By: Darcy Larum, Landscape Designer
While growing citrus fruits at home is usually a very rewarding activity, things can sometimes go wrong. Like any plant, citrus trees have their own specific diseases, pests and other issues. In this article, we will go over the common reasons why twig dieback of citrus trees may occur.
Citrus twig dieback can be caused by common environmental conditions, disease or pests. One simple reason for any citrus dieback, including twig dieback, limb decline, and leaf or fruit drop, is that the plant is stressed from something. This could be a pest infestation, disease outbreak, old age or a sudden environmental change such as drought, flooding, or extensive root or storm damage. Basically, it is a plant’s natural defense mechanism so that it can survive whatever threat it faces.
In old, large citrus trees that have not been properly maintained, it is not uncommon for top branches to shade out lower branches. This can cause lower limbs to experience problems such as citrus limb dieback, leaf drop, etc. Shading out or overcrowding can also create an ideal environment for pests and diseases.
Yearly pruning of citrus trees can help prevent this by opening up the tree’s canopy to let more sunlight in and improve air circulation. Dead, damaged, diseased, crowded or crossing limbs should be pruned out annually to improve citrus health and vigor.
In the last few years, citrus growers in California have experienced a major outbreak of citrus twig dieback. As consumers, you have probably noticed an increase in the cost of some citrus fruit. This outbreak has severely impacted the yields of citrus growers. Recent studies have concluded that this twig dieback of citrus plants is caused by the disease pathogen Colletotrichum.
Symptoms of this disease include chlorotic or necrotic foliage, thinning of citrus crowns, excessive sap secretion and twig and shoot dieback. In severe cases, large limbs will dieback. Though this is a disease, it is likely spread by insect vectors.
Steps being taken to control the disease in citrus orchards include pest control and the use of fungicides. This disease is still being studied to determine the best control and management options. “The acute toxicity of fungicides to humans is generally considered to be low, but fungicides can be irritating to the skin and eyes. Chronic exposures to lower concentrations of fungicides can cause adverse health effects.” extension.psu.edu
Note: Any recommendations pertaining to the use of chemicals are for informational purposes only. Specific brand names or commercial products or services do not imply endorsement. Chemical control should only be used as a last resort, as organic approaches are safer and more environmentally friendly.
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Read more about Citrus Trees
You can revive a waning citrus tree by establishing a regular care schedule and performing routine maintenance.
Also, what's wrong with my citrus tree? Overwatering: Citrus may become stressed and more susceptible to pests and diseases as a result of poor drainage or standing water. Trees may also become chlorotic as a result of wet or waterlogged soils, anaerobic soils, root rot diseases, or damaged roots.
Also question is, how do I know if my lemon tree is dying?
Scratching into the bark on branches or the trunk to 1/16-inch deep to 1/8-inch deep reveals a green, damp layer on lemon trees that are still alive. Dead bark shrinks tightly and cracks.
How do you fix yellow leaves on citrus trees?
When the tree runs out of nutrients its older leaves may become yellow. Apply a complete citrus food twice a year aiming for mid summer and late winter. Water well before and after applying fertiliser. Spread the fertiliser over the soil around the tree.
Names link to more information on identification and management.
Phytophthora root rot
Identification tip: Causes of leafless, dead branches include citrus red scale, inappropriate irrigation, fungi, and Tristeza virus. When Phytophthora citrophthora or certain other fungi are the cause, bark may exude resin.
Dry root rot
Identification tip: Before tipping over, this tree exhibited pale foliage and an unusually heavy crop of lemons. A girdling canker on the lower trunk (not shown here) and the absence of any oozing gum are other indications that the cause is infection by Fusarium solani.
Bark oozing, cracking, peeling or distorted growth
Identification tip: Cracked, dry bark on the lower trunk may be due to Dothiorella gummosis, Exocortis, Hendersonula tree and branch wilt, Psorosis, or infection by Phytophthora spp. Look for other symptoms, such as discoloration beneath cankered bark and the presence of oozing gum, and have samples tested by a laboratory to help you diagnose the cause.
Identification tip: Phytophthora gummosis is the most common cause of profuse dark exudate from bark. Dothiorella gummosis, Hendersonula tree and branch wilt, and Psorosis also produce gum. But sometimes there is no obvious oozing when these diseases are present.
Identification tip: Psorosis is due to a viral infection that causes a scaling and flaking of bark on the scion. It cracks and peels bark high up the tree, but Psorosis does not cause symptoms below the graft.
Photograph not available.
Hendersonula tree and branch wilt
Identification tip: Infection by Nattrassia mangiferae (=Hendersonula toruloidea) causes bark cracking and peeling or dead bark that remains tightly attached to dead limbs. Black, sooty growth may develop beneath infected bark injured limbs may bleed profusely leaves on infected limbs suddenly wither, turn brown, and dry up. Dead leaves typically remain attached to the twigs.
