Citrus in the garden
Series: Agfact H2.1.7 Edition: Fourth edition Last updated: 23 Apr 2003
Apart from the convenience of having fresh fruit readily available, citrus trees make their own contribution to the home garden with their shiny green foliage, pleasant-smelling blossom and attractive fruit colour. Home-grown fresh citrus fruits are nutritious to eat, or to juice for healthy and refreshing drinks.
Citrus are considered subtropical but will grow in most areas of New South Wales, from the coast to the western inland and as far south as the Murray Valley. However, they will generally not grow on the tablelands, where severe frosts may damage the trees and fruit. The coastal areas north of Sydney are the most favourable for growth and early maturity because of their high summer and winter temperatures. Provided adequate irrigation is available, trees are tolerant of hot conditions, although exposed fruits may be sunburnt.
The cold-hardiness of fruiting citrus types varies significantly. This often influences where certain varieties or types can be grown successfully, as regular frost damage must be avoided. In summary:
- Very sensitive: citron, lime and lemon. These usually grow continuously and are susceptible to injury even by very light frosts.
- Moderately susceptible: grapefruit, pummelo and sweet oranges.
- Cold-hardy: seville, mandarin and Meyer lemon.
- Very cold-hardy: cumquat.
Of the common rootstocks, rough lemon is frost-sensitive and sweet orange moderately susceptible, while citrange is cold-hardy and Poncirus trifoliata very cold-hardy.
Where to establish
Citrus trees grow best in deep (50 cm), well-drained, sandy loam soils. They will not tolerate very acidic or alkaline conditions, preferring a soil pH in the range 6–7. The maximum amount of sunlight is desirable for the growth, setting and maturity of fruit. Avoid positions that are low and frost-prone, or exposed to strong winds, particularly sea winds.
If trees are planted in tubs they will require exposure to the sun for part of the day to maintain healthy growth. Patios and small courtyards are suitable for tub trees such as Meyer lemons, cumquats and calamondins.
Citrus belong to the family Rutaceae, which includes the genera of true citrus fruit - Citrus, Fortunella, Poncirus, Eremocitrus and Microcitrus.
A wide range of citrus types and varieties is available in Australia, and a good range can usually be planted in any climatically suitable home garden site. Some varieties that could have appeal to the home gardener and may even be considered ‘exotic’ are not grown commercially in Australia.
By having a number of varieties growing, fresh citrus can be available over an extended period. Varieties ripening in the summer and autumn period are, however, at greater risk of attack by Queensland fruit fly and some fungal diseases.
Varieties of the following citrus fruits are described below:
|Grapefruit||Other citrus varieties|
Oranges (Citrus sinensis)
An early, seedless variety that matures in May and June. Its fruit holds on the tree for several months under favourable conditions. The juice of this variety should be used quickly as it becomes bitter, even in short-term storage. In some coastal areas excessive summer fruit drop or shedding of developing fruit occurs, resulting in very poor navel crops (see the section ‘Problems’ for more information).
Matures slightly earlier than Washington. It is also seedless with a fine rind, but is not as juicy as the Washington.
This is another early-maturing variety which is very juicy, but the thin rind is prone to splitting. This navel tends to produce small fruit, and excessive preharvest drop can also be a problem. It is suitable for inland areas only.
Lane Late navel
Matures later than the other navels — it is not coloured until August. It can be very slow to reach satisfactory cropping levels in coastal climates.
Joppa, Parramatta, Homosassa, Siletta, Hamlin and Mediterranean Sweet
All these varieties are mid-season common seeded oranges that mature in July and August and have good juice content. Trees are easy to grow and cropping is regular, but fruit will only hold on the tree for a short time. All are suitable for home juicing.
A late, seeded variety that matures in September and October. Fruit will remain on the tree for 6 months under favourable conditions, but only if it is not damaged by fruit fly or black spot as may occur in coastal districts. A seedless selection is also available. Valencia is the main growing and juicing variety in the State.
Three varieties are currently available: Ruby, Maltese and Harvard. Although popular in some Mediterranean countries owing to their distinctive flavour and appearance, they have only a limited appeal in Australia. The amount of rind and flesh pigmentation, particularly under mild coastal growing conditions, is usually disappointing. The hot, dry inland areas are more dependable for pigmentation development.
Lemons (Citrus limon)
This is a suitable variety for the coastal home garden. It bears several crops of fruit — in winter, spring and summer. The tree has few thorns and grows vigorously. Eureka is not compatible with P. trifoliata or citrange rootstocks and is normally propagated on rough lemon or the newer rootstock Benton citrange.
