orange tree with yellowing foliage
Image credit - http://www.gardeningknowhow.com/

If you live in a northern European climate but have accepted the challenge of growing an orange tree then be prepared for certain amount of heartache, some of it justified, some not. One of the things that is alway a concern is the yellowing of the new growth. The young leaves emerge in the spring a wonderful burnished, bronze colour, but once they fade to their regular vivid, mid-green colour they can often continue to change to a rather unhealthy yellow as the leave grow to their full size.

Iron Deficiency - http://www.yates.com.au/
The most common reasons as to why the leaves on your orange tree are turning yellow are also the easiest to correct which is usually down to a lack of availability of one nutrient or another. Citrus plants are heavy feeders in general and orange trees are no exception. If you are not feeding weekly during the growing period then nutrient deficiency is going to be just around the corner.

If the new growth is showing green veins with the rest of the leaf appearing light yellowish to white in colour then this is iron deficiency. If the new growth is pale green to yellow in colour the this is sulphur deficiency. If the leaves have a weird-looking inverted green V-shape at the base surrounded by yellowing then this is magnesium deficiency. If it is the older leaves that are turning yellow first and then followed by the newer growth then it s likely that nitrogen deficiency is the problem.

yellowing leaves on a orange tree due to nitrogen deficiency
Nitrogen Deficiency - http://idtools.org/
Although they are not ericaceous plants, orange trees will appreciate being kept in soil or compost that is slightly on the acidic side. This will improve the uptake of iron and magnesium which if in short supply will result in the characteristic yellowing colour known as chlorosis. You can acidify the soil or compost by feeding with a liquid soluble ericaceous fertiliser, or add more iron and magnesium to the rooting medium to increase it availability.

Before commercial acidic fertilizers were available, epsom salts (magnesium sulfate) were a popular method of treating yellowing citrus leaves. A tablespoon of Epsom salts dissolved into half a gallon of water, should do the trick and this too can be applied as a foliar spray. The magnesium in epsom salts addresses magnesium deficiency while the sulphur helps to acidify the rooting medium. What about iron deficiency I hear you say? Well it was traditional to add iron nail or iron horse shoes to the ground before planting.

Magnesium Deficiency on orange tree
Magnesium Deficiency - http://www.grantsgardens.com/
Citrus trees will also respond well to a seaweed based fertilized which are usually high in micronutrients. Not only are ericaceous seaweed fertilisers available you can also apply them as a foliar spray to help the orange tree receive its nutrients directly to the point of concern.

If your orange tree is not only showing a yellowing of its leaves but also other worrying characteristics such as a gummy inner bark, dry cracked bark with sap oozing lesions then it is probably suffering from one of several fungal diseases. Spray immediately with a systemic fungicide and withhold watering. If your orange tree is also suffering from die-back and dropping leaves then it is possibly under attack from honey fungus. Scrape away at the surrounding soil and if you uncover thick, licorice-like, bootstrap roots then your plant has been infected with honey fungus. Dig up and burn your orange tree to control the further spread of the fungus. Drench the surrounding soil with armillatox.

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