Intro

Care of Young Tree

Care of Established Tree

watering
fertilizing
pruning
weed control
freeze protection

Bloom and Fruit Set

Citrus Diseases

Trunk Diseases

Foot rot
Gummosis
Heart rot

Diseases of Fruit & Foliage

Melanose
Greasy Spot

Citrus Pests

Citrus Rust Mite
Texas Citrus Mite
Citrus Leafminer
Citrus Blackfly
Brown Soft Scale
Armored Scale
Aphids, Mealybugs, and Whitefly

Pest Control

THE NEWLY PURCHASED CITRUS TREE       

A commercially grown citrus tree consists of the rootstock, which is the roots and lower trunk, and the scion of fruiting wood, which is grafted to the rootstock. Citrus rootstocks are selected on the basis of adaptability to soil conditions and resistance to soil-borne disease organisms. Sour orange is the principal rootstock in the Lower Rio Grande Valley (LRGV).

In producing a citrus tree, seeds of the rootstock variety are first planted. When these seedlings are about pencil thickness, buds of the desired scions ( grapefruit, sweet orange, tangerine...) are grafted to them. With proper care these buds sprout and grow into healthy, vigorous shoots. The top of the rootstock is later cut off just above the scion shoot. The point where rootstock and scion meet is called the budunion.

If the rootstock is to serve its purpose, the budunion should be about 6 inches above the soil level. locally, citrus trees are sold balled and burlapped, or as container-grown stock. the planting hole should be dug so that the top of the soil mass that came with the tree is level with the soil surface. Irrigation immediately next two irrigations are critical especially if the tree is planted in May-June (summers are hot and dry usually).

CARE OF THE YOUNG CITRUS TREE

During the first couple of years after planting, it is most important to provide adequate water to the young tree. A circular area 4 to 6 feet in diameter immediately under the tree should be kept free of grasses and weeds. A ring of soil can be hoed-up around the tree and fill with water as needed. In the winter, watering once a month may be adequate, but in the summer more frequent watering is needed. In the first year this may be up to 25 gallons a week; in the second, up to 50 gallons a week.

The newly planted citrus tree will not benefit from any special fertilizer treatments, starter solutions or pellets. A one-year-old tree needs about 2 oz of nitrogen, a two-year-old tree about 4 oz. These amounts refer to actual nitrogen, not simply to nitrogen fertilizer. For example, ammonium sulfate contains 20% nitrogen, therefore, 10 oz must be applied to supply 2 oz of actual nitrogen. Soils of the LRGV are, in general, quite fertile. A recommendation is made only for nitrogen.

CARE OF THE ESTABLISHED CITRUS TREE

The majority of the citrus roots are in the top 18 inches of soil and extend out twice the diameter of the tree canopy.

Watering

With yard plantings, there is considerable competition for moisture between grass roots and the tree roots. A proper irrigation should thoroughly wet the soil to a depth of 24 inches. Sprinkling often results in frequent light waterings which benefit the grass at the expense of the tree. A slender metal rod, pointed at one end, can be used to determine the depth of moisture penetration following irrigation. The rod will penetrate the wetted soil easily but stop when dry soil is reached.

Fertilizing

Nitrogen fertilizers that are applied to the lawn will usually take care of the established citrus tree. Different forms of nitrogen (ammonium sulfate, ammonium nitrate, etc.) may be used, but the amount of material will vary because of the different percentage of nitrogen each contains. it is normally not necessary to apply other plant nutrients. To maintain healthy grass and trees, apply about 10-15 lb. of nitrogen to 4000 sq. ft. For example, 50 lb. of ammonium sulfate, which contains 10 lb. of nitrogen, might be an adequate annual application for an average city lot. On a single tree basis, apply about 1-1 ½ lb. of nitrogen to each mature tree. 

Timing of the fertilizer application is not critical. The entire supply for the year may be applied at one time (Nov., Dec. or Jan.), or split it into 2 or more applications a year. Application just prior to irrigation is advisable.         

Pruning

It is not necessary to prune citrus trees to stimulate fruit production. Pruning should be carried out only to remove damaged or dead branches, to remove branches that are in the owner's way, or to shape the tree to a more attractive form.

When pruning limbs an inch or more in diameter, avoid leaving stubs as these will not heal over and will be an entry way for disease organism. Cut branches back to the next larger limb and make a smooth cut. After the cut is made, any ragged edges should be smoothed with a sharp knife and the wound painted with a good Latex pruning compound. Small twigs that are clipped to shape the tree do not need to be painted.

