Aphids do little damage in small numbers. They are extremely prolific, however, and populations can rapidly build up to damaging numbers during the growing season. Damage occurs when the aphid sucks the juices from citrus leaves. The aphid is unable to digest fully all the sugar in the sap, and it excretes the excess in a fluid called honeydew. The honeydew often drops onto the leaves and fruit below. Plants or objects beneath the tree may also be coated with honeydew. A sooty mold fungus may develop on the honeydew, causing the leaves and fruit to appear black and dirty. Ants feed on this sticky substance and are often present where there is an aphid infestation. Repeated aphid attacks may greatly slow the growth and development of young citrus trees.
Apply an insecticide labeled for aphids as soon as the insects appear. Repeat as directed on the label if the plant is reinfested.
Dialeurodes citri This tiny sucking insect lays eggs on the undersides of leaves, where they hatch into flat, oval-shaped, semitransparent larvae. The larvae feed by sucking the plant juices from the leaves. Citrus whiteflies cannot fully digest all the sugar in the plant sap, and they excrete the excess in a fluid called honeydew, which often drops onto the leaves below. A sooty mold fungus may develop on the honeydew, causing the leaves to appear black and dirty. Several generations of citrus whiteflies occur each year, with peaks of activity in March-April, June-July, and September- October. In the past, this was one of the most serious pests of citrus in Florida. Recently a parasitic wasp (Encarsia lahorensis) has been introduced and is succeeding in controlling the whitefly.
Apply a horticultural oil or insecticidal soap as directed on the label.
Citrus plants are frost tender and are easily damaged by temperatures below 32°F. Although many citrus plants can recover from a light frost, they cannot tolerate long periods of freezing weather. Damage to fruit occurs when the juice-filled cells freeze and rupture. The released fluid evaporates through the rind, leaving the flesh dry and pulpy. In addition to causing leaf and twig dieback, temperatures of 20°F and lower promote bark splitting, which may not become apparent for several weeks or months.
In danger of frost, cover trees with fabric. If possible, place a lamp under the cover. Turn the light on during cold nights. Insulate trunks and main limbs of young trees with cornstalks or other material. Do not shade the foliage. Keep the soil moist during a freeze, Limit fertilizer. Remove damaged fruit immediately following the freeze. After the danger of frost is past, prune blackened shoots and withered foliage. Wait for frost damage to the trunk and main limbs to appear, then prune out the deadwood.
Citrus trees frequently suffer from deficiencies of iron and other minor nutrients such as manganese and zinc, elements essential to normal plant growth and development. Deficiencies can occur when one or more of these elements are depleted in the soil. Often these minor nutrients are present in the soil, but alkaline soil with a pH of 7.5 or higher or wet soil conditions cause them to form compounds that cannot be used by the tree. An alkaline condition can result from overliming or from lime leached from cement or brick. Regions where soil is derived from limestone or where rainfall is low usually have alkaline soil. Some citrus trees turn yellow naturally in cold weather, but if iron is available, the foliage will turn green again when the weather gets warmer.
Improve soil drainage. Feed trees with a fertilizer formulated for citrus.
These pests, related to spiders, are very damaging to all types of citrus. Several species of mites attack citrus, including citrus red mites, citrus bud mites, purple mites, and citrus rust mites. Mites cause damage by sucking sap from the leaves and young fruit. As a result of their feeding, the plant''s green leaf pigment disappears, producing a yellow, stippled appearance. Mite webbing traps cast-off skins and debris, making the plant messy. Feeding damage also causes tissue death, resulting in browning and silvering of the fruit and foliage. Mites are active throughout the growing season but are favored by hot, dry weather (70°F and up). By midsummer, they have built up to tremendous numbers. A severe mite infestation weakens the plant and can seriously reduce the size and quality of the fruit.
Apply an insecticide labeled for mites, following the directions on the label.
This plant disease, also known as quick decline, is caused by a virus that affects the rootstocks of citrus plants. Aphids, especially the brown citrus aphid, spread the virus to trees grafted onto susceptible rootstocks. Orange and grapefruit trees grafted onto sour orange rootstocks are especially susceptible to this virus. The virus attacks the food-conducting vessels in the rootstock bark, impeding the flow of nutrients and causing the starvation and death of the root system. Trees often live for several years after they have been infected but continue to decline in vigor and fruit yield. Sometimes trees die rapidly after infection, especially during periods of drought.
Remove infected trees. Replace with citrus trees on rootstocks tolerant of tristeza. Gardeners with citrus trees on sour orange rootstock in Florida should consider planting replacement trees on tristeza-resistant rootstock.