Citrus are a genus of subtropical fruit that grow on trees. They are generally subdivided into five species: sweet oranges, mandarins (tangerines), grapefruit, lemons, and limes. Species of the genus citrus are evergreen perennials, which may live for hundreds of years. However, the oldest citrus trees in Florida that still produce fruit are 100 years old.
The sweet orange (Citrus sinensis) is considered to be a form of sour orange. It is by far the most widely grown fruit tree in the world. Oranges are an important crop from China, India, to South Africa, the Mediterranean region, the subtropical regions of South America, and the southern and western parts of the United States.
Most oranges grown in California are Washington Navel and Valencia. Florida grows Hamlin, Valencia, and Pineapple oranges. The Washington Navel orange has outstanding flavour, is nearly seedless, and features a thick, easily removable rind.
Valencia is the other most popular orange, and is far juicier and richer in flavor than Washington Navel. It also has a thinner, tighter rind, not so easily removable. Most Valencia are seedless, except the ones grown in Chile.
Hamlin oranges are small, pale in color, and seedless. The juice they produce is also pale. The fruit quality is poor to medium. This is offset by the fact that the tree is high-yielding and cold-tolerant. The fruit is harvested from October to December. Hamlin is the leading early orange in Florida.
The favorite mid-season orange in Florida is “Pineapple.” It has the scent of pineapple, rich orange color, very good flavor, and a fair amount of seeds.
More Fruits – Blood Oranges
Blood Oranges come from the Mediterranean area and Pakistan. Growers in Florida do not grow them, because the red color rarely happens there, except after a cold spell. California growers include them only as a novelty.
Another orange type that is a favorite around Christmas is the mandarin orange, which is also called tangerine. Most of these are grown in China, but Japan is also a large producer, as is Florida, Spain and Latin America. Tangelos are a cross between tangerines and grapefruit.
Some of the popular mandarins and hybrid cultivars grown in Florida are Robinson, Sunburst, Temple, Orlando, Minneola, and Ambersweet. They all have specific harvest dates, stretching from September to December and January to March.
Grapefruit cultivars grown in Florida include Duncan, Marsh, Redblush, Ruby Red, Henderson, Ray Ruby, Star Ruby, and Flame. Some of them can be harvested from October to June, while others have a shorter harvest period.
Grapefruit accounts for only about 8 percent of the citrus market world-wide. The slightly bitter taste of grapefruit is not for everybody. However, with recent technological advances in obtaining the juice from this fruit and selling it in glass bottles to improve the taste, sales of pink grapefruit juice, for instance, have increased tremendously.
How Hardy Are Citrus Trees? – The Hardiness Of Citrus Fruit Trees
Commercial citrus cultivars need a temperature-range between 55° and 100° F (approximately 13° to 38° C). Their best growth occurs between 80° to 90° F (c. 27° to 32° C), provided water is readily available. During winter dormancy, temperatures can drop as low as 35° to 50° F (2° to 10° C). Below freezing temperatures can permanently damage citrus fruit and trees.
Florida has a humid, subtropical climate. California also has a subtropical climate, but it’s arid or dry. Aside from the southernmost areas of Texas and Arizona, where a small percentage of commercial cultivation does exist, most citrus production in North America takes place in Florida and California.
In terms of hardiness zones, most commercial citrus growing takes place in Zones 9 and 10, which cover most of Florida, the southern tip of Texas, south Arizona, and California. Zone 8, which goes from the northern panhandle of Florida to the Mexican border is still questionable as a desirable area for citrus production, since the temperature does drop below freezing more than once during the winter.
Even though “subtropical” goes as high as the 40th parallel, which is the southern border of Pennsylvania on the east coast, cold winters preclude any large scale citrus growing. Some hardy souls do grow orange trees in their backyards. Provided they receive winter protection, these trees might even bear fruit.
Freezing cold is by far the worst enemy of citrus fruit and trees. The United States was the leading producer of citrus fruit until devastatingly cold temperatures in Florida in the 1980’s caused widespread damage. Today Brazil is in first place, and the U.S. is second.
