We received three questions about problems with citrus trees. We've combined the answers to make sure all the useful information is in one place.
Q: Citrus trees in the backyard - how can I control bugs (stink beetles) and grass hoppers? How do I ensure a good fruit crop and how often should I water lime, lemon and orange trees? Majella - Kelvin Grove
Q: Some of the leaves on my lemon tree have curled and something is eating them. Can you tell me what to do? And if you've got any hints on how to prune a lemon tree, I'd really appreciate it! Thanks. Marg - Runcorn
Q: We recently bought a hectare of land in Dayboro, and have planted many citrus and native trees, about 50 in all. Some have been invaded by insects, especially the white cedars. What is the best insecticide we can employ on the citrus and the natives? Thanks for any advice. Bill - Dayboro
A: Pests and diseases
‘Leaf curl’ … ‘something is eating the leaves’ … ‘bugs (Stink Beetles)’ … ‘grass hoppers’ … ‘invaded by insects’ … these are the several problems being experienced.
WOW! A few problems here! When planting tea in Sri Lanka I thought WE had pest and disease problems across there, but I soon learnt that Queensland certainly has very many more.
Leaf curl
Leaf curl in citrus can be caused by several cultural and pest and disease problems.
Under-watering AND over-watering can bring about ‘leaf curl’, and so you need to check those conditions first. Please check your soil. Is it too wet or too dry? Compensate accordingly, for what you require is a ‘slightly moist’ soil at all times, 50mm below the surface.
One pointer to either of these ‘watering’ conditions is that the leaves of the plant will not be marked or damaged in any way, but will be a ‘dull, greyish’ green, and will also droop.
Wind injury can cause crinkling and curling. Is your plant situated in a location exposed to strong winds?
Leaf Miner is a very common citrus insect problem, and causes the leaves to distort and curl. It is very easily identified, for you will see whitish lines meandering or crisscrossing across the leaf. It is caused by an insect larva (teeny-weeny caterpillar) that eats away at the innards of the leaf between the upper and lower surfaces of the leaf.
Aphids, Thrips and Scale can cause leaf curl, particularly the young leaves in the case of the first two, and the more mature leaves if scale.
Have a very close look – do you see an infestation of tiny little insects on the young leaves, or funny looking ‘bumps’ on the mature leaves?
Mites can cause leaf curl. Mites are virtually impossible to see with the naked eye.
Fungal problems cause leaves to curl, particularly Powdery Mildew and Scab. Powdery Mildew is fairly easy to identify for you will see a white powdery substance on the surface of the leaves. Scab can be identified by the presence of patches of raised ‘rough tissue’ on both the leaves and the fruit.
Control measures
Very obviously, either increase or reduce your watering if that is the problem, and depending on what your soil-check tells you.
If wind is the cause … either move your plant or give it some wind protection.
In the case of a pest or disease (Bill – for plants other than citrus as well) you need to first identify what it is that is causing the curl. To control Grass hoppers, Caterpillars, Leaf Miner, Aphids, Thrips or Scale – try Dimethoate, Confidor, or Lorsban. White oil also works on Scale. If Mite is the problem – try Dimethoate, Kelthane or Wettable Sulphur. If Powdery Mildew – try Wettable Sulphur, and if Scab – try Copper Oxychloride.
Pruning
Citrus trees do respond to some trimming and pruning.
Examine your tree and remove weak and spindly branches, dead branches, branches that cross over and touch or rub against each other; any suckers that arise from low down on the plant, from below the grafted union.
Many citrus trees become overburden with fruit and it is a good idea to remove some of the fruit bearing branches. Be ‘cruel to be kind’. This will prolong the life of the plant, will open it up for better aeration and light and will give you larger fruit as well.
When does one prune? The best time would be between mid autumn to late winter, but definitely after the last frosts of winter can be expected … if you happen to be in a frost pocket in any of the Brisbane suburbs. The tree will go into some dormancy and slow growth into winter and will spring back into life once the weather warms up again.
Watering
If there is any one thing that is ‘all important’ for best results with citrus – it is watering.
To ensure a good crop and get the best out of citrus one needs to keep them well watered. They will not die off on you if they receive less water, but they will certainly not perform at their best.
The size of the tree, its age and soil type obviously need to be considered in determining the quantity of water to use. A really good soaking once a week in summer, and once every fortnight in winter is a good guide to follow.
Please remember … mulch around your trees will help retain moisture in the soil. A lack or shortage of water at critical times can affect the onset of flowering, and the quality of fruit at maturity … they can shrink and lack juice and flavour. A lack of water can also result in flower and young fruit drop.
Fertilising
The best times to fertilise citrus are in August, November and February.
The first application prepares the plant for its spring flush, and will assist in flower and fruit set. The others sustain the plant during growth and fruit maturity.
For best results – you must fertilise regularly.
If I have trimmed my citrus trees to the bare branch, does this mean that I have killed them?? They had leaf curl so I thought this would be a good way to get rid of it.