Like any other crop, many destructive pests and diseases limit the production of citrus. The common pests which attack the citrus plant include rind borer, greenbug, barkborer, fruitfly, scales,citrus psylla,aphids, and mites. Citrus diseases are either caused by bacteria, fungi or virus and virus-like particles. A systemic virus-like disease known as leaf mottling and its vector Diaphorina citri caused the decline of more than half a million citrus trees. Likewise recommended are control measures outlined to guide citrus growers on what to do.
Aphids (Toxoptera citricidus, T. Aurantii, Aphis gossypii)
Damage : These insects are vectors of tristeza, a viral disease of citrus. Toxoptera citricidus is the most efficient vector. Leaves curl and become yellowish. Sooty molds are produced.
Control : Spray with organophosphate insecticides during the flushing stage.
Barkborer (Agrilus occipitalis Exch.)
Damage: The adult beetles feed on the leaf edges and later lay their eggs on the bark. The grubs then feed on the cambium layer and pupate on the wood. The injured portion exhibit gumming and in severe cases the bark may peel-off.
Control: Spray on the trunk and branches with copper sulfate. Catching adult beetles with insect net to prevent them from laying their eggs is also helpful.
Caterpillars (Papilio demoleus)
Damage: Feeding on young leaves and shoots.
Control: Hand pick if population is low. Avoid removing weaver ants. In severe cases, spray selective insecticide.
Fruit fly (Dacros dorsalis Hend.)
Damage: Females puncture the skin of fruits with their ovipositor and lay a batch of eggs. Soon the eggs hatch into maggots. The maggots feed inside the fruit causing decay and fruit drop.
Control: Trap adults by using attractants on cotton buds and place on an improvised container to be hung under the citrus tree. Collect and bury infested fruits for proper sanitation.
Greenbug (Phynchochoris longirostris Stal.)Greenbug (Phynchochoris longirostris Stal.)
Damage: Both nymphs and adults puncture the fruit at various stages of fruit growth causing heavy fruit drop.
Control: Temporary removal of covercrops when infestation is heavy and persistent.
Spray with insectides or use parasitizing wasp such as Telenomus latisulcus Crawford and Anastatus stantoni on female bugs and eggs.
Jumping plant lice or citrus psylla (Diaphorina citri Kuway)
Damage: Sap feeding by this insect effectively transmits causal pathogen of mottling disease. The leaves curl and turn yellowish.
Control: Regular spraying of appropriate insecticide during flushing period.
Leaf miners (Phyllocnistis citrella)
- Tunneling on the underside of the leaves causing defoliation and curling of leaves.
- It also causes the falling of premature leaves.
- Leaves become discolored and malformed.
- Fruits drop and sooty molds are produced.
- Flowers drop in heavy infestation.
Control: Spray appropriate insecticides during the flushing period.
- Red spider (Panonychus citri McGreg) causes tiny scratch like marks on the upper surface of the leaves. Abundant scratch like marks give the leaves a pale or grayish appearance.
- The leaves drop leaving the petiole attached to the green twig.
- Dying and browning of leaves giving appearance of pin injury is also observed.
Control: Spray appropriate acarricides or a sulfur-based fungicide.
Rind borer (Prays endocarpa Meyr.)
Damage: The caterpillar bores inside the fruit and feeds on the rind tissue. Young fruits attacked by rind borer often drop while mature fruits develop lumps.
Control: Observe sanitation in the surroundings. Spray with the use of insecticide during the pre and post bloom stage.
Scales (Lepidosaphes beckii)
Damage: Leaves turn yellow. Dieback occurs and sooty molds are produced.
Control: Prune infested twigs and burn it. Spray with suitable insecticides.
White flies (Aleurodicus dispersus)
Damage: Leaves turn yellow and fall prematurely. Sooty molds are produced.
Control: Spray with suitable insecticides. In small areas, soap and detergent solutions help to provide effective control, together with pruning and mulching which helps the plants fight moisture loss due to the infestation.
1. Diseases caused by fungi
Brown Rot Gummosis or Foot Rot
Foot rot is one of the most dreaded diseases of citrus caused primarily by two fungi: Phytophthora parasitica Dastur and P. citrophtora. These fungi are present in the soil and infect trees whenever conditions are favorable. Infections occur below or above the bud-union. The fungi gain entrance to the tree by directly penetrating the bark and through wounds in the bark. Soil containing the fungus which is splashed on the trunk of the tree during rains is a common and usual means of trunk infection.
Brown rot gummosis attacks the trunk near the soil and the larger main roots. Initial symptom is the profuse gumming on the surface of the infected bark. When the infection is below the ground this gum is absorbed by the soil. The infection extends both upwards and laterally when scraped, the affected part is brown in contrast with the surrounding healthy green tissue. The bark is killed through the wood. Removal of the infected bark will reveal a dark brown staining of the cambial surface. At a later stage, the dead bark dries, shrinks and cracks, and sometimes patches of it become loose and fall off leaving an open canker.