Protecting Citrus Trees From Freeze Damage – How To Protect Citrus Trees From Freeze Damage
The best cold protection for a young citrus tree is to build a soil bank around the trunk, all the way up to the lowest branches. This method can be used for the first two to four winters. Even if the cold kills off the parts of the tree above the soil bank, the trunk and roots below ground will survive and are capable of sprouting new shoots.
Certain types of tree wrap, some made out of burlap, some out of fibreglass, have been used at different times to protect young citrus trees from freezing temperatures, as well as from rodent damage. However, they harbor insects such as ants, and may be harmful in that way.
For the older citrus trees in the grove, many methods have been tried. Prior to the 1960’s, the prevailing wisdom was to spray water on the trees from the top causing the water to freeze, and ironically to protect the tree’s temperature from falling below the freezing point. However, in Florida this was implemented in 1962, with disastrous results. Since the water was sprayed on at the wrong rate, the sprayed trees suffered from freeze damage, while adjacent trees went unharmed.
Orange groves in Texas have tried covering their mature treetops in blankets or plastic sheets to keep them from freezing. In Florida, huge fans have been tried to take advantage of the air inversion, whereby the temperature near the ground is lower than the layer of air further up. Also, stacks of tires have been burned in the grove to keep the trees from freezing. Huge gas or diesel oil heaters are a more expensive and less polluting way to heat the air and the ground of orange groves.
Planting Citrus Trees – How To Plant Citrus Trees
Citrus trees can be grown from seed, but the seedlings take forever to flower, are prone to soil borne diseases and nematodes, and may produce fruit of inferior quality. Most citrus trees, therefore, are propagated as two-part trees—a scion above ground, that is grafted (or budded) onto a root stock, which is mostly below ground.
Rootstocks are true to type in that they are genetically similar to the maternal (seed) parent. Within the genus citrus, rootstocks are chosen for their characteristics, rather than for matching the scion. Thus for many years a rough lemon rootstock was used to bud sweet orange scions, since it imparted great vigor to the scion.
Some years later it was discovered that sweet orange trees that were budded on rough lemon rootstocks developed “lemon root decline,” which resulted in losses of 80 percent in some groves. Furthermore, rough lemon rootstocks were susceptible to nematodes and to major citrus viruses.
Sour orange is the most common rootstock in the Texas citrus industry. Sour orange rootstocks are, unfortunately, susceptible to scab, which can easily be controlled with fungicide sprays. It is also vulnerable to nematodes, and CTV, the Citrus Tristeza Virus. On the positive side, seedlings grown on sour orange rootstocks are the hardiest to cold of any commercial rootstocks.
Other rootstocks in use include Cleopatra Mandarin, Sweet Orange, Sweet Lime, and Trifoliate Orange. Several promising rootstocks, called Sour Orange substitutes, are showing CTV resistance. Long term performance is too early to judge. Gou Tou, Citrus obvoidea, and Smooth Flat Seville are popular selections.
Rootstock selection depends on a number of factors, such as soil quality, environmental conditions, relative freedom from pests and diseases, and scion compatibility. Some rootstocks are superior in some qualities and inferior in others. None are superior on all counts.
Budwood selection is equally important. Ask the nursery whether their trees are registered, which means they have been inspected and certified free of certain diseases and viruses.
The type of budding widely used in Florida is called “shield budding,” because the scion piece is cut in the shape of a shield (oval). A T-shaped incision is made on the rootstock bark to receive the shield. In Florida, this incision is an inverted-T, while in California it is an upright-T. Neither is better nor worse.
For many years, it was thought that this incision should be 2 or 3 inches above the ground. However, much foot rot can be avoided by making the incision as high as 6 inches above. The bud shield is gently pushed up under the bark, so that the cambium tissues of both pieces are in close contact.
Protecting The Bud – Prevent Bud Drying
To prevent drying out of the bud and to maintain close contact, a waterproof material is used to wrap the rootstock. Strips of polyethylene plastic are often used now, as opposed to the waxed cloth strips of the past.
About three or four weeks after budding, the wrap may be removed and the bud examined. If it is green still and callous tissue has formed, the bud has taken. Otherwise, another shield may be inserted and the budding tried again.
All horticultural tools used in this process should be sterilized in a bleach solution to prevent the spread of plant diseases. Depending on what time of year the budding takes place, a soil bank should be used to prevent freeze damage, not just in November, but also in early spring.
