Improving an existing citrus orchard

Citrus growing is a long-term investment. Orchards can be productive for up to 50 years or more. The useful economic life span of the plantation depends on climatic and management conditions. Organic farmers aim to achieve tree longevity as part of their sustainable land use strategy.

Almost every orchard can be improved back to full production by applying organic methods. However, these practices also apply to newly established citrus orchards.

Increase biodiversity

One of the most important organic approaches to start with is to increase biodiversity. A high degree of diversity decreases the risks of disease infestation and enhances ecological pest control through the appearance of a highly diverse bird and insect community.

There are many ways of increasing biodiversity:

Intercropping - in the citrus orchard with annual crops. In existing high density citrus orchards, rows of citrus trees can be removed to increase ventilation and light intercepion. In the remaining spaces legume annual crops like beans or peas can be planted.
Cover cropping - Between the rows legume pasture or cover crops can be planted. Flowering plants such as Alfalfa (Medicago sativa), African blue basil (Ocinum basilicum), Centrosema pubescens, Desmodium, Cassina obtusifolia, Alysicarpus vaginalis) provide useful habitats for beneficial insects. They also improve soil conditions without competing with citrus trees.
Agroforestry - In some African countries, citrus trees are mixed with other fruit trees like mangoes, jackfruit, banana, palm trees, coffee, and cocoa, as well as with leguminous trees (e.g. Albizia) or other permanent tree species. In windy areas, the trees act as windbreaks for citrus which is sensitive to strong winds.

Ensure proper pruning

As the trees grow, the inner and lower branches become shaded. This leaves fruiting to the outer periphery of the canopy whereas the inner parts have poor or no fruit set at all. This problem can become especially severe in high-density orchards. Annual pruning maintains light and air penetration through the canopy of the citrus trees and reduces the humidity around the plants contributing to pest and disease prevention. The tree height should be maintained at less than twice the planting distance in the row.

Three major types of pruning include:

Heading back to control size and form of the tree
Thinning out to improve aeration
Selective pruning to cut off infested parts of the tree, especially after Phytopthora infections or citrus greening

Pruning should be done during the vegetative phase (before flowering) of the trees.

Recommendations to farmers regarding proper pruning:

Maintain a single stem up to a height of 3 feet (100 cm), and pinch or break the main shoot to encourage side branching
Allow 3 to 4 main branches to form the framework of the tree
Remove any extra side branches including those growing inwards
Ensure all diseased and dead branches are removed regularly

Excursion: citrus tree pruning

Take the farmers to a citrus orchard. Inspect some trees and discuss how to prune them.

Improving soil fertility

The building up and maintenance of a fertile soil is a central goal in organic citrus growing. Careful soil management is especially important in the humid tropics, where heavy rainfall and strong solar radiation accelerate soil degradation, leaching of nutrients and erosion.

The basic tools of organic soil management are interdependent and influence tree health, tree development, fruit yield and quality. Relevant soil management techniques for organic citrus orchards include:

Soil erosion control
Use of cover crops (understorey plants) or mulching
Agroforestry and intercropping methods
Application of compost

Soil erosion control

Especially in sloping areas, soil erosion control structures are very important. Water conservation structures like terraces should be constructed to prevent the loss of topsoil and organic matter.

Soil cover systems

A permanent soil cover is an important component of the organic orchard cultivation system. Locally adapted leguminous crops, such as velvet bean (Mucuna spp.), sunhemp (Crotalaria spp.) and lablab (Lablab purpureus) help restore degraded soils very fast. They successfully suppress weeds, fix nitrogen and prevent erosion. To avoid strong competition between the cover crop and the citrus trees, the cover crop should be regularly cut.

Application of compost

Most tropical soils are low in organic matter. To improve soil organic matter content at least 20 kg or 1 bucket of well-rotted cattle manure or compost should be applied to each tree every year. If possible, a handful of rock phosphate should be added. Compost should be added especially 2 to 4 weeks before flowering to encourage good fruit formation and development. According to citrus farmers, application of manure or compost also makes fruit sweeter.

Discussion on soil fertility management in citrus orchards

Discuss with the farmers about their approaches to soil fertility management and plant nutrition. You may ask them the following questions:

How do you commonly improve soil fertility in your orchards?
What else do you think needs to be done to sufficiently supply nutrients needed for the proper growth of citrus trees?

Proper weed management

Noxious perennial grasses such as Panicum spp., Paspalum spp., Amaranthus spp., couch grass (Cynodon dactylon) and spear grass (Imperata cycylindrica) are suppressed by sowing cover crops.

Cover crops must be slashed when they start to compete with citrus trees. Hand weeding may be necessary when specific weeds start to dominate.

If tools or machines are used, injuries on the tree trunks and roots must be avoided, as they may become entry points for soil-borne diseases.

In orchards with newly planted citrus trees, the soil is best when covered with mulch.

