Orange trees, as with any fruit-bearing trees, are prone to both specific and common pests and diseases. Pests that commonly attack orange trees include mealy bugs, aphids and spider mites, and these are fairly easy to treat but, with young trees, can cause a lot of damage.

Then you have diseases, including root rot and citrus canker, which are either fungal or bacterial. One of the more common problems with diseases on orange trees is Black sooty mould, as it is on most citrus trees.

Here’s how to figure out the orange tree pests and diseases you have and what to do about them, starting with pests.

Pests

Mealybugs

What are mealy bugs?

Mealybugs are one among many different kinds of sap-sucking insects. This means that they live on your orange tree and suck the sap out of the leaves and stems. This weakens the tree, leading to diseases and other pests moving in. The lack of strength in the orange tree means that it can’t maintain its existing growth, let alone produce new foliage and fruit.

How to recognise mealy bugs?

These pests are oval, shaped flat insects that are pink. They have a white wax coat as protection. While mealy bugs aren’t that small, they hide away in out-of-sight parts of the orange tree. They’re easier to see when they get together in groups. Look under the outside layer of the stem and in the leaf sheafs.

However, mealy bugs secrete a clear, sweet, sticky substance called honeydew as they’re sucking on your plant leaves. This substance is visible to you if you look closely at the plant leaves. As several different bugs also secrete honeydew, you need to do more to identify which type is causing the problem. But if you see honeydew, then you do have some kind of bug present.

Note that honeydew attracts a disease called black sooty mould. See the next section for details on this.

What to do

Catch the mealy bugs as soon as you see a few of them. Just a few is OK, but they quickly expand into large groups that are more different to handle. First of all, isolate the orange tree as mealy bugs do move between plants, though they don’t go far.

Then cut off the parts of the tree that are infected. That’s all you can do, as they’re no insecticides that work on these pests. If the tree has been overrun by these bugs, you have to destroy it to save your other plants from being affected.

Aphids

What are aphids?

Orange trees are susceptible to several different types of aphids. However, all aphids can be treated in a similar way. It’s important to catch and control aphids early in their life cycle as this lowers their numbers later on. When they’re in the garden, aphids are a key part of the ecosystem, being food for other insects and birds. But in your home, on your orange tree, not so much.

How to recognise aphids?

Aphids are varied in colour, from yellow to green to black. Blackfly, greenfly and whitefly are the most common aphids. They look like tiny flies (1mm to 2mm long) with small bodies. They’re difficult to see individually, but a whole infestation of them takes up enough space so you can easily catch sight of them.

Aphids are another insect that sucks the sap out of the leaves of your orange tree, leaving it in a weakened state. Over time, the leaves turn yellow and start to curl, and the tree begins to wilt.

But if you look carefully before this happens, you’ll see the honeydew substance that the aphids leave. This is a clear sticky substance on the leaves. This attracts black sooty mould (see Disease section) and also ants. So if you see ants on your tree, look for honeydew.

What to do

If you can see just a few aphids, remove them from your orange tree using sticky tape to pick them up. Also, hang bright yellow fly paper from your tree – though you may catch the good insects as well as the bad ones. Or carefully spray water on the plant to knock the bugs off. But don’t use so strong a stream that it carries them to nearby plants.

Prune away the parts of the orange tree that are affected. Sterilise the cutting tool between cuts.

Another solution is to apply neem plant oil (a natural oil) to the plant carefully. Follow the manufacturer’s instructions, and don’t use it if your orange tree is really stressed out.

As a last resort, use a commercial insecticidal soap product. Again, apply according to the manufacturer’s instructions.

Spider mites

What are spider mites?

Not exclusive to orange trees, spider mites are a common pest for all household indoor plants. They’re very tiny pests (0.05cm), so you need to look closely to find them with a magnifying lens. Spider mites are also sap-sucking and live on the sap on the leaves of your orange tree.

How to recognise spider mites?

If you do see a group of spider mites on your orange tree, they may appear to be green or brownish-red, or even translucent. They each have two dark spots on their body. Instead of seeing the spider mites, you may come across their spider webbing instead.

And as they’re sap-sucking, they too, leave behind the clear sticky substance called honeydew. As it’s this that’s usually the most easily spotted.

The leaves may turn yellow and appear ravaged. They may also fall off the orange tree. The oranges may also shrivel up and drop off the tree as well.

