Citrus Tree Problems – Pests and Diseases

Many gardeners have a lemon tree and likely one or two other Citrus trees so now is an opportune time to look at the problems that can occur.

Insect problems include:

Aphids, usually clusters of black or green insects on young leaves. Spray with Neem Tree Oil and Key Pyrethrum to control. Also sprays of Sunlight yellow bar soap lathered up in warm water.

Image from Wikipedia

Mealy bug, often the presence of black, sooty mould will be the first noticed signs. Small, mealy insects found in protected cavities. Spray with Neem Tree Oil and sprinkle Neem Granules under the tree and water in to kill the root mealy bugs.


Image from Wikipedia

Scale, poor growth, pale dehydrated leaves will be noticed. Fruit will be small and dry.
The cause is hard scale-like insects on woody and green stems. Scale numbers build up in dry seasons, spray with Neem Tree Oil over summer months.

Leaf roller caterpillar, leaves tightly rolled and foliage and surface of fruit eaten. Spray with Neem Tree Oil as needed from October to March.

Soft wax scale, snow white, soft scale likely sooty mould present. Treat as Scale.

Thrips, show as silvering of foliage and fruit. Black spots of excrement maybe seen especially under leaves. Spray with Neem Tree Oil spray from November to March.

Image: from Wikipedia

Spider Mites, leaves go yellow and hydrated. Minute insects under leaves. Common in hot dry weather, spray with Neem Tree Oil or  Liquid Sulphur spray.

Whitefly spray all over with Neem Tree with Key Pyrethrum added late in the day. Repeat every few days till under control.

Lemon tree borer, tree shows poor growth, are dehydrated and branches die.
Holes found in branches and sawdust indicates presence of borer.
Remove infected wood where practical and burn, sprinkle Neem Tree Granules under the tree, Wrap felt pad soaked in Neem Tree Oil around base of trunk or affected branches.
It is a good idea to fill in the borer holes with an acyclic paint. This prevents adults entering the holes and laying more eggs. It also means that it is easy to detect new holes and further problems.

Citrus brown rot, fruit develops brown rot and drops from tree. Prune out lower branches of tree to increase air circulation, spray with Liquid Copper and Raingard.
Verrucosis/scab, Irregular, grey, scabby, wart-like growth on fruit or stems.
Spray with Liquid Copper at monthly intervals.

Brown Spot, spots on leaves, fruit and stems of mandarins. Common in damp weather.
Spray with  Liquid Copper, prune dead material and burn it.

Melanose, small dark, red-brown spots on leaves and fruit, often merging. Skin may crack. More common on older trees in warm humid weather. Prune off dead twigs and branches. Spray with  Liquid Copper at monthly intervals.

Mineral Deficiencies:
Lack of food, pale small leaves, Mulch with an animal manure based compost and blood and bone.
Sheep manure pellets, Bio Boost or Break Through are very good also.
Lack of Iron, light green leaves fading to pale yellow or white. Veins remain green.
Apply mulch and Sulphate of Iron.
Zinc, new leaves small and narrow, growing close together. Spray foliage with Matrix Reloaded and apply Ocean Solids to the drip line.
Magnesium, older leaves yellow from outer edge and yellow area between the main veins on younger leaves. Spray foliage with Matrix Reloaded and sprinkle Fruit and Flower Power under tree to the drip line.

Citrus trees are pruned only in summer by removing total branches from within the frame work of the tree to open the tree up and allow for better air circulation. Do not trim off the ends of branches as this causes further branching and a denser tree.
Citrus trees are mulched in spring. Beware of mulches in winter that prevent wet soil drying out and causing roots rots. During flowering and fruiting periods apply Fruit and Flower Power once a month. This will create fruit with great flavour and ample juice.

Winter Time:
Lemon trees can be grown throughout New Zealand in home gardens, but in some areas, which are more exposed than others, some little tricks are needed.
Most home gardeners know about protecting young citrus trees from air frosts with a tent of clear plastic or sprays of Vaporgard.

Gardeners can be tricked in winter by lemons and other citrus fruits which grow quite yellow or orange, appearing to be ripe. But usually the white pith is thick; lacking in juice the fruit can be dry. Sheer cold will turn green fruit a bright yellow/orange in some conditions on some soils.
Give the plant more warmth — because there is some warmth in the low winter sun if it can be trapped — and the fruit will ripen better.

An almost total plastic enclosure, making a small glasshouse round the trees, is worthwhile if the gardener is serious about ripening fruit in midwinter. At the price of horticultural plastic these days it is not an expensive option either.

But clear plastic on the soil round the tree, out beyond the drip line, will also make a tremendous difference to the growth of the tree.

The clear plastic allows the sun’s rays, weak though they be, through to warm up the soil and then traps that heat so that the tree roots get the benefit of it.

Weeds should be eliminated first, before pinning down the plastic. If the gardener prefers organic methods, boiling water will kill surface weed seeds without harming the tree roots if not applied too liberally.

You can also get a comprehensive list of citrus diseases from Wikipidia

8 comments on “Citrus Tree Problems – Pests and Diseases

  1. ray metcalfe on said:

    greetings. 3 years go,I planted an orange, and the next season it flowered andad 2 fruit. the last 2yars ther has been no flwers [even after a thinning] now it has 2 ain stems coming from right at the bottom[ close to ground]and bright green leaves[Sept2010]. It had Citrus fertilizer last autumn, and a month ago , the old fashioned urea! Any ideas please?

  2. Tina Johnston on said:

    Hello I have 2 lemon trees but one of them every year about this time gets infested with blow flies it is as if they are nesting in it, I have checked to see if it has Aphids and no it doesn’t. This lemon tree is very healthy except all the fly poo on it.
    Regards Tina

  3. Spray with Pyrethrum

  4. starbutton on said:

    I have two small meyer lemon trees that just can’t grow any leaves – one was bought with leaves and they have all disappeared, and the other was like that when we bought the house (in a pot on a deck).  They seem to be eaten as soon as they grow, and I can’t find any evidence of bugs.  What could be doing it? I have another large lemon tree with no problems.

  5. Murray Stevenson on said:

    I have a very old plum tree which has a couple of problems.
    1) Some brances in full foliage and others which have hardly any leaves at all. The branches with hardly any leaves have borer showing and i sprayed those braches last year. This tree provides a much appreciated screen from our neighburs so i am reluctant to prune to many branches away but also want the tree to live. What should i do here wregards the booere and the braches not coming inot leaf very well?

    2) A lot fo the branches are covered in a geen moss -do i need to be concerend about this?

    3) Last year the tree seemed to loose its leaves very early. Does this mean anything?

    Any help would be appreciated.

  6. Brucelevien on said:

    My lemon trees go through the flowering stage but once the small fruit buds begin to form they get to the size of a pea then drop off, I assume there is a mineral deficiency can you advise thanks

  7. Swardys on said:

     I have two lemon trees growing next to each other. One is perfectly healthy but the other is loosing its leaves and I have noticed that some of the smaller branches have what looks like swellings at different intervals… any ideas?

  8. Alinjay on said:

    My lemons are being eaten by a bug that looks like a tiny seashell.

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