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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Taking Care of Lemon Trees and Other Citrus Trees

Lemon tree leaf showing soil deficiencyListen to any radio gardening talk-back program particularly in the U.S. and Australia, at any time of the year, and you will undoubtedly hear a question about a lemon tree, orange tree, lime tree, mandarin tree, grapefruit tree or any other variety of citrus tree. Citrus trees, especially lemon trees, have become a national obsession in Australia and the U.S.  information on how to successfully grow and care for them is always welcome.

Taking care of citrus trees involves staking, mulching, fertilising, watering, pruning and identifying and fixing pest and disease problems as follows:

Staking
In windy exposed sites, some staking will be needed to stop your citrus tree moving excessively. Insert two stakes, one each side about 8 inches (20 cm) from the trunk. Place a flat band of material (or old hosiery) around the stake and stakes in a figure 8. This will allow some flexibility while the tree root system establishes during the first 2 to 3 years.

Mulching
Mulching the soil can add nutrients, deter weeds, conserve moisture, help to spread and retain water and protect the roots of your lemon tree.
Mulch from a couple inches beyond the canopy to a few inches from the trunk. Do not mulch right up to the trunk—give it a little room to breathe! Cover the area under the canopy with 3 to 4 inches (8 to 10 cm) of mulch. Under normal circumstances you should need to re-mulch every six months, but if your washes or blows away in stormy weather, go ahead and re-mulch right away. Do not use pine bark nuggets as they can attract ants, and do not use any inorganic material for mulch.

Fertilising
Citrus trees need high-quality compost plus a number of trace elements to meet their specific nutritional needs. Citrus trees in containers are susceptible to malnutrition even when planted in good compost, as every time you water some of the nutrients leak out, and they can only be replaced by you. You can recognise plants with poor nutrition by stunted growth, yellow leaves, and low or no fruit production. Happily, nutritional deficiencies can be easily addressed with soil amendments.

Watering
Careful watering is of the utmost importance when it comes to lemon trees. Citrus trees need lots of water. They need a minimum of 3 to 4 cm (1 – 1.5 inches) per week from spring until fall/autumn. The way to see whether you’re watering correctly is simply to use a cup, turn on a sprinkler and see how long it takes to get 3 to 4 cm of water in the cup.

Pruning
Lemon trees and other citrus trees don’t need as much pruning as other fruit trees do. While your trees are first getting established, limit pruning to just removing crossing or damaged branches. Because citrus fruits are so heavy, you’ll need to encourage strong branches that can hold them. In the spring, the main branches can be cut back to an outward-facing bud if you wish.

Pests and Diseases
One of the difficulties of growing citrus trees 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.

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 in Grow Citrus: The Insiders Secrets to Growing Great Citrus


All about lemon, lime, orange & mandarin trees

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