Once it is warm enough, remove the soil bank and provide a stake for each budling. Remove shoots on rootstocks, to provide more nourishment to the budlings. When the young tree reaches 18 inches in height, it should be topped to promote the formation of 3 to 5 lateral shoots. By the end of the growing season, the scion will no longer need support, and the stake can be removed.
How to water, fertilize, and cultivate the young, budded tree? Please see the next segments for the answers.
Nourishing Citrus Trees – How To Nourish Citrus Trees
Fifteen elements are essential for citrus trees. These elements can come from an organic or synthetic source, or a combination of both.
Nature provides three of these—carbon, oxygen, and hydrogen. 95 percent of the dry weight of citrus trees is made up of these three elements.
Carbon dioxide from the air is taken into leaves, so the carbon and oxygen molecules combine with the hydrogen and oxygen molecules in the water taken up by the roots, to produce carbohydrates or sugars. This process is known as photosynthesis and it takes place in cells containing chlorophyll in the presence of sunlight.
Unless the other 12 elements are supplied by the soil naturally, they must be added to it. The sandy soil of central Florida, for instance, lacks these elements. The macronutrients necessary for optimum tree growth are the primary ones (nitrogen, phosphorus, and potassium) and secondary ones (calcium, magnesium, and sulphur).
The micronutrients that are essential to citrus growing are iron, copper, manganese, zinc, boron, and molybdenum. Even though they are used only in minute amounts, they are necessary for satisfactory growth and fruiting.
Micronutrient deficiencies usually show up as distinctive patterns of leaf chlorosis, or yellowing. Macronutrient deficiencies are usually not symptomatic, but excessive amounts of nitrogen, for instance, result in poor fruit yield and nitrates leaching into the groundwater.
For newly planted citrus trees, wait until the budling begins new growth, before applying a fertilizer that should contain no more than 8 percent of the primary nutrients (8-8-8). A slightly higher number fertilizer may also be used, if you decrease the amount accordingly.
Then the young tree should be fertilized once a month, until October. Scatter fertilizer on the soil in a circle a foot away from the trunk and water it in thoroughly. The quantity of fertilizer increases each year, from one cup to two cups to four cups in the third year. However, the frequency of application decreases, from six to five to four times a year, in the third year.
Citrus trees generally prefer soil with a pH of 6 to 8. Coastal Florida has alkaline soil, while central Florida, the home of many orange groves, has sandy, acidic soil. Certain chemicals can be added to correct the pH reading of any soil. The Advanced Nutrients company makes two excellent products, pH Up and pH Down, depending on which way you want to go with your correction. Remember, the lower your pH reading, the more acidic the soil. Conversely, the higher your pH, the more alkaline the soil.
Advanced Nutrients Harvest Pride (9-6-6) is a superior, all-purpose fertilizer that is heavily organic based with just a touch of synthetic chemicals. It contains Cottonseed Meal, Blood Meal, Earthworm Casings, Bat Guano, Alfalfa Meal, Ureaform, Urea, and compounds of the macronutrients and micronutrients necessary for vigorous tree growth and fruit production.
Watering Citrus Trees – How To Water Citrus Trees
Young citrus trees require a thorough drenching two or three times during the first week, and one or two times a week during the next few weeks.
A watering ring should be dug around the trunk of the tree to allow for an even distribution and a slow release of moisture to the root system. This ring will erode after the first four to six months of tree life, after which the tree can be watered as a mature tree with a soaker hose or sprinkler system. Remember, all watering depends on rainfall, soil type, and time of year.
Good water drainage is absolutely essential for citrus trees. Coastal areas tend to be quite level and poorly drained. These sites need to be artificially drained to protect the roots of the trees planted there. Trees in these areas are planted on raised beds, and a system of ditches need to be dug to remove the excess water from the rooting zone of the trees.
Shallow soils have a greater need for irrigation, than deep, sandy soils. In drought conditions, a shallow soil must be irrigated more frequently and can absorb and store less water at each irrigation, than its deep soil counterpart.
Mulching keeps the water in the soil from evaporating too quickly, but it also causes problems, such as insect infestation. If you must mulch, be sure to leave a circle of 12 inches in radius around the trunk of citrus trees mulch free.
