids encourages sooty
mould. If pest numbers are very high
the mould can completely smother
leaves and stems, blocking out sunlight
and reducing photosynthesis. This
weakens the plant and decreases its
vigour. It is less of a problem outdoors
where rain may wash off honeydew
and mould, but is more of a problem in
an indoor or greenhouse environment.What are the signs you have
psyllids?
Typical symptoms of the disease in tomatoes,
potatoes and other solanaceous plants
are yellowing and purpling of the leaves,
stunting, upward rolling of leaves throughout
the plant and basal cupping of leaves.
The plant nodes will be enlarged and
the terminal internodes will be shorter and
thickened, creating a rosette effect. Other
symptoms include leaf scorch, poor fruit set,
and lots of small, misshapen and poor quality
fruits. Black sooty mould may also be present.
The underground symptoms in
potatoes include a collapsed root system,
a cross section of the stems will be brown,
and the tubers will have brown flecking
and streaking. The potato will eventually
collapse and look burnt. Infected potato
seed can transmit this disease.
If potatoes are infected early in the
season, there will be many small tubers
produced. When plants are affected later, the
bacteria prevent the plant from converting
sugars into starch for storage. The potatoes
become rich in high concentrations of sugar,
and when fried, they exhibit dark, blotchy
stripes and streaks, called ‘zebra chip’, due to
the caramelisation and burning of the sugars.Organic control of the TPP
There are three main organic methods that
can help combat TPP: cultural control,
covering up, and biological control.Cultural control
- When buying potatoes always buy
certified seed that is free of the zebra
chip disease. Don’t plant ones you buy
from the supermarket or even your
own home-saved potatoes; these could
be infected and carry forward this
disease. (Non-organic potatoes may
have been sprayed to prevent them
sprouting during storage, so will not
sprout properly.)- Carefully check any plants from
nurseries before purchasing: turn the
leaves over to see if psyllids are hiding
there. - Choose early varieties of tomatoes,
potatoes and other susceptible plants,
and plant them as early as possible in
the season to avoid the peak period
(usually from Christmas onwards)
when psyllids are most active. In
colder areas, delay planting until the
threat of frosts has passed. The use of
cloches or frost cloth may be helpful. - Māori potatoes seem to be resistant to
this disease. - If your plants become severely infested
with TPP, carefully remove the plants,
put them into sealed paper bags and
bin or burn them. - Before winter, remove and destroy
any volunteer potatoes, tomatoes and
other solanaceous crops in case they
don’t get killed off by frosts. Remove
all solanaceous weeds, as psyllids will
overwinter on these plants in sheltered
spots in your garden or greenhouse. - Poroporo and tamarillos are
perennials; they can harbour
overwintering psyllids, so carefully
remove and burn or bin any infested
leaves or plants before winter.
Covering up
In recent years horticultural mesh has
become a popular way of protecting plants,
and is used by commercial growers and
home gardeners. The Biological Husbandry
Unit (BHU) at Lincoln University was
the first to look into its effectiveness in
preventing TPP from attacking potatoes.
Horticultural mesh has tiny holes
(0.6 mm) that prevent small insects from
getting through and attacking the crops
within. It also fends off larger pests like
cabbage white butterflies, root flies, flea
beetles, shield bugs, birds, wasps, possums,
rabbits, cats, dogs and even deer!
The mesh is permeable to rain, wind
and sunlight, so there is no concern about
shading or watering crops adequately. The
microclimate underneath is also improved,
so less watering is necessary. It is not designed
for frost protection. Any sprays of foliar
fertilisers, fungicides or other liquids applied
to the plants can penetrate this mesh.
Many crops can be protected, including
brassicas, carrots, peas, lettuces etc. Taller
crops like sweetcorn are less easy to cover.
As main-crop potatoes are planted later in
the season, the mesh is ideal as they are
Above: Psyllid eggs on the edge of a tomato leaf. Photo: Duncan Smith more vulnerable to this pest at that time.Above: 0ņRULSRWDWRHVVHHPWREHUHVLVWDQW
to the tomato-potato psyllid. These potatoes
were grown at the Eastern Institute of
Technology’s maara kai in Ruatoria, and include
kowiniwini, karu pareia, hua karoro, moemoe,
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Photo: Philippa Jamieson