Cultivation and Weed Control

It is not necessary to work the soil to obtain good tree growth. In fact, disturbing the soil can damage shallow roots and thus be detrimental to tree growth. On the other hand, heavy weed growth under or around the tree should be avoided. Where grass is allowed to grow, it should be mowed frequently allowing the clippings to fall to the ground.

Citrus trees tolerate may different herbicides and their use is practical for keeping down weeds. Check the label to see if a particular herbicide is registered for citrus use and the rate at which it can be applied.

Freeze Protection

Citrus trees can tolerate a few hours of frost or freezing; however, temperatures from 26 to 22° F for 4 to 8 hours will damage tree foliage and wood. Freeze injury can range frpom a few nipped leaves to death of the tree, depending on the minimum temperature, its duration, and the variety, age and dormancy condition of the trees. Dormancy, induced by continuous cool temperatures, is usally greatest in january. Kumquat and calamondin trees are most cold hardy, followed by tangerines, oranges, grapefruit, lemons and limes in rank of hardiness. Weedy trees freeze easier.

Small trees may be protected from freezing by placing a covering over them and putting a lighted electric bulb underneath for a source of heat. Covering must reach all the way to the ground. If you black plastic coverings that are in contact with the foliage should be removed when the sun comes out and temperatures warm up, otherwise the leaves may be scorched. Use of clear plastic for covering foliage canbe left on the tree or taken off when the sun comes out.

Wrapping with insulting material or mounding soil around the trunk 15-18" up from the ground will prevent damage to that part of the tree. If the top  is frozen, the wrapping or soil may bre removed and soon new spouts will grow to replace the frozen top.

Large citrus trees are more cold hardy than young trees and usually require temperatures below 22°F for several hours before substantial damage occurs to the major branches or trunk.

FLOWERING AND FRUIT SET

Citrus trees first bloom when two or three years old. Heavy crops of oranges and grapefruit may not be produced until the trees are four or five years old. proper care throughout the growing season is necessary in order to have a good bloom and fruit crop the next year.

Oranges, grapefruit, and tangerines bloom only once a year and then 93-97% of the blossoms or young fruit will fall from the tree without setting fruit. The 3-7% remaining are usually enough to make a good crop.  Navel oranges, while often desired by the homeowner, have a reputation for setting vvery light crops. Lemons, limes, calamondins and kumquats usually bloom several times a year, but the main bloom is in the spring.

Sometimes a heavy bloom is followed by a poor fruit set. the fruitlets may fall from the tree even after they are as large as a nickel. Weather, tree condition, or other unknown factors can be responsible for a heavier than normal fruit drop.

The mature size of citrus trees should be considered when planning the home landscape. Mature tree is being used here for fertilizer and care purposes only not for production rate. Although growth rates differ between varieties, and on different soil types, citrus tree canopies can increase in size from ½ to 1½ ft a year. When grown under good care and soil conditions, the home owner can expert the mature tree diameter and height, respectively, for the varieties to be: grapefruit — 12 and 10 ft; oranges and tangerines — 18 and 16 ft; lemons — 13 and 10 ft, and limes — 13 and 13 ft. Closer spacings, even hedges, can be achieved by pruning.

CITRUS DISEASES

Trunk Diseases

Foot rot is a disease caused by a soil-borne water mold called Phytophthora which frequently affects young citrus trees. Affected trees often have profuse gumming at or near the budunion of the tree. The bark is usually split open, revealing the discolored and diseased cambium. To find the full extent of the infection, the bark must usually be removed to expose the lesion. The discolored tissue does not extend into the wood of the tree since Phytophthora lacks the ability to decay internal wood. This characteristic is diagnostic for foot rot and separates this disease from internal decays caused by other fungi. The tree may eventually be girdled and movement of water and minerals between roots and leaves stops and the tree declines. The sour orange rootstock is tolerant to the disease so that damage is usually confined to the area above the budunion. The budunion needs to be at least 6 inches above the soil line to avoid foot root problems. If foot rot is already present, scrape away the dead bark and gum to the living tissue before applying systemic fungicide. The systemic fungicide usually used is Ridomil 2E^® mixing one gallon of Ridomil with three gallons of water and spraying the entire trunk of the tree. Ridomil 2E should not be applied to bare roots.