Pruning Citrus: Pros And Cons – Do Citrus Trees Need Pruning?
According to some growers, pruning and training of citrus trees is not necessary, since they are sold already properly shaped. However, even they admit that shoots below the crown should be removed, whether on the scion or the rootstock, as soon as they appear.
Early citrus growers had more experience with apple trees, and pruned citrus accordingly. However, they soon noticed that trees that were only minimally pruned did just as well as rigorously pruned ones. Since pruning is labor intensive and costly, they abandoned the practice.
Pruning of fruit bearing trees is still done for size control, for rejuvenation, to improve fruit quality, and to remove freeze damaged limbs.
Sprouts should be removed from the scion and especially the rootstock, once a year. Scion shoots should be removed, if they are not desirable as permanent branches.
All sprouts should be removed when they are young enough to be cut with hand clippers. Woody sprouts may require long handled pruning shears, a hand saw, or in the worst case scenario, a chainsaw.
A certain amount of deadwood is present in every tree. Twigs that die because they are too shaded may be left to fall off naturally. Removal of infected twigs is very important in disease control. Larger pieces of deadwood that are not diseased might be allowed to fall off, but they rub against fruit and cause damage, so they should also be removed.
Covering pruning scars with pruning paint may make the grower feel more secure, but it is not necessary in the summer months, when scars will heal naturally. Most pruning takes place at that time of the year, since labor is more readily available.
Some older groves were planted so closely together (20-25 feet apart) and the trees have grown so big, that they might require hedging. This is cutting the side of the tree facing the operating middle with circular saws at an angle. This practice exposes the bottom most branches of the tree to light, increasing fruit production, and allows air to penetrate the canopy.
Topping is often used in contemporary citriculture. If trees are allowed to grow unchecked, they will become too tall to spray and harvest. Moreover, the shade they provide will reduce fruiting in the lower canopy. Citrus trees today aren’t allowed to grow higher than 15-18 feet.
Harvesting Citrus Fruit – Tips On How To Harvest Citrus Fruit
Commercial growers have tried different methods of mechanized harvesting, but the labor intensive hand picking is still the most common harvesting process used throughout the world.
If you pull the citrus fruit from the twig carelessly, plugging often occurs. This means that some rind tissue is left with the fruit stem. Citrus fruit picked in this way is injured and often decays on the way to market.
For many years, clipping each fruit from its stem was used as the primary method of harvesting. This is a much slower process. With the labor shortage during World War II, growers returned to the pulling of oranges and grapefruit for faster harvesting. They found that if the pulling was done carefully, leaving only the button on the twig, no more injury was done to the crop than the clippers caused (clipper cuts or stem punctures).
By the time citrus fruits are ready to harvest, they should separate readily from the twig, when pulled properly. By turning the fruit at right angles to its stem and exerting a sideways motion—without pulling straight away from the twig—a clean split should occur.
If you have only a few citrus trees, it is easy enough to tell when the fruit is ready for harvesting. You may sample one or two and find the optimum time. Commercial growers, in their haste to turn a profit, used to send immature fruit to market. Government citrus commissions have put a stop to this. However, limes are an exception to this. They are usually picked only in an immature state, but ripen in transit.
Legal maturity for Florida citrus is governed by regulations that prescribe the proper color, minimum juice content, and minimum percentages of soluble solids and acids. Inspectors make sure that Florida citrus meets these requirements, before it is certified for handling and shipping.
Citrus Pests, Pathogens, And Diseases – How To Deal With Foot Rot, Root Rot, And Nematodes
“Lemon root decline” has been found to occur in groves with rootstocks other than rough lemon. Although the cause is still unknown, this disease is now called Citrus Blight. Numerous scientists are hard at work trying to find the cause of this mysterious citrus malady.
Foot and Root Rot have been troublesome since the late 1800’s. Budding sweet orange on sour rootstocks has helped, since that particular rootstock seems to be resistant. Trees that are budded high on resistant rootstocks, planted in wet soil and given good air circulation are less likely to be infected.
When over a quarter of the trunk’s circumference is effected, decline is visibly apparent. The leaves on one side of the tree yellow fast due to the cutting off of mineral nutrients, and eventually twigs and branches die back extensively.