Discussion on appropriate weed management

Ask the farmers to propose ways how to manage weeds efficiently in citrus orchards under their conditions. Discuss the different propositions considering their benefits and inconveniences.

Pest and Disease management

Citrus are attacked by a wide variety of economically important pests and diseases. Major pests are citrus whitefly, thrips, mites, aphids, ants, black flies, citrus psyllid, fruit flies and false coddling moth. Major diseases include the greening disease, damping off, citrus tristeza virus, anthracnose, phaeoramularia fruit and leaf spot, gummosis, canker and scab.

Effective management of citrus diseases first requires proper implementation of preventive measures to avoid heavy infections:
Choice of suitable varieties for the scions and rootstocks that are adapted to the local climatic conditions, increases plant resistance to pests and diseases.
Regular monitoring of the orchard allows early identification of infections and timely intervention before major damage occurs.
A diverse habitat including organic orchard design, hedges, flower strips and agroforestry trees enhances natural enemies of pests (see section 4.1 also).
Proper soil fertility management improves nutrition and health of trees.
Pruning and de-suckering increases aeration of the orchard.
Removal and destruction of infected plant parts reduces pest and disease pressure in the orchard.

For direct control, natural pesticides such as pyrethrum, derris, neem, soaps, mineral and plant oil as well as mass trapping techniques are used in organic citrus production. Nursery and field plants can be protected by routine sprays with protective copper based fungicides such as Bordeaux mixture or Copper oxychloride. However, such sprays must be approved by the certifier in case of certified organic citrus production.

Major pests and diseases of citrus in Africa:

Leaf miner (Phyllocnistis citrella)

This is a very serious pest of citrus direct from the nursery to the field, usually attacking young leaves and shoots. The adult moth deposits tiny eggs on the underside of the leaves. Upon hatching, the larvae enter the leaves, where they remain until they mature. At maturity they come out of the leaves and form a pupation cell by folding over a part of the leaf margin. They mine the underside of young leaves, but attack both sides of the leaves in heavy infestations, and occasionally also the fruit. Feeding of the leaf miner causes serpentine mines that have a silvery appearance. As a result of the leaf mining, the leaves become twisted, show brown patches of dead tissue and eventually fall off.

The leaf miner can be controlled by spraying neem. Application is most useful on young seedlings in the nurseries, as they are most susceptible to leaf miner attacks.

Aphids (Toxoptera citridus and T. aurantii)

The shiny black or brown insects feed by sucking on new blossoms causing severe curling and deformation of young leaves and stunted growth of leaves and twigs. High numbers are found on the leaf surfaces during the period of flushing (production of new shoots) and stems of attacked young shoots die back. Aphids excrete large amounts of honeydew, leading to the growth of black sooty mould on the leaves. They also transmit tristeza and other virus diseases in citrus.

Aphids are kept in check by natural enemies, especially ladybird beetles, lacewings, hoverflies and parasitic wasps. Therefore, by encouraging natural diversity around and within the orchard, aphids can be well managed. However, in heavily infested plants, neem sprays should be applied around the aphid populations, especially around new shoots and under the leaves.

Scales

Scales are small insects, which resemble shells glued to the plant. They suck sap from all plant parts above the ground. Their feeding may cause yellowing of leaves followed by leaf drop, poor growth, dieback of branches, fruit drop, and blemishes on the fruit. Leaves may dry when heavily infested and young trees may die. Some scales (soft scales) excrete honeydew, causing growth of sooty mould. In heavy infestations, the fruit and leaves are heavily coated with sooty mould turning black. Heavy coating with sooty mould reduces photosynthesis and thus plant growth. Fruit contaminated with sooty mould loses market value. Ants feed on the honeydew that is excreted by soft scales, preventing a buildup in sooty moulds, but also protecting the scales from natural enemies.

Scales are attacked by a large range of parasitic wasps and predators. These natural enemies usually control scales in well balanced systems. Botanical sprays should target young stages of the scales. Oil sprays for example should be applied after harvesting and not during flowering or during periods of excessive heat or drought. Heavily affected branches and leaves should also be removed and burnt.

Damping off

Damping off is a common infection in recently germinated seedlings of any citrus cultivar, where the seedbed shows poor drainage. Damping off attacks and kills seedlings at the stem zone above the soil level. However, damping off fungi can also cause seed rot or a pre-emergence rot, resulting in sparse stands of seedlings. Damping off is caused by Rhizoctonia solani, Phytophthora spp. or Pythium spp. Infections can be prevented by avoiding planting into infected soil, by hot water treatment of seeds before planting and by avoiding overwatering of seedbeds. Another successful method for controlling damping off in the nursery involves using a special planting media using manure and sand at a ratio of 1:2. Such a media increases aeration and improves drainage in the seedbed. The media is set to make a bed, and a 2-inch depth of sand is spread on the surface of the bed. The seeds are placed into the sand zone and when they germinate, they plug their roots into the sand manure mixture. But their stems will remain in the sand zone. Sand on the surface will help to drain away water from the stem zone and thus create unfavourable conditions for damping off at the stem.