A vigorous, thorny variety that is more cold-tolerant than Eureka and more suited to cooler growing situations. The main crop matures in autumn/winter, but light crops ripen in spring in coastal areas. Selected strains are compatible with P. trifoliata and citrange rootstock.
A hybrid that is more cold-tolerant than other lemon varieties. The fruit, which is produced throughout the year and is almost orange in colour, has a high juice content and a mild, low-acid flavour.
An Australian seedling originally located in Queensland. A vigorous, heavy-cropping tree with fruit similar in appearance to a lemon but with lower acid and a distinctive ‘lemonade’ flavour.
Mandarins (Citrus reticulata)
A large number of mandarin and related hybrid varieties (tangor and tangelos) are available. Some develop rind colour early, but each variety is mature and retains its fruit quality for a relatively short period (5–6 weeks). Compared with most other citrus types, some varieties are hard to pick and require clipping to reduce rind damage at harvesting. The following popular varieties are listed in their approximate order of maturation, from very early (April) to very late (September).
The Silverhill selection is a very early variety and the first to mature (April and May). The tree is very cold-hardy and small. Fruit is seedless, medium-sized, juicy and has bright orange flesh. Also known as C. unshiu.
An early variety that matures in May. The fruit has an excellent flavour, but tends to be small when heavy crops are produced. Thinning fruitlets will improve fruit size.
The Algerian selection is widely grown in some Mediterranean countries. It is early-maturing, commencing in mid-May. Fruit is well coloured (red-orange), medium to small in size, sweet and juicy, but seedy.
A mid-season variety that matures between June and August; a late-maturing selection is also available. It has good flavour and is easily peeled. The fruit is subject to brown spot in coastal areas, and the trees tend to overcrop, resulting in smaller fruit size and tree decline.
Also a mid-season variety. Fruit is small, sweet and juicy with good flavour, but seedy.
A mid-season hybrid, maturing in early July. The fruit has a distinct neck and thin bright reddish-orange rind, which is difficult to peel. Fruit is also subject to brown spot when grown in coastal areas.
A late variety, maturing in August in the warmer districts but sometimes harvested as late as October in cooler areas. It is not a true mandarin but a natural tangor of good eating quality.
An attractive late hybrid mandarin with reddish-orange rind. It is very juicy with a distinctive flavour.
This is a good quality, very late maturing variety (end of September).
Grapefruit (Citrus paradisi)
Grapefruit require hot, dry growing conditions to ensure good flavour and quality.
A nearly seedless variety with straw-coloured flesh, it can be harvested from July but is sweeter if left until October in the Sydney area. A natural physiological preharvest fruit drop is a common problem and can occur just as the fruit is colouring or before it is fully mature (see the section ‘Problems’ for more details).
A natural hybrid with light, straw-coloured flesh, a distinct lemon flavour and many seeds. It grows vigorously and produces large fruit but tends to have the problem of alternate cropping.
Of the pink-fleshed varieties, Thompson is similar to Marsh but has a faint red-pink flesh. Ruby (shown at right) and Red Blush have an orange-pink flesh and some colour on the skin under favourable hot and dry inland climatic conditions.
A hybrid with large, pale orange rind and flesh. This variety is more suitable for growing in colder districts than other true grapefruit varieties.
A newer imported variety that is apparently a natural hybrid of grapefruit and orange. Fruit size and appearance is similar to grapefruit but with an orange flesh that is juicy and sweet, lacking the acid and bitterness of normal grapefruit.
Limes (Citrus aurantifolia)
Limes are suitable for growing only in frost-free sites. They do well in the more tropical areas.
Tahiti (or Persian)
A variety similar in size and shape to a small lemon, but the flesh is seedless and pale green in colour. It should be harvested while the rind is still green, or it may develop stylar end rot and drop from the tree. The juice is not as acidic or aromatic as that of the Mexican lime, which is preferred for fresh juice drinks. The trees are upright, of medium size and vigour. This variety has good tolerance of tristeza virus disease.
Mexican (or West Indian)
Fruit from this variety is very acidic with a strong lime aroma and flavour, but is smaller than that of Tahiti. The trees are also bushier and not as vigorous. They are very likely to decline at a young age from tristeza virus disease, which is spread by black citrus aphids. The tree is also attacked by the melanose fungus.