If trees are regularly examined for the first signs of foot or root rot, and steps taken to correct the situation, loss of the trees may be avoided. Pull away all soil to expose all diseased areas to air. Diseased bark could be cut away and the wound painted with a safe disinfectant and pruning paint.
In order to avoid root rot, many growers recommend Advanced Nutrients Root Health Package that contains their products Sensizym, Voodoo Juice, Piranha, and Tarantula. They contain beneficial enzymes, fungi, and bacteria that colonize the roots and keep destructive pathogens from invading.
The Burrowing Nematode (Radopholus citrophilus) has been shown to be the cause of spreading decline in citrus groves. The nematode moves through the soil searching for new roots after the roots it has been feeding on become diseased and damaged. Milam, a citrus hybrid rootstock, is recommended as a biological barrier to stop the spread of this nematode.
Buffer zones have been established to prevent the spread of this nematode from one area of the grove to another.
The Citrus Nematode (Tylenchulus semipenetrans) is not as troublesome, since citrus trees can endure even a heavy infestation. It does damage many rootstocks, however, and badly infested trees exhibit stunting and produce unmarketable fruit.
Advanced Nutrients Scorpion Juice acts as a vaccination for the tree by inducing systemic acquired resistance (SAR) in its cells. Once this mechanism is triggered, the plant quickly builds an immune response to a diverse variety of pathogens.
Viral Diseases Of Citrus Trees – What To Do About Viral Diseases?
The principal viral diseases of citrus trees are psorosis, citrus tristeza virus (CTV), citrus exocortis viroid (CEV), and cachexia.
Psorosis, also known as “scaly bark,” is most serious on sweet orange and tangerine, but it can infect all types of citrus world-wide. Not all trees show the scaly bark symptoms, but a symmetrical yellowing of the leaves, either in small flecks or large blotches is apparent in all psorosis infections.
The only way this virus is transmitted is through budwood. This disease can easily be avoided by careful selection of budwood. Once your tree is infected, there is no known cure.
CTV wiped out the citrus industry of Brazil almost completely in 1940. It is transmitted by budding and grafting and by aphids that feed on citrus. Florida citrus growers have stopped using sour orange rootstocks, since the brown citrus aphid, which caused the Brazil devastation, seems to prefer them. More virulent strains of CTV have been found in citrus growing regions of the U.S.
Sweet orange scions seem to be able to stand up to CTV infection, unless they are rooted on sour orange or other susceptible rootstocks. The virus creates a barrier where the scion and the rootstock join, preventing the movement of metabolites from the top to the roots. Soon the rootstock dies, followed by the scion.
Excortis is a viroid that infects trifoliate orange and some of its hybrids (citranges), lime and lemons. It is transmitted to rootstocks by infected budwood or contaminated pruning tools. Stunting is the major symptom, and no cure is known. However, the disease can be prevented by the proper selection of rootstocks and scions.
Cachexia (xyloporosis) is a viroid transmitted only by budwood. Symptoms include bark scaling, gum infiltration, leaf dwarfing, chlorotic patterns, leaf drop, tree stunting, and small fruit with poor yields. Infected trees must be replaced, making it a costly disease. To avoid this calamity, cachexia-free budwood must be chosen (which is also free of psorosis).
Preventing Fungal Diseases Of Citrus – What To Do About Fungal Diseases?
First of all, Pirhana, made by the Advanced Nutrients company, will not only introduce an expert selection of beneficial fungi into your root zone to help roots absorb nutrients, but it can also be used as a foliar spray to combat many different types of fungal infestations. It acts as a bio-fungicide that can resist harmful fungi. The best cure always is prevention. Scab is a fungal disease of Temple, Murcott, and many types of lemons. There is also a sweet orange scab, prevalent in South America. Young tissues are most susceptible and peak spore release occurs in the early spring months. Spray during the spring-flush period.
Melanose is caused by a fungus that likes to inhabit dead citrus wood. The removal of deadwood is very important. Groves that prune regularly have less trouble with melanose, than groves that don’t. Melanose affects all citrus cultivars, but is most troublesome on grapefruit. Spray applications should be made in late April or early May.