Greening Disease

The greening disease leads affected oranges to completely change colour, leaving patches of green peel. A more typical leaf symptom is a leaf mottling, a blend of light and dark green patches on the leaf that are not bound by veins. Leaves on affected branches usually fall off readily, defoliating the affected trees. Flowering and flushing occur out of season and the fruit on affected branches, if any at all, are small and deformed. Even the maturing fruit colours are different. Healthy oranges start to change colour from the free end, while greening affected fruit start changing from the stalk end. The green patches do not change colour even after picking.

Greening disease is mainly introduced into the orchard from neighbouring infected orchards through vector insects such as Diaphorina citri or with infected plant material. Prevention thus bases on good nursery management and use of healthy plants. Bud sticks should not be obtained from trees showing signs of infection. Production of healthy nursery material includes using healthy bud sticks only and protecting the seedlings and plants from insects. Organic farmers are recommended to focus in the control of the vector Diaphorina, as control of the HLB Bacterium is nearly impossible. Farmers should, therefore, avoid the proliferation of Diaphorina in the orchard and in the region. In organic agriculture, Diaphorina can be controlled by application of biocontrol agents such as Tamarixia radiata or other natural enemies and predators such as Cycloneda sanguinea. Local producers of biocontrol agents may have other solutions also. Neem, tephrosia or pyrethrum botanical mixtures can be used as well.

Heavily infected trees should be removed and replaced by new plants, since the soil is not affected by the greening disease and replanting presents no problem. In orchards with a very low level of Diaphorina activity and where only isolated branches are infected, they should be removed with a saw. The cut should be made as close to the trunk as possible.

Phaeoramularia fruit and leaf spot

This disease is caused by the fungus Phaeoramularia angolensis. It is favoured by wet and cool conditions. The fungus produces circular, mostly solitary (single) spots with light brown or greyish centres on the leaves. Each spot is usually surrounded by a yellow halo. Occasionally, the thin necrotic tissue in the centres of old spots falls out, creating a shot-hole effect. During rains, leaf spots on young leaves often join together ending in a general paling of the leaves or what is known as chlorosis. Premature defoliation takes place when the leaf petioles are infected. On fruit, the spots are circular to irregular in shape, which may join together and are surrounded by yellow halos. The disease has been observed on all citrus species including grapefruit, lemon, lime, mandarin, pummelo and orange. Grapefruit, mandarin, pummelo and orange are very susceptible. Lemon is less susceptible and lime is least susceptible.

The disease can be effectively controlled by using fungicides including copper based products such as Bordeaux mixture or copper oxychloride, which are allowed for use in organic farming.

Citrus Tristeza Virus (CTV)

The citrus tristeza virus is a major threat to global citrus production. The virus is transmitted through the use of tristeza-infected vegetative material, both scion and rootstocks and through the insect vectors, which is primarily the aphid Toxoptera citricidus.

Typical tristeza symptoms in older orange trees include yellowing of leaves with clear veins, yellowing showing poor new growth. As the disease progresses, the older leaves begin to fall, breaking between the petiole and the leaf blade, leaving defoliated twigs or with few young leaves. Twig growth becomes weaker each season until the tree dies, but some trees seem to linger on for many years. In some cases, affected trees quickly collapse after a sudden wilting and drying of the leaves.

Control is achieved by avoiding the use of sour orange rootstock, but using CTV tolerant rootstocks such as sweet orange, mandarin or rough lemon instead. Additionally, infected trees must be removed and destroyed. If these measures are combined with the control of aphid vectors, the multiplication of disease can be significantly reduced.

Field visit for identification of citrus pests and diseases

Take the farmers to a citrus orchard and identify any pests and diseases. Share knowledge on symptoms and preventive management measures.

Water management and irrigation

Citrus trees can conserve water well and are capable of withstanding long periods of drought, as their leaves are covered with wax. However, even in humid subtropical and tropical regions with sufficient total rainfall, irrigation is important during dry periods, especially during and after flowering, to ensure sufficient water for fruit setting and growth. Regular, moderate irrigation reduces physiological fruit drop, improves flowering, fruit set, fruit size and juice content.

The water should be of good quality. It should be free of chemicals, heavy metals, toxic bacteria and contain little salts. Regular water analysis is compulsory for organic certification. Soil water conservation practices such as mulching are also very necessary especially in young trees.

A low cost water irrigation practice, especially for newly planted citrus seedlings is the use of a jerrycan, or plastic water bottle placed near the stem of each plant to provide water. The jerrycans/bottles are filled with water and perforated at the bottom or in the lid so the water can drip continuously into the soil around the plant. When the water is finished, the jerrycan or bottle is refilled.

Discussion on water management

Discuss with the farmers the different possibilities of managing limited water resources in citrus production. How can farmers keep water in the soil? How can they irrigate citrus orchards using little water only?