And if you haven’t noticed the spider mite infestation, the black sooty mould (see the section below) that’s attracted to the honeydew may be your first clue.

What to do

Use a jet of water to dislodge the spider mites, the honeydew and any black sooty mould build-up. Do this in an open outdoors space (or your bathtub), so you don’t spray the bugs onto any other plants.

If this doesn’t work, trim off the infected leaves (sterilise the cutting tool after every cut) and cut off the infected branches. Spray the rest of the plant with neem horticultural oil or a natural insecticidal soap to prevent more bugs from moving in.

Diseases

Several of the diseases – black sooty mould and root rot –  are common to many plants, not just orange trees. However, citrus canker loves orange tree. Here’s how to recognise these diseases and treat them.

Black sooty mould

What is black sooty mould?

Black sooty mould is a fungal infection that affects many plants indoors and outdoors. Orange trees aren’t exempt from unfortunately. Black sooty mould is fond of growing on the honeydew left by bugs and pests (see previous section for more details). While it doesn’t harm your orange tree by itself, it does prevent photosynthesis from taking place and so weakens the orange tree.

How to recognise black sooty mould?

As its name says, this mould looks just like black soot. It starts out as powdery spots on orange tree leaves and then spreads across the whole leaf. Look for it also on the oranges themselves. On the leaves, it covers the green pigment that performs photosynthesis and so the tree is unable to make enough energy to maintain itself and to produce new growth.

What to do

Black sooty mould is treatable. Wash off the black soot with soap and water. This reveals the green pigment that’s still on the leaf underneath, and the tree can get back to normal. However, if you’ve left it a while to treat this sooty mould, your leaves may be yellow. While you’re doing this, scrape off the sticky honeydew as well.

But realise that you still have the underlying problem of the pests that created the honeydew in the first place. Re-read the first section of this article to figure out what they are and how to get rid of them.

Root Rot

What is root rot?

Pretty much every plant or tree that you water is susceptible to root rot if you overwater it. Orange trees are no exception. Root rot is a fungal disease that moves into roots that are saturated with water. The roots aren’t able to breathe and can’t send water and nutrients up through the orange tree.

How to recognise root rot?

Root rot begins underground in the roots. They turn brown or black and become mushy. By the time the above-ground symptoms of yellowing and wilting leaves come into view, the disease is well along.

Root rot often turns into stem rot as it moves up the tree. In this iteration of the disease, the tree stem becomes soft and flexible.

If you suspect root rot, dig around in the roots of your orange tree and check if they’re squishy (root rot) or firm (OK).

What to do

You can’t reverse root rot. You can’t really treat it either. But you can save the part of the plant that’s not affected. Remove the tree from its pot and gently take away all the potting mix – it’s probably wet. Check all the roots and cut off those which are black or brown and mushy. Sterilise your cutting tool between each cut, so you don’t transport the disease around the healthy roots.

Check that there are enough drainage holes in the pot by running water through it. Make new drainage holes in the bottom of the pot if needed.

Repot the remaining roots (and tree) in the pot with an entirely new potting mix that’s dry. Water in well.

And change your watering habits. Wait until the top 3cm of soil is dry before thoroughly watering the tree. Let all excess water drain off.

Citrus canker

What is citrus canker?

Unlike root rot and soot black fungus, citrus canker is a bacterial disease. It thrives in warm, wet environments. So look out for it in the summer if you’re prone to over-watering your plants. It’s spread within trees and between trees on pruning equipment.

How to recognise citrus canker?

This disease causes discolouration and blemishes on the oranges themselves. The circular lesions it forms on the stems, the oranges and both sides of the leaves have raised scabs in the centre. These lesions are brown or tan with yellow halos. As the disease takes hold of the tree, the lesions turn darker in colour.

The infected leaves drop early from the tree, as do the immature oranges. The oranges are still safe to eat even though they don’t look very appetising.

What to do

If your tree already has citrus canker, there’s no cure. You need to remove the infected parts and be careful not to spread the disease to the rest of the tree.

But you can prevent the disease from appearing in the first place. Use liquid copper fungicide on the tree. Spray this on according to the manufacturer’s instructions. It stops citrus canker from taking hold.

If you think your orange tree is dying, read my guide here in possible reasons

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