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Rio Grande gummosis - The cause is not known. Infection may occur through uninjured bark or through wounds. The first symptom is the appearance of a small drop of gum on the surface of the bark, and the bark may split slightly. The young lesion appears to heal and a thin layer of barks sloughs, leaving a buff colored scar. Later in the year, or perhaps the following year, gum again appears and the cycle is repeated until in time the wood is exposed. In older infections the disease penetrates deep into the wood, which will be found to be buff colored, with salmon colored bands. Gum collects in lens-shaped pockets in the wood. The gum pockets may occur a considerable distance from the nearest bark lesion. Diseased branches can be removed, but little can be done once the decay has entered the principle branches and trunk. 

Heart rot is caused by wood-rotting fungi. These fungi invade living trees through wounds made by pruning, cultivation equipment, growth cracks, wind splits at crotches, frost damage, bark diseases. Trees affected by heart rot are often characterized by die-back of the upper branches. A decline of either the whole tree or a part of it becuase of wood decay interfers with the movement of sap and the utilization of food reserves stored in the trunk. Decayed trunks and main branches, which are often hollow, do not have the mechanical strenght of healthy wood and are easily shattered by winds. Upon exposure the decayed wood is seen to be discolored, brittle, soft, and punky.

Heart rot is readily distinguishable from other tree declines by the hollow trunk and the formation of fungus structures on the surface of the trunk and branches. When the rot breaks through to the surface, the bark over the diseased area dies, cracks and a depressed canker develops, usually at a crotch or around a pruning wound.

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Diseases of Fruit and Foliage

Melanose is a fungus disease which produces raised, brick-red spots on the fruit, leaves, and twigs. Small spots may occur separately or may coalesce to form a tear-streaking or mudcake pattern on the fruit when the attack is severe. Infection usually occurs during rainy periods in the spring and early summer. Grapefruit and kumquats are most severely damaged, but oranges and tangerines are also affected. An application of Cupric Hydroxide in late April or early May will provide control of the disease.

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Greasy Spot is a fungus disease which causes premature leaf drop and a strippling or blotch on the fruit rind. The most common symptoms are oily brown to black spots on the underside of the leaves. In the early stages each spot is marked by a small, transluent yellow dot. the spots later become swollen and darkened and resemble dirty spots of grease. Because disease developement is slow, sysmptoms are usually first noticed on older leaves. Severely affected leaves fall prematurely, giving the tree a sparse apperarance especially in late winter. An application of Cupric Hydroxide in August will provide control of the disease.

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Citrus Pests

Citrus trees are subject to attack by many different insects and mites. Control of these pests is important to maintain the health and beauty of the tree and to produce attractive, good-tasting fruit. While many of these pests are difficult to identify, there are often tell-tale signs or symptoms that indicate their presence in the tree. For example, some citrus insects secrete a sticky "honeydew" that serves as a medium for fungal growth. Foliage and fruit become blackened with growth of this sooty mold fungus. Following are descriptions of some of the more important citrus pests:

Citrus rust mite — A mite, so small that it can be seen only when magified 10 to 20 times, causes severe damage to citrus. Citrus rust mite feeding injury on immature fruit gives the peel a silvered and rough sandpaper texture, called "sharkskinning." Affected fruit usually doesn't develop to normal size. Late season mite infestations on mature fruit cause the peel to turn dark brown or bronze. This mite is a most important pest on citrus in the Lower Rio Grande Valley.

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Texas citrus mite — Spider like in appearance, this mite has a brownish-green oval body with four long slender legs. It can be seen without the aid of a hand lens and is usually found feeding on the upper surface of the leaves. Leaves become silvered or gray appearing. Heavy infestations can result in serious defoliation of citrus trees. Little or no damage is done to the fruit by this mite.

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Citrus leafminer — The adult leafminer is a small moth about 2 mm long with a wingspan of about 4 mm. Moths have silvery and white iridescent forewings with brown and white markings, and a distinct black spot on each wing tip. The adult are most active from dusk to early morning; they spend the daytime mostly resting on the undersides of leaves. Adults feed on nectar and live from 2 to 12 days. Leafminer larvae form mines predominantly in leaves, but also in succulent stems and sometimes fruit. The larvae bore through the leaf epidermis, ingesting the sap and producing a chlorotic leaf patch. Leafminer may prevent young leaves from expanding, causing them to remain twisted and curled. After the leafminer have finished feeding, other insects such as aphids and mealybugs often continue feeding on the damaged area.

