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 Scab.
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 spray.
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