Sweet limes or sweet lemons (C. limettioides)
These are widely grown in India, where they are considered to have medicinal value. They have an insipid taste because the fruit lacks acidity, and they are not grown commercially. The fruit is medium-sized with a smooth, thin rind which is greenish to orange-yellow. It has good juice content and few seeds.
An attractive hybrid mandarin–lime variety with orange-coloured fruit which is very acidic and suitable only for drinks.
Calamondins (Citrus madurensis)
This fruit has frequently been confused with the round cumquat, and many home gardeners do not know it by its correct name — both cumquats and calamondins are attractive ornamental trees and produce distinctive small fruit which have a wide range of uses.
Although resembling a cumquat, the calamondin is similar to a very small mandarin and has a loose, bright orange skin when ripe. The tree blossoms in the spring at the same time as mandarin trees, whereas cumquats flower in early summer. Variegated calamondin leaves and fruit are shown at right.
The calamondin has been used mainly as an ornamental tree in the garden or in a tub, but the fruit, although it lacks sweet flesh and skin taste, can be made into a tasty marmalade.
Cumquats (Fortunella species)
Nurseries have often confused round cumquats with the calamondin. The round cumquat resembles Nagami but is more rounded and has more segments and a sweeter flesh and rind flavour. The round cumquat available in the Sydney area is thought to be the Meiwa selection. As with the oval cumquat, blossoming is delayed until after November because of the tree’s dormant habit, and this makes it particularly well suited to colder climates with late frosts.
A variegated (light green and pale yellow) foliaged cumquat selection is also available.
Other citrus varieties
Fruit from this variety is bitter and not pleasant to eat fresh but is sometimes used for candying or crystallising. The tree has unusually shiny small leaves on thornless branches, and with its large, orange fruit (shown at right) it makes an attractive tub specimen.
Seville (Citrus aurantium)
This variety has a very bitter, orange-coloured fruit known as ‘sour orange’. It is used commercially for marmalade. Both smooth-skinned and rough-skinned selections are available.
Pummelo (Citrus maxima)
Indigenous to the Malayan peninsula, this variety was carried to other countries by traders. It is also known as a ‘shaddock’. It has very large, yellow-green fruit with coarse pink or white flesh and a very thick white rind, which makes it suitable for marmalade. It can be used as a substitute for grapefruit. The local selections are highly variable and have little use in the home garden, except for the novelty value of the very large fruit.
Citron (Citrus medica)
This is one of the oldest citrus species known and grown. It has special religious significance. The fruit is large, oblong, thick-skinned and very aromatic, but it is bitter and virtually non-edible; however, it can be used for marmalade or the peel can be candied. The tree is small, weak-growing and short-lived because it is very sensitive to wind, frost, heat and the common tristeza (‘quick decline’) virus disease.
This variety is an exotic type with an unusually large-winged petiole leaf that is dried and used widely as a seasoning in cooking, particularly in South-East Asia. The fruit is very bumpy, full of seed and with very little juice. The leaves from the Kaffir lime tree are shown at right.
Citrus trees grown from seed are normally very variable and do not produce true-to-type trees or fruits. Common varieties, available from local nurseries, are normally propagated by budding the required scion or variety onto specially grown and suitable rootstocks (understocks).
The most commonly used rootstocks for citrus are P. trifoliata and rough lemon, but Troyer, Carrizo and Benton citrange, Cleopatra and Emperor mandarin, and sweet orange are also used to propagate trees in some localities. Different rootstocks will adapt citrus to various soil conditions, such as light (sandy), heavy (clay), alkaline, saline, or soil containing citrus nematodes.
P. trifoliata and the citrange rootstocks have resistance to phytophthora root rot and are used in heavier soil types where drainage problems are expected or where trees are planted to replace others that have been removed. They may be used for all varieties except Eureka lemon. Benton citrange, however, can be used for this variety.
Rough lemon is very susceptible to phytophthora root rot but is suitable for all varieties except Ellendale and Imperial mandarins.
Citrus nursery trees are normally sold in containers, although some may be available as bare-rooted trees. Prepare the site by digging over several months before planting. Add organic matter and remove troublesome weeds.
The best time for planting a citrus tree is when the risk of severe frost is past, usually in August or September. Advanced trees that have been grown in containers may be planted during spring, summer or early autumn provided there is little disturbance to the main root system and they are watered frequently. Where summers are hot and dry, and winters are mild, trees may also be planted in autumn.
