Citrus Agronomic Principles
There are a large number of agronomic factors which can influence this, many are within the control of the grower, under given climatic and soil conditions.
Citrus grows well in the tropics. Sweet oranges proliferate in this region, but their skins remain pale greenish and do not develop a bright orange color due to a lack of lower temperatures before harvest. Sweet oranges have a constant floweringfruiting cycle, which makes it difficult to distinguish between mature and immature fruit. Fruit drop throughout the season is a problem with sweet oranges grown in the tropics.
Grapefruit is largely restricted to the tropics. Limes are also limited to tropical regions, and the warm, humid-subtropics.
In subtropical climates with hot, humid summers and mild winters, large, sweet fruits with high juice quality and content are produced. This makes them ideal for processing or fresh consumption.
Sweet oranges and mandarins predominate in the subtropics. Satsumas also suit cool subtropical climates.
Semitropics and Mediterranean
In semitropical and Mediterranean climates citrus fruits have the brightest skin color, smoothest skins and an optimal blend of sweetness and acidity for fresh fruit production. Navel and blood oranges as well as lemons are predominantly restricted to Mediterranean climates.
Physiology and Physiological Manipulation
Individual fruit growth and eventual size relies heavily on the health and vigor of the nearest leaves. The greater the number of leaves, the better the tree’s ability to utilize water and produce carbohydrates for fruit fill.
Thus, in order to produce a good crop of evenly-sized fruit it is important to manipulate tree structure to provide evenly-spaced, abundant, well lit and active leaves. On a whole tree basis the optimum leaf/flower ratio for oranges are around 60:1 for oranges and 25:1 for satsumas.
While irrigation and correct nutrient management can ensure leaves are growing well, most trees need pruning and thinning to ensure an optimum leaf to flower/fruit ratio and allow light into and air movement through the tree.
In an un-pruned tree, the growth of fruitlets located inside the tree canopy is usually slower than those on the outside branches. In addition, the potential size of fruit is normally set by the end of fruit set and before cell expansion. Thus it is important to manipulate tree shape prior to this period.
Pruning aims to ensure that water and nutrients are available to an optimum number of well positioned fruit. Weak bearing wood that is long, thin, sparsely foliated and which comes from the underside of branches is removed by pruning. This wood often produces fruit that is 15% smaller and 40% less heavy, than thick, leafy bearing branches. Pruning also allows more light and air flow into the tree.
Winter application of phyto-hormones such as gibberellic acid (GA), reduce flower formation and also increase leafy inflorescences. Summer applications of phyto-hormones will increase peel resistance to creasing and pitting.
Auxins are used to thin out smaller fruits and increase the leaf/flower ratio. However, they may reduce juice content and cause early granulation of fruit. Alternatively, hand thinning of fruitlets after flowering, prevents overload of the tree.
Girdling, the practice of cutting around limbs with a knife during flowering, helps increase fruit set by manipulating sap flow.
Once properly manipulated, trees need the right nutritional package to reap the full benefits and produce a heavy, even crop.
Soil Type and Management
Citrus crops are grown on a range of soils varying from sands to clay loams with different water holding capacities, drainage conditions and irrigation needs.
An ideal citrus soil is well structured, with good drainage to allow proper root aeration and to minimise root diseases.
Citrus has a shallow root system, which concentrates under the tree canopy. Tree roots are easily destroyed by tillage or poor water management.
Root growth occurs when soil temperatures range from 53.6 to 95°F with most active root growth occurring when the soil temperatures are between 77 to 86°F.
Soils with low organic matter contents are often low in nutrients especially nitrogen and sulfur. A low organic matter content may also impact on soil structure. In compacted soils, aeration can be restricted, affecting root growth, nutrient uptake and overall crop development.
Organic matter levels can be improved by manuring and/or by cover cropping with legumes or grass.
This practice – with the residues used as a mulch – can also help improve soil structure, water holding capacity and moderate soil temperatures.
Citrus prefer well-drained soils with a pH of 5.5 to 7.0 and low salinity. Yields are almost halved when pH drops to 4.5.
