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Citrus Pests & Diseases

Aphids & Mealy Bugs.  The presence of these small, sap sucking
pests is usually indicated by black sooty mould.  Aphids may be evident
clustered on the new growth but mealy bugs are usually on the underside of
the leaves.   Use synthetic pyrethroid products, Pest Oil or Confidor
on these pests.

Black Spot is a fungal problem showing as circular, dark, sunken spots on
the fruit.  Spray the young fruit and foliage with a copper spray in
early spring, and then repeat again in 6 weeks time.

Boron Deficiency is indicated by fruit with dry, brown flesh.  Dissolve
1 tablespoon of borax in hot water then mix into 9 litres of water and apply
to the soil between the trunk and the outer foliage line.  This treatment
should not be used in quantities larger than recommended and never more than
once a year.  Sometimes, if the soil is very acidic, this may limit
the boron available to the plant and the addition of dolomite may assist.

Bronze Orange Bug, or citrus bugs are either brown or bronze/orange in colour
and emit a strong smelling liquid if disturbed.  These bugs cause significant
loss of fruit by sucking sap from the stalk of the fruit so that the fruit
is shed.  They also cause damage to new growth.  Remove with kitchen
tongs and drop into a bucket of water mixed with 1 cup of kerosene.

Citrus Bud Mite causes damage to the flowers whilst they are still in bud.  Usually
only a few lemons are affected by this pest.  Remove any deformed fruit
and dispose of it.

Collar Rot is a fungal disease which affects the bark on the trunk of the
tree.   The bark begins to rot or flake just above ground level
and this band of soft decay gradually encircles the trunk.  If not
detected before it fully encircles the trunk, it is too late and the tree
will die.  This problem can be caused by injury from mowing, weeding
etc, or by incorrect mulching, where the mulch is too close to the trunk,
allowing harmful fungi and bacteria to invade the bark.  The first signs
may be loss of vigour and possibly small fruit which turn black and drop
off the tree.  If collar rot is discovered, use a knife to gently scrape
away soft, diseased bark, down to clean wood.  Mix copper spray to a
paste (Bordeaux or similar) and paint generously on to the affected section.  Make
sure any mulching material, plants or weeds are kept well clear of the trunk
so that there is good air circulation.  Remove any weak or congested
branches from the tree, and all fruit and blossoms.  Preventing collar
rot is easier than trying to cure it so make sure the lower section of the
trunk is kept clear of all mulch or plant material.

Crinkle Leaf is a disorder of some lemons and grapefruits.  It will
not cause problems with the tree or fruit and there is no remedy for this
problem.  Keep the tree healthy and stress free, but bear in mind that
aphids and leaf miners can also cause similar symptoms, so check that it
is not these pests that are causing the problem.

Defoliation can be caused by several factors.

Salt winds in coastal areas.  Unfortunately citrus cannot tolerate salt
winds and the only solution to this is to provide complete protection.

Manure which is too fresh.  Only composted manure should be used on
all plants.

Poor drainage.   Citrus cannot tolerate poor drainage and the plant
should be moved to a raised bed containing good soil and aged manure and
watered well with a solution of seaweed extract.  All flowers and fruit
should be removed and no fruit permitted to form for a period of 12 months.

Insufficient water in hot weather.

Dried Flesh Usually a sign of over mature lemons which have been left on the tree too
long.  Remove the fruit before they change colour fully, and store
the lemons.

This problem may also be an indication of a condition called ‘Internal
Decline’ which occurs in older trees.  Older trees should be rejuvenated
by radical pruning in late spring or early summer.  The trunk and exposed
branches should be whitewashed as citrus bark is susceptible to sunburn.  The
tree should be fed with an organic fertiliser, well watered and mulched.  No
fruit should be allowed to form for 12 months after radical pruning.  Maintain
regular deep watering throughout the warmer months.

Another possibility is stress caused by hot, dry conditions.  Moisture
from the fruit is withdrawn to assist the tree and the fruit becomes dry.  Ensure
that trees have sufficient water in the warmer months.

Failure to Fruit in the first few years is normal as citrus are usually
not productive until the main branches have formed.  Young citrus trees
should have all fruit removed for at least the first 2 years.  When
large, healthy plants fail to set fruit it can be caused by excessive nitrogen.  Apply
1 handful of superphosphate per square metre of area below the tree and just
beyond the outer branches and water in thoroughly.

Frost Damage to the canopy will result in dead, curled foliage and fruit
may turn black and dry.  Do NOT prune until all frosts are over for
the season as the damaged leaves will afford some protection to the plant.

Fruit Drop is most often caused by dry conditions and insufficient watering
of plants.   For treatment of fruit drop at embryo stage, see Lemon

Fruit Fly is usually not a problem except occasionally with Meyer lemons.  A
registered chemical for fruit fly control can be used, or baits hung in the
trees.  Rogor should NOT be used on citrus, especially on Meyer lemons
or cumquats as it can cause severe defoliation and sometimes death of the
plant.  Remove and dispose of all affected fruit according to local
government requirements.