When planting any citrus tree, prepare the site well beforehand. Dig the planting hole deep enough to allow the tree to be placed at the original nursery or container soil level, with the bud union at least 15 cm above the ground level, and wide enough for the roots to spread out. Remove the tree from the container and gently tease the external roots to free them for planting (or for removal if they have spiralled in the container).
Do not put artificial fertilisers in the planting hole. If the tree is planted too deep or allowed to settle below ground level, the value of the rootstock will be lost. Make sure the soil is pressed firmly around the tree’s base, and form a basin around the tree to make watering easier. Pour about 10 L of water slowly into the basin to consolidate the tree and provide a reserve of moisture. Rewater several times a week until new shoots appear.
In the first year of growth in the garden, water the tree at intervals of 4 - 7 days, depending on the weather. Regular watering is essential during hot, dry, windy periods.
When the tree is well established, watering may be limited to a good soaking with a hose and sprinkler every week. Tub-planted trees will need to be watered more frequently, depending on the amount of soil and the growing position.
The root system of citrus trees is usually concentrated in the top 30 cm of soil and within the ‘drip ring’. If a mulch has formed or is maintained under the canopy, many of the important fibre roots will be found within 10 cm of the surface. Take care when digging around the tree not to damage larger roots, as an injury may result in invasion by wood-rotting phytophthora fungi. Heaping grass clippings or other material around the base or trunk of the tree can result in similar problems.
Remove weeds by light chipping or mowing until the canopy develops and reduces light under the tree, thus inhibiting weed growth. Problem weeds such as couch, kikuyu or summer grass may also be controlled by the careful use of registered herbicides. Some that are safe to apply near citrus may be injurious to other nearby fruit trees, shrubs or plants in the garden.
Trees planted in tubs may require repotting into larger containers after several years, or root pruning if the tree is to be kept small. Artificial fertilisers may damage the roots and burn foliage, so it is safer to use blood and bone or slow-release, mixed fertilisers (pellets or tablets) for these trees. (See the section Nutrition for more information.)
A light pruning of foliage may be necessary at planting if the root system has been badly damaged or reduced to fit in a tub. If the tree was grown in a shallow container it may be necessary to cut the tap root to remove spiralled growth before planting.
Apart from removing suckers and shoots from the rootstock and trunk, there is no need to prune the tree until the dead wood has to be removed. The citrus tree is self-shaping but in time the shaded inside twigs and limbs should be removed to open the centre and reduce the risk of fungal diseases like melanose.
- Cut out all dead or diseased wood cleanly; don’t leave stubs.
- Remove crowded or crossed-over branches.
- Paint cuts that are larger than 2 cm across with a bituminous-type wound dressing.
- Prune in the spring when the danger of frosts is over.
The lower growth (up to 30 cm above ground level) can also be removed if necessary to assist in the tree’s soil management. This pruning is called ‘skirting’.
If individual branches impede access or are crowding other trees, fences or paths, they can be removed completely or cut back to any convenient position. Citrus trees readily produce new growth from near major pruning cuts.
Orange and lemon trees that have lost vigour but have a sound root system and trunk may be rejuvenated by a skeleton pruning, that is, a severe cutting back in the spring. This can rejuvenate some citrus trees into completely new growth. Remove all the leaves and twigs cleanly, and cut all the main branches that form the framework at 2 - 3cm diameter so that only the tree’s skeleton remains. Seal the main cuts as described earlier and whitewash the limbs and trunk to protect against sunburn.
Most garden soils require some annual fertilisers for good tree development, fruit size and regular cropping. Citrus trees, being evergreen, have two main growth flushes (spring and autumn) and therefore require more than most deciduous trees or vines.
The main nutrition requirement is nitrogen to promote growth and fruit size. Phosphorus is also usually lacking, and, if the soil is a light sand, regular additions of potassium are needed to maintain tree health and fruit quality. Magnesium and calcium may be needed in some soils, together with small amounts of the trace elements zinc, manganese and copper.
For more detailed information, see Agfact h3.3.11 Citrus nutrition.
Type and amount of fertiliser
The main fertiliser needs of citrus trees can be applied as organic/animal manures or artificial/chemical fertilisers.
For non-bearing trees, organic manures can be convenient and supply adequate plant elements and organic matter to the soil. Provided these manures are not too fresh, they may be applied safely to young trees in the autumn or winter. As a guide, apply 4–8 kg for 1-year-old trees, increasing annually up to 16–32 kg by the eighth year.