Soil pHs can range from 4.5 to 9.0 and this has a distinct impact on the availability of certain nutrients. Extreme soil pHs should be adjusted where it is practical to do so.
At lower pH values, trees can suffer from aluminum and other heavy metal ion toxicity as well as restricted P availability. At pH values above 7.5, nutrient availability, in particular of phosphorus and micronutrients, can be reduced, even though high total amounts of these elements may be present in the soil.
Liming can ameliorate undesirable, low pH values. Dolomitic limes will add Mg to the soil. In Ca-rich soils the availability of K and Mg can be restricted. During periods of active growth, Ca rich soils may not supply sufficient Ca to meet the plants uptake demand. In saline soils (high pH and NaCl levels), or sodic soils, Ca availability is also reduced.
Where soil pH is too high or too low, resulting in a ‘lock-up’ of nutrients (frequently micronutrients) in the soil, foliar application of those nutrients will ensure rapid response in the trees and prevent growth from being restricted.
Rootstocks differ in their sensitivity to specific conditions such as soil acidity, salt/water stress and disease. They also influence nutrient uptake as well as fruit size and quality on the selected scion. There is no perfect rootstock. For example, vigorous rootstocks such as rough lemon are susceptible to frost damage.
Sour orange rootstocks suit alkaline and saline soils (Texas); rough lemon has excellent drought tolerance (Africa/Australia); Poncirus trifoliata is well adapted to cooler climates and acid soils (Japan), etc. Several rootstocks may be used in a citrus grove to match the varying soil types or slope and aspect. Local advice is essential to ensure the best rootstock is chosen for the given environment. Rootstock selection, to suit the soil and climate, is essential to good citrus production.
Correct water management is critical for high yields, as citrus is sensitive to water stress. Irrigation is essential in arid or semiarid regions and increasingly practiced in the humid subtropics and tropics. Conversely, waterlogging for as little as three days, in high temperature conditions, can result in significant root die-back and leaf loss. Citrus trees on poorly drained soils - including some shallow sands – have few leaves, are stunted and low yielding. In these conditions, Phytophthora spp. And Pythium spp. may rise to levels that can cause death in nurseries and young groves.
Phytophora can also be a problem in old groves causing a slow decline in the tree’s vigor. In general, mature citrus trees require 11.8-39.4in (320,000 gal to 1 million gal/ac) of water a year to replace that lost by evapotranspiration. The lower rates are used for early mandarins and higher rates are applied to lemons and late oranges.
The water rate varies with climate, in Mediterranean climates the water rate ranges from 11.8–29.5in and in tropical climates the range is 2 to 4in. Irrigation practice should aim to ensure that locally derived soil moisture deficits are not exceeded. Soils should not be allowed to dry out to more than one-half of their water holding capacity.
On saline soils, irrigation is necessary to leach accumulated salts and minimize salt stress by frequent water application. Care is needed if irrigation water is calcareous or of high pH, because this can affect nutrient uptake. cidification of the water will help in such cases. Good drainage using raised beds, or installing a suitable drainage system, will ensure optimum citrus production. Fertigation provides nutrients into a ‘wetted zone’ and the roots feed from this restricted area. By providing a constant supply of water and nutrient into this zone, the efficiency of nutrient uptake is increased.
Weed competition is most significant in younger trees and control usually involves the use of herbicides. Mechanical mowing and cultivation (but not such that roots are damaged) is also practiced. Pest problems include mites, aphids, leaf miners, insects and nematodes, which feed on citrus roots, trunks, leaves and fruit. Many need to be controlled with pesticides once populations reach economic thresholds.
Citrus bud, rust and spider mites can damage fruits, thrips scar peel tissue and fruit fly larvae feed on the fruit pulp. Scale also reduces leaf effectiveness and fruit appearance. Thus, effective pest control is particularly important to minimize deterioration of several external quality characteristics. Bacteria, fungi and viruses can significantly affect productivity and quality. Most postharvest decay problems are due to fungal infestation and a number of other fungi cause fruit blemishes. Viral diseases have been responsible for severe tree loss in the past.