Management of pests and diseases:Orange

1.1. Alternaria Brown Spot (Alternaria alternata or A. citri)

 

Conidia are released by rain events or sudden changes in relative humidity. Optimum temperatures for diseases development are 23-27°C. Plants can get infection between 17-32°C. Infection can occur with as little as 4-6 h of leaf wetness but disease severity increases with leaf wetness.

Management:

• Disease-free nursery trees;
• Careful choice of planting site;
• Air drainage is important;
• Wider spacing;
• No vigorous rootstocks;
• No over-fertilization or over-watering;
• No overhead irrigation.
• Chemical control: Copper based fungicides. Maintain protective coating.

1.2. Melanose (Diaporthe citri)

The disease causes lesions on fruit and leaves. All citrus are susceptible but grapefruit and lemons are the most susceptible. Optimum temperature for the disease development was determined to be 24-28 °C.

Management:

•  Remove dead wood from canopy.
• Copper based fungicides.

• Copper to be applied every 3 weeks until fruit are fully developped.

1.3. Pseudocercospora Fruit and Leaf Spots
 (Pseudocercospora angolensis; Syn.: Phaeoramularia angolensis)

This is a quarantine disease affecting all Citrus but the most susceptible are grapefruit, oranges and mandarin; while the less susceptible is lemon. Yield loss due to this disease ranges between 50-100%.

Young leaves are highly susceptible to infection from lesions older tissues. Young fruit up to golf ball size are also highly susceptible.

The disease spread is so far restricted to humid tropics of Africa between 800-1500 m and is favoured by prolonged wet weather followed by dry periods with temperatures between 22-26°C. Leaves are the main source of inoculum. The disease is long distance spread by windborne conidia. Infected planting material may also contribute to long distance spread. Within the orchard, spread is by splash dispersed conidia.

Symptoms:

Fruit lesions are circular to irregularly shaped. Young fruit has nipple‐like lesions with yellow halo. And can become mummified. Mature fruit lesions are dark brown to black and generally flat or sunken with a yellow halo.

Leaf symptoms are circular to irregularly shaped lesions that can coalesce. Leaf lesions with brown or grayish center surrounded by a yellow halo. Young  flush can be killed and leaf drop can occur.

This is a quarantine disease affecting all Citrus but the most susceptible are grapefruit, oranges and mandarin; while the less susceptible is lemon. Uield loss due to this disease ranges between 50-100% .

Management:

• Inoculum control via collecting and destroying all fallen fruits and leaves in affected orchards;
• Plant windbreaks around the citrus orchards. Wind is the primary dispersal agent spores;
• Discouraging inter-planting in affected orchards with mature producing trees;
• Prevents creation of a microclimate of relatively cool temperatures and high RH;
• Judicious pruning of shoots to allow light penetration and aeration within the tree canopy;
• Fungicides: Alternate Benomyl and copper sprays every 2 weeks from a week following the onset of rains.

1.4. Citrus greening

Citrus greening disease is a major cause of crop and tree loss in many parts of Asia and Africa. Before it was identified as one disease, it became known by various names: yellow shoot (huanglungbin) in China; likubin (decline)in Taiwan;  dieback in India; leaf mottle in the Philippines;  vein phloem degeneration in Indonesia; and Yellow branch, blotchy-mottle, or greening in South Africa.  As it became clear that all these were similar diseases the name "greening" was widely adopted.

Nature of the causal agent:

The demonstrations that greening is a graft- and insect-transmissible disease led to the conclusion that a virus was responsible. In China some researchers believed tristeza virus to be the cause. In South Africa it was shown that tristeza and greening could readily be distinguished since the aphid Toxoptera citricidus transmitted tristeza but not greening, and psylla vice versa. Other researchers suggested that it should be classified as a true bacterium.

Citrus greening is caused by phloem-limited bacteria. The African form of the disease is heat sensitive, and the Asian one is not. The bacteria are transmitted by insect vectors and by grafting.