As bearing trees would require large amounts of organic manures (20–40 kg per tree), they are instead usually given a complete ready-mixed citrus fertiliser such as 10:4:6 or 10:3:6* at the rate of 500 g per year of tree age until the tree is 10 years old, then at the rate of 5 kg per year applied in the later winter. Give high-producing trees a supplementary application of nitrogen (700 g ammonium sulfate) in November.
*These ratios represent the proportions in which the constituent elements are present in artificial fertilisers:
nitrogen : phosphorus : potassium (N:P:K)
Mixed fertilisers can also be used for non-bearing trees, at the above rate. When used for non-bearing trees, three-fifths of the annual amount can be applied in the late winter and two-fifths in the summer. For very light soils a ‘little and often’ program in the growing period can be followed, particularly for non-bearing trees.
Application of fertiliser
Most citrus trees blossom once per year in the spring, and the applied nitrogen should be available at that time.
Whatever type of fertiliser is used, spread the required amount very evenly around the tree, avoiding the trunk, and water after each application. This spreading should be 0.5 to 1 m wide for young trees, and to the drip ring or foliage edge for bearing trees.
Correction of low soil pH to 6 - 7.5 with dolomite, magnesite or agricultural lime will improve the tree health generally and ensure that other fertilisers applied become available to the tree.
Deficiencies of nitrogen, magnesium, zinc and manganese are fairly common in citrus trees and can normally be corrected easily, but the correct identification of any nutrient deficiency is important. (See the later section on Common nutrient disorders.)
For all varieties except limes, fruit should be left on the tree until they have developed full colour and flavour. Unlike many other fruits, mature citrus can be held on the tree for some time, but quality will eventually deteriorate and pest and disease problems may increase losses.
Citrus fruit should be handled carefully and picked by a twisting–pulling action which breaks the stalk but does not damage the button or the fruit. Do not pick cold or wet fruit, as shelf life can be severely reduced.
Surplus fruit can be stored for several weeks in the crisper of the refrigerator. Lemons can also be cured and stored for long periods:
- clip dry fruit at first colour change from dark green to light green (silver);
- handle carefully;
- store in paper-lined or dry sand-filled boxes in a cool, dark place away from draughts but with good ventilation.
Summer fruit drop
Citrus normally shed large numbers of fruitlets shortly after blossoming in the spring and at early fruit set (pea size). However, it is also common for a sudden drop of small fruit (20 mm diameter) to take place in the summer, when warmer weather places stress on the tree. The problem is particularly severe in young navel orange trees and may be related to lack of water at, or soon after, fruit set. Root diseases and lack of nitrogen or trace elements may also be responsible.
Rind splitting of fruit, particularly in navels, also occurs before or near maturity as a result of climatic factors, specifically a drop in average temperatures and an increase in relative humidity at a time when the rate of fruit growth is decreasing. There is no control for the disorder.
Preharvest drop in the autumn before fruit is fully mature is a common problem with navels and grapefruit. Some of this drop is natural, but in coastal areas fruit stung by Queensland fruit fly during the late autumn colour-change period is also very prone to drop. Some mandarin and lemon varieties will also shed fruit when damaged by the spined citrus bug. Brown spot infection in mandarins will result in fruit drop.
Alternate cropping (biennial bearing)
Alternate cropping (biennial bearing) is a common problem with many citrus varieties, such as Valencia orange, Wheeny grapefruit, and mandarins. After a heavy crop the tree often responds by carrying a light (or nil) crop. Once this cropping pattern is established it is difficult to return to regular annual cropping. Pruning or thinning of the heavy crop and early harvesting will assist in reducing the problem.
Second crop fruit
Sometimes orange trees will produce blossoms in the autumn after a stress period, and set a second crop. This fruit is often of poor quality (thick skins and low juice content) and is susceptible to fruit fly attack. With lemon trees, however, these intermediate crops are desirable.
Pests, diseases and nutrient disorders
Pesticide residues may occur in animals treated with pesticides, or fed any crop product, including crop waste, that has been sprayed with pesticides.
It is the responsibility of the person applying a pesticide to do all things necessary to avoid spray drift onto adjoining land or waterways.
ALWAYS READ THE LABEL
Users of agricultural (or veterinary) chemical products must always read the label and any Permit before using the product, and strictly comply with the directions on the label and the conditions of any Permit. Users are not absolved from compliance with the directions on the label or the conditions of the Permit by reason of any statement made or not made in this publication.
A wide range of pests and diseases can affect both the fruit and the tree. Citrus can also suffer a number of nutrient disorders. Most of these problems can be resolved where required by spraying with registered and recommended pesticides.