The infected leaves become chlorotic, the yellow color appearing first in the midrib and spreading to the lamina. Later, the yellow leaves drop and twigs and even branches die. The leaves of the new shoots are small and fruits are reduced in size and quantity. Eventually, the tree dies when the trunk is infected with the fungi.
Conditions Favoring Disease Development
The fungi causing brown rot gummosis live in the soil. Their spores can be dispersed by irrigation or splashing water. In case of floods or heavy rains, they can attack parts of the trunk that lie above the ground. The plant will be infected when the bark is in contact with water for at least five hours. Thus, the disease is more prevalent in wet or heavy soil that does not drain easily.
- Select well-drained land or provide adequate drainage in the orchard.
- Avoid too low budding and too deep planting.
- On established plantation, remove the soil around the crown roots to dry them.
- Remove dead and badly infected trees.
- Inspect the trees daily where the disease is prevalent.
- Remove the soil that adheres to the trunk and examine the trunk down to the first lateral roots. By following this, it is possible to detect gummosis at its earliest stages.
- When the affected area does not exceed half of the circumference of the trunk, then follow these corrective measures:
- Remove the affected bark with a strip of 2 mm wide of healthy tissue. It is not necessary to cut the wood.
- Disinfect the exposed part of the trunk or roots with copper-based fungicide.
- After the cut, edges of the bark begin to heal. Paint the exposed wood with any suitable sealing material.
- Cut back or thin the tops of treated trees.
The causal fungus is Elsinoe fawcetti Jenkins.
The disease occurs on fruits, leaves and branches. It is characterized by the appearance of raised lesions in the form of scabs or corky warts.
Small translucent spots which rapidly become pustular, two to three cm in diameter sometimes coalescent, pale rose and glossy then turning chamous beige in color.
Each lesion develops on only one side of the leaf.
The warts are often larger and completely encircle the stem.
- The development of young lesions is identical to that on the leaves.
- Serious deformities
- Premature fall of affected fruits.
Scab is often confused with citrus canker, however, the latter can be distinguished by the presence of corky lesions on all parts of the leaves.
Conditions Favoring Disease Development
- The causal fungus thrives well during the dry season on affected leaves and twigs which serve as source of infection on the
- It is disseminated by agents such as wind, rain, dew and fogs, and insects. The disseminated spores are capable of infecting the young and tender leaves, twigs and fruits of infected citrus.
- The leaves are most susceptible during the earliest stage of their growth but they gradually become resistant with growth.
- The young fruit buds are most susceptible to infection immediately after the falling of the petals and they become progressively resistant until they reach immunity, which occurs by the time they have attained a diameter of about 3/4 of an inch in the case of mandarins.
- Abundant moisture is essential for the infection of scab. This moisture maybe in the form of rain or heavy downpour occurring during the time that the flushes are appearing or when the fruit buds have just been set.
Since scab attacks and gains entrance only when the leaves, twigs, and fruits of susceptible citrus varieties are very young and tender, the disease can be effectively and economically controlled by an early regular spraying using any copper-based fungicides or benomyl dithiocarbamates.
A. To protect the growing leaves and twigs of non-bearing trees:
Spray once shortly before every flush or growth appears.
B. To protect the fruits
Spray only and repeat if necessary before they flower and later, at one to two weeks interval commencing when about 2/3 of the petals have fallen until the fruits shall have attained a size of about 3/4 of an inch.
It is caused by Corticium salmonicolor Berk & Br.
This disease is caused by a bacteria scientifically known as Xanthomonas citri.
The disease begins with the appearance of minute, yellow flecks, less than a millimeter in diameter and first visible on the lower surface. At later stage, these develop into shiny raised lesions, whitish pale yellow in wet weather and browner in dry weather. The lesions at first are surrounded by a glazed margin with an oily appearance and a yellow halo, eventually become rough and cracked.
The symptoms in affected fruits are similar to the leaves. The yellowish halo is often evident but the lesions have a characteristic cratered look which is more pronounced on fruits than on leaves.
These lesions never affect the pulp but they deform the fruit and leads to premature falling. Lesions are only found on the stems of the most susceptible varieties.
C. Young Shoots
Symptoms in young shoots are similar to those on leaves, but on the trunk and branches the cankers can reach a diameter of 15 cm.
Conditions Favoring Disease Development
- Infection usually starts in sap pockets on the twigs and branches.
- The appearance of cracks on the bark and the secretion of gum are the first signs of the disease.
- The infected twigs or branches manifest a sudden yellowing followed by wilting of the leaves.
- Death of the infected part.
- At certain stages of disease development during humid or rainy days, the infected parts are covered with masses of pinkish mycelium that often extend in strands over the bark.
- During the dry weather, the mycelium dries considerably and will not be so evident since its color changes to dirty white to grayish. F The disease, if neglected, often kills the entire tree in a few days or weeks, depending on the weather condition.
