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 http://en.wikipedia.org/wiki/Aphids
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 http://en.wikipedia.org/wiki/Mealy_bug
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 http://en.wikipedia.org/wiki/Scale_Insect
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.
(BUT NOT THESE TWO PRODUCTS TOGETHER)
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.
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.
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 http://en.wikipedia.org/wiki/List_of_citrus_diseases