Identify insect pests correctly and do not spray unnecessarily. Small numbers of pests may not cause any significant damage or crop losses in the home garden. Some can be picked off the tree and be destroyed without the need for any spraying. Many pesticides are also toxic to bees and other beneficial insects that give natural control of some pests on citrus trees.
With most diseases, infection takes place before visual symptoms appear, so routine protective sprays are normally needed. In coastal districts, diseases can be more common and damaging owing to the more favourable (wetter and more humid) seasonal conditions. In the drier and hotter inland areas, fewer citrus disease problems occur.
Nutrient disorders are corrected by fertiliser or trace element applications, but these can be harmful if used incorrectly or unnecessarily.
See also Citrus pests, diseases and disorders.
Scale insects, Queensland fruit fly, bronze orange bug, spined citrus bug, mites, citrus leaf miner, aphids and citrus butterfly caterpillars are common pests of citrus.
There are several types of scale insect:
- White wax scale and black scale mainly affect the tree by producing honeydew, on which sooty mould develops.
- Citrus red scale blemishes fruit and can harm tree health.
- White louse scale, which is mainly on the trunk and larger limbs, can also seriously impair tree health.
Queensland fruit fly occurs in coastal districts and the central and northern inland districts of New South Wales. It can attack citrus from September to May, depending on the fruit variety. Cover-spray treatments reduce fruit losses, but do not leave fruit on the tree beyond full maturity.
In coastal districts and in the central west and north-western districts, watch for bronze orange bug and spined citrus bug, which reduce fruit production. Bronze orange bug is a native pest of coastal citrus. In spring and summer the bugs suck sap from young shoots, leaves and flowers, or fruit stalks, causing them to fall.
New growth can be damaged by the citrus leaf miner in Central Coast and North Coast districts, and black citrus aphid and the caterpillar of the citrus butterfly (Museum Victoria images) in all areas (the citrus butterfly is also known as the ‘small citrus butterfly’ or the ‘dingy swallowtail butterfly’).
Melanose, citrus scab, black spot, brown spot and brown rot are common coastal fungal diseases which can be reduced by spraying with recommended fungicides. Fungal spores can live on dead twigs and wood. As trees age, the amount of dead wood increases as does the level of disease. In order to reduce disease problems, keep your trees clean by pruning out dead twigs and wood annually.
Melanose occurs in coastal areas and produces dark brown spots (or tear stains) on the fruit, leaves and twigs. Infection is reduced by spraying with protectant copper fungicides after petals fall.
Citrus scab occurs in coastal areas and appears as scabby areas mainly on lemon fruit and leaves. Infection is reduced by spraying with recommended fungicides at half petal fall.
Black spot occurs in coastal areas and appears as dark spots on mature citrus, particularly Valencia oranges. Infection is largely prevented by spraying with recommended fungicides at petal fall. A second spray is required 16 weeks later. Harvesting fruit early in the season reduces black spot problems.
Brown spot fungus disease affects mainly mandarins, especially the Emperor and Minneola varieties. Both leaves and fruit can be infected, particularly in cool damp weather during early spring to early autumn. Up to four sprays may be necessary to control heavy infections under favourable conditions.
Brown rot occurs on fruit, usually under wet, humid conditions, and is prevented with a protectant fungicide spray applied in March or April. This is also useful in preventing sooty blotch and sooty mould.
Trunk or collar rot and root rots caused by the fungus Phytophthora citrophthora occur in poorly drained soils and may cause decline or death. The first symptom of trunk or collar rot is death of a section of bark on the trunk near ground level. This is often accompanied by production of gum, and cracking as the dead bark dries out. Trunk rots, if not too extensive, can be treated by removing affected bark and then painting with a recommended fungicide paste. Lemons are very susceptible to this disease and tree losses are common.
Common nutrient disorders
Nutrient disorders are most commonly deficiencies of nitrogen, magnesium, manganese or zinc.
Nitrogen is the most important nutrient for citrus, and for most soils it is needed as the major component in fertiliser or manure applied regularly around the tree. Nitrogen-deficient trees are recognised by small, uniformly pale leaves; they are thinly foliaged and stunted, with reduced flowering and fruit set.
Magnesium deficiency (occurring most commonly in coastal districts), zinc deficiency (occurring most severely in inland districts), and manganese deficiency (occurring in both coastal and inland areas) can all adversely affect tree growth and cropping. They can be recognised by distinctive colour patterns in the foliage.