Symptoms:

• Citrus greening disease is characterized by leaf symptoms reminiscent of severe nutritional deficiencies, yellow shoots , twig dieback, tree decline, and reduced fruit size and quality. Symptoms resembling zinc deficiency occur on younger leaves, and older leaves develop a characteristic mottle.
• Fruit yield is severely reduced, and what little fruit is produced is greatly reduced in quality, poorly colored (hence the name greening), and have a bitter, sour flavour.
• Root system is poorly developed with relatively few fibrous roots. New root growth is suppressed and the roots often start decaying from the rootlets.

Losses due to greening are not easy to assess. Sometimes only sectors of a tree are affected and losses are small, but in other cases the entire tree is infected and crop loss is total.

Management:

To date, there is nowhere in the world where citrus greening disease occurs that it is under completely successful management. In every place where the disease occurs, life expectancy of citrus trees is vastly reduced and production losses are significant.

• The most successful management efforts combine production of clean stock with psyllid control and inoculum suppression once plantations are established.
• Psyllids must be controlled both within plantations and on any alternative host plants.
• Inoculum suppression involves removal or severe pruning of any affected plantation trees several times each year. Non-commercial citrus and any alternative hosts of the pathogens may also need to be removed.

1.5. Aphids: Black citrus aphid (Toxoptera aurantii), Cotton or melon aphid (Aphis gossypii), Spirea aphid: Aphis spiraecola

Aphids feed on buds and on the underside of leaves , causing leaves to curl toward the stem. Spirea aphid, black aphid and cotton aphid can all transmit citrus tristeza virus.  Insecticide applications for aphids are not recommended because it is difficult to prevent transmission of citrus tristeza virus by controlling aphids with insecticides unless an area wide treatment program is conducted.

Management:

• Natural enemies normally control aphid populations and an insecticide application is rarely warranted.
• Biological Control: A number of coccinellid and syrphid predators, parasites, and fungal diseases usually keep aphid populations below damaging levels.
• Treatment Decisions: On newly established trees and on new growth flushes on mature trees, it is not uncommon for aphids to cause curling of leaves and produce honeydew.
• Treatment is usually not warranted because citrus can tolerate extensive leaf curling without yield effects. Pesticide applications are reserved for special situations such as area wide treatment programs.

1.6. Citrus mealybugs (Planococcus citri)

The citrus mealybug is a sporadic, and extremely damaging and difficult to control pest of citrus. They prefer humid conditions. In citrus, mealybugs spread by crawling from tree to tree, wind, on bird’s feet, machinery, and labor crews.

On hatching, the nymphs are light yellow, but they soon excrete a waxy covering. There are 2-3 generations per year.

Damage:

• Mealy bugs suck plant sap reducing vigour and causing fruit to drop.
• They also contaminate bunches of citrus by their presence or by the sooty mould growing on honeydew.

Management:

• Pruning & hedging. Hedging trees to prevent touching between trees will help prevent within grove spread of infestations. Additionally, pruning will aid in opening up the canopy to maximize spray penetration and coverage.
• Equipment Sanitation. Thorough cleaning of equipment and harvest materials will help prevent the spread of mealybug from an infested grove to others.
• Ants must be controlled to allow natural enemies to do their job. Sowing vetch (Vicia sp) as an intercrop will attract ants away from the mealybugs.
• Natural conytrol: Mealybugs have a lot of natural enemies: parasites and predators. One of the best known is Cryptolaemus montrouzieri, the Mealybug Destroyer, which is a type of ladybird. Its larva is similar to a mealybug but is larger and has long waxy filaments. It can be imported and introduced.
• Chemical: Treatments are most effective against ‘crawlers’. No treatments are effective against mealybugs in closed fruit bunches. Imidacloprid is highly systemic and can be applied to the soil before irrigation at bloom.