Greasy Spot is a fungal disease that infects leaves, and occasionally, fruit (greasy spot rind blotch). It can attack all types of citrus, but it’s worst on grapefruit. Blotchy fruit is hard to market, but the most serious damage is caused to the leaves which drop in autumn and reduce the fruit yield the following year.
Alternaria Brown Spot affects Minneola tangelo and Dancy tangerine, as well as Orlando tangelo and Honey and Lee tangerines. Spray applications should be timed to coincide with the emergence of new growth and several sprays may be necessary to control this disease.
Postbloom fruit drop (PFD) is a relatively new disease to Florida. Severity is related to the time of bloom, in relation to rainfall. Cultivars that bloom over long periods and young trees are especially vulnerable.
Symptoms are blotches and distortion of flower petals. The fungus is spread in droplets of rain. The growth of young fruit is prevented. PDF used to be a problem for limes and navel oranges, but several other orange cultivars have shown infestation. Timely application of Pirhana during the blooming period will control PDF.
Pests That Bug Citrus Trees – Annoying Pests Of Citrus
Scales, Whiteflies, and Mealybugs are the most troublesome in the southern U.S. There are many different types of scales—purple scale, red scale, citrus snow scale, glover scale, yellow scale, chaff scale, soft brown scale, black scale, etc. Most of these are armored scales and their name usually betrays the color of the pest on citrus leaves or stems.
Several species of Wasps have been introduced, which attack the scale insects on tree and fruit, thus providing a means of bio-control in the grove. Cottony cushion scale is rarely a problem, due to the activity of two beetles and a parasitic fly. Many native wasps are predators of a range of other scale insects, as are the tiny ladybird beetle and lacewing insects.
Remember, spraying with an insecticide will kill the beneficial insects, as well. Better to spray with a summer oil, such as Advanced Nutrients Genius Oil, which contains organic, concentrated Neem Oil, one of nature’s miracles. It will help control pest infestations.
The Common Citrus Whitefly, which lays yellow eggs, and the Cloudywing Whitefly, whose eggs are black, are serious pests. The damage to the trees are done by the feeding of the larvae stages or nymphs, which are much more like scale crawlers. Spraying with Genius Oil will deprive them of their food supply.
Whitefly larvae, along with aphids, mealybugs, and soft scales produce a sweet secretion called “honeydew,” which serves as a nutrient for the sooty mold fungus, seen as a black film coating on leaves or fruit. Ironically, this serves as a protection from various other pests.
Aphids are a problem, especially when large numbers of them devour young shoots. The Green Citrus Aphid is the most destructive, but the Melon and Black Citrus Aphid do similar injury to new growth. The Brown Citrus Aphid is especially destructive in vectoring CTV.
Grasshoppers may feed on citrus leaves, if drought kills off their food supply in nearby fields. The citron bug, the leaf-footed plant bug, and the stinkbugs often feed on citrus fruit in late September, after their regular food supplies have dwindled. Best control measure is to remove the primary plant hosts from the grove in September, before the citrus fruit become attractive to these bugs.
The Citrus Leafminer (CLM) is a very small nocturnal moth that lays eggs on newly expanding citrus leaves. The eggs hatch into larvae, which tunnel into the leaf tissue leaving white “mine” lines in a pattern. CLM ends up pupating in the leaf margins, causing them to roll up. Biological control is sufficient for this pest, rarely is chemical control needed.
Root Weevils cause severe damage to citrus trees, particularly in the larval stage. Larvae of these weevils feed upon citrus roots. Again, the Advance Nutrients Roots Package will help to control these pesky pests.
Spider Mites come in many varieties—rust mites, citrus red mite, Texas citrus mite, six-spotted mite, and they all have natural enemies. Biological control is much more preferable to chemical control of these insects.
Sometimes the tiniest of bugs are the most destructive. Citrus Canker is caused by a bacterium. In the early nineteen hundreds, a large number of citrus trees were burned in Florida to get rid of this pest. It was unknown in that State from 1926 to 1984, when a Citrus Canker scare caused the burning of millions of nursery trees. Turns out it was a different disease caused by the same organism.
It was spotted again in 1995 and 1997, resulting in quarantine, destruction, and the spraying of trees in several southern Florida counties. The disease has not yet been stopped, but it is hoped that stricter measures will once again put an end to Citrus Canker, at least for another sixty years.