Pink disease is a malady of citrus trees only during the rainy season, especially in a neglected orchard. Greatly favored by cool weather and rain, the pathogens usually gain entrance through sap pockets of the twigs and branches of the tree.
- Practice proper crop sanitation.
- Regular spraying of copper fungicide.
- Prune first and properly dispose infected branches prior to any spraying, since it will be useless to spray once the fungus has gained entrance into the bark. Prune well the infected twig or branch to prevent the infection to occur into the healthy wood.
- Strict field sanitation is a very important factor in the successful control of the disease.
This disease is caused by Oidium tingitanium Carter.
- Young actively growing leaves and twigs are the most commonly attacked. Usually the young shoots near the center of the tree and nearest to the ground show the disease symptoms.
- Pale, whitish patches of mildew, mostly on the upper surface of the leaves. Often starting at the edges of the midribs, single lesions appear to be circular although the fungus does not spread actually in perfect circular manner, but the hyphae may be seen to radiate from the original center of infection.
- The mycelium has a coarse appearance but is not dense.
- The leaf tissues beneath the fungus mycelium are at first darker and watery green, as the disease progresses they lose their color and become yellowish-white.
- The seriously infected leaves shrivel, dry and adhere to the twig or fall.
- In older leaves, there is no shriveling but a buckling of the tissues resulting to malfunction of the leaves.
Conditions Favoring Disease Development
- The disease develops best during the driest and coolest period of the year. However, the spread of the disease during an otherwise dry season is closely associated with infrequent showers of short duration, with high humidity and excessive dew.
- The weather conditions at high elevation are usually conducive to the propagation of the fungus althroughout the year.
- Heavy rains of long duration which maintain leaf surface wetness for prolonged periods.
- Bright, sunny, hot weather are unfavorable to the fungus.
Copper-based fungicides have been found to be most effective and economical for the control of powdery mildew.
2. Disease caused by bacteria
- The causal organism chiefly survives in the cortical tissues of infected stems, in plant debris, and in the soil.
- The bacteria which exude from wet cankers are effectively ispersed by the flow of rainwater into the leaves and fruits of the lower branches.
- A cloudy and wet climate is very favorable.
- Timely and regular spraying with any copper-based fungicides.
- Start spraying when the trees are on their dormant stage. Repeat when new flushes of growth develop and continue spraying every two weeks thereafter until the leaves, twigs or fruits are maturing.
3. Diseases caused by virus and virus-like particles
Citrus Tristeza Virus (CTV)
The virus is transmitted by Black Citrus Aphid (inset). Other means of spread is through budding and grafting.
- Leaf color changes from normal green to olive green with a characteristic hardening in the appearance of leaves.
- Thinning of foliage is observed and growth of water sprouts.
- Clearing of veins and stem pitting on sensitive varieties like calamondin, limes and pummelos.
- Root decay which begins at the root tips and progresses back to the large roots.
- Top of the trees may die within a few days after observed, declines or may extend over several months.
Two types of Tristeza
If the tree is affected with this type, the plant especially young ones will collapse and the leaves suddenly wilt and dry on the tree. This effect suggests the name “Quick Decline”.
The tree will be able to live with few functional sieve tubes for normal growth and continue to produce many fruits much earlier, one or more years than the healthy trees of the same age but commercially unproductive for several years.
- Control insect vector by chemical spraying.
- Quarantine and eradication for very limited infestations.
- Use disease-free budwood and tolerant rootstocks like trifoliate Troyer Citrange, sweet orange, rough lemon, rungpur lime and cleopatra mandarin.
- Cross protection by using a mild strain of Tristeza.
- Disinfect tools used for budding and pruning with formalin solutions.
- Use disease-free budwoods from indexed trees.
- Avoid susceptible rootstocks.
Huanglongbing Disease (Yellow Mottling or Greening)
This is transmitted by Diaphorina citri (Citrus psyllids), an insect vector. Other means of transmission is by budding and grafting.
- Reduction of leaf size in upright position similar to that caused by zinc and manganese deficiency.
- Affected trees produce leathery leaves with tendencies to roll, and develop into dull, yellow-green color similar to boron deficiency.
- Midribs of the older leaves developed during the dry season turn yellow similar to nitrogen deficiency.
- Leaves showing different types of chlorosis may occur in the branches have dying foliage, resulting on death of tree.
- Fruits are small body-shaped and poorly colored with seed abortion.
- Control of insect vector with any chemical spray.
- Start with disease-free planting materials with scion taken from budwoods which are indexed or certified parent trees.
- Establish the citrus nursery in a place far from existing citrus plantation to prevent the planting materials from contamination with the disease.
- Roguing of diseased plants.
Physiological and Nutritional Disorders of Citrus
Aside from pests and diseases, citrus also suffer from various physiological and nutritional disorders. Table 1 describes the signs, symptoms and observations of the disorders and recommends appropriate control measures.
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