New Zealand Gardener Magazine put together a list of 10 of the most common problems with growing Citrus trees, so read on if you want to know how to grow luscious lemons and awesome oranges without the usual mould and infestations.
1. Yellow Leaves: According to NZ Gardener, discoloured leaves is one of the problems most suffered by citrus trees. It signifies a lack of nutrients and to fix it, you must focus on improving the magnesium available to the tree. Applying Sequestron or Yellow Leaf Remedy is a good quick fix or dissolve Epsom Salts in some water and apply to the soil (about 500g per young plant) for a more long-term solution (but don't over do it!).
2. Insects: We all know infestations of bugs and insects can cause immense harm to our plants and trees. The best way to tackle these is with a low-toxicity spray. Nz Gardener suggests applying a soapy spray or a pesticide such as Yates' Target or for an organic option, try neem extract, either sprayed on foliage or applied as granules to the soil. Regular applications will see a reduction in pests.
3. Sooty Mould: Sooty mould is a black fungal growth that lives off sugar secreted by scale or aphid insects. This also attracts ants. Although the mould does no damage, it isn't great to look at so by controlling the insects first, you can control the mould.
4. Scale: Scale are waxy brown insects that disguise themselves as bits of bark! Their tough little exteriors tend to make them resistant to most pesticides but using neem oil regularly should soften their outer shells and help get rid of them. You can also wipe off smaller infestations with an alcohol soaked cloth.
5. Leaf loss & Curling: NZ Gardener tells us that plants lose their leaves when under stress. Cold, bad conditions or poor drainage are often the culprit or you might be killing your tree with over-watering. Figure out exactly what the problem is to ease the stress on your tree.
6. Little Fruit: It takes a lot of energy to make a big, juicy crop and its not uncommon for some citrus trees to only produce fruit every two years. You can avoid this by thinning the crop with secateurs, removing half the fruit while still very small so the tree has enough energy to produce again the next year. If a tree doesn't produce year after year, it may not be in a warm or sunny enough spot.
7. Pale: New leaves that are very pale, even white, signify an iron deficiency. You can add iron quickly and efficiently by applying a liquid feed to the foliage or by drenching the soil with Sequestron. Also, it is suggested that you try to lower the pH of the soil by replacing it with a bag of citrus potting mix and water the tree with rain water instead of tap to encourage a more acidic environment.
8. Scabby Fruit and Leaves: This is cause by a fungal infection called verrucosis. It looks like raised warts on the surface of the fruit and the leaves. NZ Gardener tell us that this is a common cosmetic problem that needs to be treated with a preventative method as once it turns up, it is too late to get rid of it. Give trees a double spray with copper and apply in mid-spring and in summer.
9. Borer: This little pest is a longhorn beetle that causes havoc with citrus, fig, wisteria and gooseberry trees. The telltale sign is sawdust exuding from active holes. You can spear the lavae with guitar strings poked directly into the holes or spray an product like No Borer Spray Injector into the holes. Avoid pruning between early spring to mid summer in hope that these little bugs will leave your trees alone!
10. Collar Rot: This is a fungus present in the soil and it rots the base of the tree trunk just above ground level. It is a nasty problem that casues the bark to split and ooze, makes leaves turn yellow and will eventual kill off your tree. To treat it, NZ Gardener recommends cutting back infected wood, spraying with an anti-fungus spray and keeping the base of the tree free from foliage and well drained.
From NZ Gardener Magazine.