Citrus Tree Pests and Diseases

‘What is wrong with my lemon tree?’ is one of the most common asked questions on gardening websites and gardening talkback radio shows. One of the difficulties of growing a lemon tree, orange tree, lime tree or other citrus tree is that there are many insect or animal pests, various diseases affecting the plant, stem, leaves or fruit, and disorders such as split fruit skins and nutritional deficiencies shown when a mineral deficiency is present in the soil.

This blog is devoted to answering some of the most pressing citrus tree questions gardeners ask the experts.

Q. Why is my orange tree is not producing fruit?
A: It is most likely that your orange tree needs fertiliser. Citrus trees require feeding on a regular basis in order to grow well. Feed your tree with a balanced citrus fertiliser every couple of weeks and see if the tree improves. If you live in a cold climate it may be better if you overwinter your tree in a warm greenhouse.

Q: Some of the fruit on my lemon tree have split. Why has this happened?
A: When citrus fruit split and inside flesh is exposed it can be due to lack of nutrients such as copper and calcium, moisture stress, frost and general health of the tree. This often happens when a tree receives a sudden soaking of water particularly after a long dry spell. Citrus trees require plenty of water while the fruit is developing, so ensure that the tree receives sufficient watering. Good nutrition, and application of lime around the tree every few years will help prevent fruit splitting. If soil is alkaline, add gypsum rather than lime.

Q: The leaves on my lemon tree are covered with oval brown shells and sticky black substance. How can I get rid of this?
A: This sounds like scale insects and sooty mold. The scale insects feed on new growth by sucking the sap and exude a sticky, sugar substance called honeydew. This accumulates on the leaves and feeds the black mold. If left untreated, the insects and mold weaken your tree. If you control the insects you will stop the mold. To do this, spray the tree with pest oil every 6 weeks until the infestation clears.

Q: Some of the new leaves on my lime tree have silvery lines on them and they have become curled and distorted. What is causing this and what can I do?
A: This is caused by the citrus leaf miner insect. These minute insects feed by tunnelling their way through the leaves. They do not usually cause a problem for large trees but it is best to treat younger trees. Prune off the infected growth and then spray the tree with a pest oil every 3 weeks during summer and autumn, making sure to cover the top and underside of the leaves.

Q: Last year, my mandarin tree gave me a huge crop but this year there is hardly any fruit at all. Why might this be happening?
A: Some citrus varieties tend to bear fruit well one year and little or no fruit the following year. This is also referred to as biennial bearing habits’. Varieties such as Valencia oranges, mandarins and kumquats are alterative. The condition may be influenced by factors such as nutrition and water supply. To reinstate regular bearing of fruit, prune during the ‘on’ year when a large crop is expected and ensure good feeding and watering and you may receive a more regular harvest.

Q: There are several large lumps on my lemon tree’s branches. What is causing this and what can I do?
A: The citrus gall wasp insect has caused the swelling on your branches. It attacks all citrus, but lemons, grapefruit and the Rough lemon suffer most. The wasp is only about 2mm long and shiny black in color. It lays eggs in soft new shoots and as the hatching larvae feed, they cause the infected tissue to swell gall. In spring, adult wasps emerge from the gall. Galling reduces tree growth, and when new shoots become galled, leaf and fruit production is severely affected. Cut off the galls and destroy them by the end of winter before the wasps emerge in spring and lay eggs in the new shoots.

You will find full details about caring for your lemon trees, orange trees, lime trees, grapefruit trees, mandarin trees and other citrus trees and great advice and photos on recognising and correcting nutrient deficiencies, pest and diseases and problems in Grow Citrus: The Insiders Secrets to Growing Great Citrus

All about lemon, lime, orange & mandarin trees

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