Gall Wasp is a pest which occurs in warmer areas.  The tiny wasp lays
it’s eggs into soft, green wood from September to November.  The
affected section of the branch then swells.  Large numbers of galls
can affect the sap flow, causing dieback in parts of the plant.  Prune
out the affected wood before the larvae emerge (usually August).

Iron Deficiency is indicated by the new leaves showing light green to yellowish
colour during the warmer months.  Apply iron chelates according to the
directions and composted cow or sheep manure.  This problem usually
occurs on alkaline soils.

Leaf Miner is a pest which affects the new leaves and causes whitish, silvery,
wavy lines on the leaves and distortion of the new growth.   This
pest often encourages sooty mould.  Spraying with Pest Oil is usually
successful, but in areas where this is a continuing problem, spray with Pest
Oil once a fortnight as a preventative measure.

Lemon Scab affects lemons and some mandarin varieties.  It presents
as raised grey to brown corky scabs on the smaller branches, leaves and fruit,
causing peaked distortions on the fruit.  It may also cause fruit drop
at the embryo stage, when fruit is very small.  Lemon scab is caused
by a fungus which attacks when the petals are falling from the flowers.    Spray
when about half the flower petals have dropped, using copper spray mixed
with white oil.  Take care to mix the white oil separately, and then
add to the already prepared copper spray.  Two tablespoons of white
oil to one litre of water, and add this to 4.5 litres of prepared copper

Magnesium Deficiency is indicated by a green ‘V’ shape through
the centre of mature leaves, with yellowed outer edges.  Leaves fall
prematurely.  Water the soil under the tree, out as far as the area
covered by its branches.  Dissolve 2 tablespoons of Epsom salts into
9 litres of water and pour onto the pre-soaked area under the tree.  For
quick results, retain about a litre of this mixture and spray onto the foliage.  Affected
foliage will probably remain unaltered but new growth should emerge green
and healthy.

Manganese Deficiency may cause yellowing or light coloured leaves between
the midrib and the veins of young leaves.   Add rotted poultry
manure or some dolomite to make the soil more alkaline and increase the plant’s
access to manganese.

Melanose is a fungal problem which affects immature growth and fruit with
round, brown pinpoint lesions.  These lesions become larger and raised,
with a rough texture, and form a tear drop shape.  Spray with a copper
fungicide in early spring and then repeat the process in 6 weeks.

Mites are tiny insects which are barely visible to the naked eye.   A
preventative treatment can be carried out with sulphur spray when the fruit
is ‘marble’ size, but mites usually don’t affect the quality
of the fruit.  Their presence is indicated by the skin of the fruit
turning brown or grey.

Scale is a sap sucking insect hidden beneath a scaly coating.  The
presence of ants often indicates that there is scale on a tree.  There
are many types of scale which affect citrus trees.  For the control
of soft scale, either Pest Oil or Confidor can be used.  For the control
of hard scale, use Pest Oil.

Septoria Spot is a fungal disease and generally a sign of high humidity
in the area around the tree.  It presents as brownish marks about the
size of a finger print on the fruit, and slightly sunken.  The quality
of the fruit is not usually affected but discontinue overhead watering and
improve the air circulation in the canopy by thinning the central branches.

Stunted Plants with lots of flowers may indicate stress caused by soil problems,
strong winds or root bound container plants.   Remove all flowers,
water and feed well.  Container plants will need to be repotted.

Thickened Skins and little juice in the fruit.  Lemons need to be stored
after harvesting so that the skin becomes thinner and the flesh becomes juicer.  Harvest
and store lemons for 2 weeks or so before using them.  This is particularly
common with Eureka lemons in the first few years of fruit bearing.  Thick
skin may also be caused by heavy applications of nitrogenous fertiliser such
as sulphate of ammonia or urea which is applied after mid summer.

Zinc Deficiency is fairly common in citrus and presents as yellow areas
between the midrib and the main lateral veins of leaves.  In severe
cases the new leaves will be very small and narrow on short stems, giving
a crowded appearance.  Some twigs may die back and some may have multiple
buds and dense shoots giving a stunted, bushy appearance.  Fruit may
be pale, elongated and small.  Feed the tree with a balanced citrus
fertiliser, watering before and after fertiliser application, or apply zinc
sulphate.  To apply zinc sulphate, dissolve 2 tablespoons in 9 litres
of water with 1 tablespoon of pest oil and thoroughly spray all parts of
the tree – branches and foliage.  Can also be treated by applying
composted cow or sheep manure.  This is best done in early spring as
new growth begins to emerge.

Ann Costelloe