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Citrus Blackfly— Blackfly adults have slate-blue wings with a median white band. The abdomen and head are bright red with white legs and antennae. The female begins egg laying 3-4 days after emergence from the pupae. Completion of a blackfly generation (egg to adult) requires about 60 days at 90°F; 75 days at 80°F; and 120 days at 70°F.

The blackfly feeds and develops on the leaf undersurface of all citrus varieties with a slight preference toward lemons. Other plant hosts for blackfly include: amngo, avocado, coffee, pesar, plum, pomegrante, guava, and ash. Blackfly nymphs extract sap from leaf tissues with their sucking mouthparts and secrete copioius amounts of honeydew. The honeydew serves as a growht media for sooty mold fungus that blackens leaves and fruit. Sooty mold fungus interferes with the tree's normal photsynthetic food production process. The combined effects on the tree of blackfly feeding and the associated sooty mold can result in serious fruit yield reductions.

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Brown soft scale — A common insect pest of citrus in midsummer and early fall, brown soft scale is flat, oval, brown in color and resembles somewhat in shape and appearance the back of a turtle. Soft scales inhabit leaves, twigs and fruit, and excrete large amounts of honeydew. Whole trees can be blackened with sooty mold growing on the honeydew from infestations of soft scale.

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Armored scale— California red scale and chaff scale are the most common armored scales attacking Texas citrus. They are difficult to kill because of the protective waxy coating or "armor" covering the insect's body. Armor of the California red scale is circular, rusty red in color, with a distinct peak or nipple at its center. Chaff scale armor is irregular in shape, brown in color, without a nipple.  Chaff scale may be particularly abundent under the calyx (button) of the fruit and in pits of the rind.Citrus spray oil is effective against armored scale by shutting off the sir supply and causing the scale to suffocate.

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Aphids, Mealybugs, and Whiteflies— Aphids usually attack in the spring when citrus trees are "flushing" or putting on new foliage. The spirea aphid is the most commonly occurring species. It is apple green, almost identical in color to the new tender citrus leaf. Aphids suck sap from the leaves and cause them to become rolled and twisted.

The citrus mealybug is a problem on trees in mid and late summer. They have soft, oval, flattened and distinctly segmented bodies. Mealybugs cover not only themselves with a white mealy wax but also foliage and fruit. Mealybugs also secrete honeydew. The resulting buildup of sooty mold, together with the mealywax, can make a fruit very unappealing.

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Whiteflies are found principally on the foliage, from which they suck the sap. Adults are small, winged, white insects that readily take to flight when foliage is disturbed. Immature whiteflies are oval, translucent, very thin and inhabit the underside of leaves. Both diazinon and oil are effective against whitefly, especially the immature or nymphal stages.

Citrus Pest Control Around the Home

Application of chemical sprays on a regularly scheduled basis will usually provide control without the need of frequent tree inspections for pest detection. Spray should be applied at the following times: Spring(after petal fall) (April 1- May 1); summer (June 1 - July 1); late summer - fall (August 15 - October 15).

The hand-pressurized sprayer or various garden hose-attached sprayers work well for home citrus plantings. If a hand sprayer is used, the recommended pesticides are measured into one to three gallons of water, depending on tank size, anad pressurized for spraying by a hand air pump. With a hose-connected sprayer, the pesticides are measured into the glass or metal spray container and the correct dilution is obtained as the water from the hose withdraws the materials from the container. Most containers hold about one quart of pesticide concentrate and will produce 15 to 20 gallons of diluted spray.

When spraying citrus trees, thorough coverage is esstential for good pest control. A mature tree requires about 10 gallons of spray. The pesticides listed are relatively non-toxic to warm blooded animals but can poison fish. DO NOT SPRAY AROUND FISH PONDS.

There are products on the market under different trade names which contain one or more of the following materials: malathion, sevin, kelthane, and diazinon. These products can give economical and adequate pest control oon citrus if used on a regularly scheduled basis. They are recommended particulary when only one or two trees are to be sprayed. Check for these products at your home garden center.

Materials Recommended For Home Use

* 1 cup equals 8 oz fluid

** Trees should not be sprayed with oil when they are in need of water

*** To prevent clogging of the spray nozzle orifice, wettable powders should be thoroughly mixed with liquid prior to application.

Chemical Activity on Pests and Diseases

x = effective against pest species

Date Last Modified: Wednesday, July 13, 2005