Identification tip: Cracked, bark that peels off (bark shelling) in small pieces, but only on old trees, is characteristic of Exocortis. Damage is limited to around the root crown. In comparison, Phytophthora gummosis affects trees of any age and damage often extends from the soil to several feet up the trunk. If Hendersonula tree and branch wilt is the cause, bark cracking can occur even higher on the trunk and also on limbs.
Vein enation (woody gall)
Identification tip: The cause of this gnarled bark on trunks is unknown. Small bumps also develop on leaves (vein enation). An a aphid-vectored virus is one suspect. This malady is rare as it is eliminated during propagation.
Discolored wood or cankers beneath bark—Top of page
Identification tip: Sunburn cankers are limited to outer branches exposed to direct sunlight, usually in the south or west portion of trees. Mechanical injury cankers can occur at any location where bark is impacted by equipment or tools. The location of pathogen cankers does not depend on sun exposure.
Identification tip: Bark cankers and limb dieback may not appear until weeks after cold weather. More immediate symptoms include fruit drop and brown dead leaves that remain attached, causing cold-damaged trees to appear scorched.
Bud union disorder
Identification tip: Cut-away bark (the two pale squares) reveals a dark horizontal line paralleling the uneven growth where the rootstock and scion meet. This "crease" at the bud union is a delayed symptom of genetic incompatibility.
Dry root rot
Identification tip: Cut under bark where the lower trunk is sunken and discolored. Wood infected by Fusarium solani will be dark and discolored, in contrast to the healthy greenish white wood, as shown adjacent here. Dry root rot's discoloration extends deeply into wood. It does not produce oozing gum.
Identification tip: Cutting underneath to expose the inner bark and cambium reveals discolored, yellowish brown wood. Unlike dry root rot, Dothiorella discoloring is lighter and infected bark may ooze dark liquid. On the surface, Dothiorella cankers can have a grayish cast with dead bark that remains tightly attached.
Identification tip: Pits in wood and bark, and brown discoloration and gumming in the phloem underneath pits, are symptoms of Cachexia. This disease is rare as the viroid is eliminated during propagation.
Mushrooms or fungal mycelia
Armillaria root rot mushrooms
Identification tip: During the rainy fall and winter, short-lived mushrooms often grow around the base of Armillaria-infected trees, such as this almond.
Armillaria root rot mycelia
Identification tip: The most reliable sign of Armillaria root rot is large white fan-shaped mycelia plaques growing beneath bark. When the entire tree aboveground declines, exposing the root crown and cutting under bark may reveal Armillaria mycelium.
Photograph not available.
Hyphoderma gummosis fruiting body
Identification tip: Pink to white fungal growth of Hyphoderma sambuci appears around wounds after wet weather. Reported in the field only on lemon, this wood decay fungus causes branch wilting and dieback that ultimately results in tree death.
Statewide IPM Program, Agriculture and Natural Resources, University of California
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Agriculture and Natural Resources, University of California
Citrus leafminer (Phyllocnistis citrella) tunnels through the young leaves of citrus trees which creates silvery lines. Finally, it curls the leaf into a shelter and pupates within. This can severely distort the leaves, but mature trees are not likely to be seriously damaged. To protect young trees, spray new growth thoroughly with Searles Pest Gun .
Watch out for these brightly coloured bronze orange bugs on citrus trees and flowers. They suck the sap from young stems and damage new growth. They give off a bad smelling substance when threatened so its best to spray them with an approved systemic insecticide to control their numbers.
Small female wasps lay her eggs inside the branch of a citrus when the weather starts to warm up in spring. By summertime new tiny wasps escape from the swollen growth leaving the branch deformed. The citrus gall wasp does not directly kill the citrus tree but when repeated attacks occur it severely deforms the tree branches inhibiting normal growth. If you see lumps starting to appear cut the branch off promptly and remove the affected branch away from the tree.
Scale found on the leaves and stems of citrus are from sap sucking insects laying their eggs underneath the protection of a hard waxy dome shell. Once the immature ‘crawlers’ hatch they spread and multiply rapidly. Severe infestations can lead to branch dieback, leaf drop and yellowing of the leaves. Control red scale, white and pink wax scale with an organic oil spray Searles Ecofend Natural Solutions Fruit & Garden . Some soft scale, such as white wax scale and black scale secrete a sticky like substance ‘honeydew’ which then attracts the fungus sooty mould and ants to the plant. The ants protect the scale from predators, letting them proliferate even further. Treat the ants first.
Melanose is a fungus that can multiply quickly in wet weather. Little dark brown, raised spots appear on immature leaves, twigs and fruit. In severe infestations, Melanose can cause fruit disfigurations and wood rot. Remove dead wood from your citrus where the spores lay and spray with Searles Copper Oxychloride when fungus is first sighted.
The small and large citrus butterfly caterpillar can strip citrus trees of their leaves and produce a strong foul odour when disturbed. Though, this smelly, spiky and unattractive caterpillar will turn into a beautiful butterfly. If infestation is severe, spray tree leaves and branches with a natural pyrethrum insecticide . Alternatively, you can handpick them and squash them if you can handle the smell.