Citrus Trees Under Attack: Greening, Canker, and Borers

The citrus tree is affected by a disease (Citrus Greening Disease) caused by the bacterium ‘Candidatus Liberibacter asiaticus’ which is transmitted (vectored) by the Asian Citrus Psyllid (ACP) insect (image: Citrusalert.com).

By John Green, Certified Texas Expert Gardener, Orange County, TX

It was almost a year ago that the U.S. Department of Agriculture’s (USDA) Animal and Plant Health Inspection Service (APHIS) requested assistance from South Texas residents due to an invasive fruit fly (Anastrepha ludens) that is migrating from Mexico and Central America. Residents were asked to inspect fruit trees in suburban yards and commercial properties, looking for signs of invasive fruit flies. If noted, residents should report signs of invasive fruit flies and other citrus diseases to local county agriculture officials.

Currently, the Lower Rio Grande Valley (Cameron, Hidalgo, Starr, Webb, Willacy, and Zapata counties) are at heightened risk of being infested by these invasive and damaging marauders. The fruit fly is innocuous to people but feed on more than 40 types of fruits and vegetables. Invasive fruit fly larvae can infest homegrown fruit maturing into adult flies, then migrating into commercial areas, destroying harvests.  All SETX gardeners should take note of this ‘new’ invasive pest. Fruit and citrus trees are under constant threat, requiring gardeners to be vigilante. Inspect fruit and citrus trees often (multiple times during the seasons) to maintain the health of the tree and notice problems early, then report questionable findings to the local agricultural extension office for assistance and guidance. Agricultural personnel will place ‘traps’ to monitor citrus trees and find new fruit fly introductions early. We can assist agricultural officials eradicate invasive fruit flies by providing them property access to inspect trees and hang traps, and if detected, allow officials permission to conduct treatments.

Preventative Measures:

• Remove mature citrus fruit (including fruit fallen to the ground) quickly
• Discard over ripe fruit (rotting) fruit by double-bagging before placing in the trash
• Never compost fruit or vegetables from quarantined areas
• Dispose of tree trimmings into plastic bags (we reside in a quarantine area)
• Never relocate or mail homegrown fruit or plants from quarantine areas
• Exercise caution when purchasing fruit from backyard vendors or social media
• Declare all agricultural products before entering the US from another country
  
  

There are numerous other pests and diseases which adversely affect citrus trees, such as Asian Citrus Greening, Citrus Canker, Citrus borers which are common in SETX. It’s important to note Texas counties (nearby) which are under citrus quarantine: Harris, Montgomery, Galveston, Brazoria, and Fort Bend. Home-grown citrus located within quarantine ‘zones’ must be consumed within the zone and citrus trees cannot be transported outside of the quarantined area.

Commercial citrus retailers within the State of Texas are required to demonstrate they follow Federal and State quarantine guidelines. Gardeners, before buying citrus trees, ask questions of the seller, validating adherence to Federal and State guidelines before purchase.

Asian Citrus Greening is caused by the bacterium ‘Candidatus Liberibacter asiaticus’ and is transmitted (vectored) by the Asian Citrus Psyllid (ACP), a tiny insect (about ⅛ inch). The ACP nymphs are yellow orange in color and feed on new growth, leaving behind a waxy fluid. Once a tree is infected, the tree can remain with undetectable symptoms for months to years. Unfortunately, during the ‘symptomless’  phase, the tree serves as a source of bacteria to infect other trees. Leaves become yellow, blotchy, and mottled, with raised veins and a corky appearance.  Fruit will taste bitter and might be stunted, or possibly lopsided, remaining green or partially green, often falling from the tree before ripening. Once a tree is infected, there is no effective control or cure for the disease.  This disease poses no threat to humans but can destroy all types of citrus trees, including orange, grapefruit, lemon, lime, kumquat, tangerine, and others.

Citrus canker appears on leaves as small, pimple-like spots which enlarge to become brown & corky with sunken centers. This highly contagious bacterial disease (Xanthomonas citri subsp. Citri) causes lesions to form on leaves, stems, and fruit that lead to premature fruit drop and diminished fruit quality (image: Citrusalert.com).

Citrus Canker (another incurable bacterial disease) causes lesions on citrus fruit, stems, and leaves are highly contagious. It is spread by wind-driven rain, unsanitary equipment, and via people who can transmit the infection on their hands, and clothing by handling infected plants or plant parts. Citrus canker is spread long distances by movement of infected plants or plant parts.

Extreme weather events can spread Citrus Canker extended distances, unfortunately, the bacteria can survive for up to 10 months in lesions on living citrus plants. Naturally, the bacteria thrive in our SETX growing environment, entering the host (citrus tree) via leaf pores or injury caused by insects (leaf miners or citrus borers). After bacteria enter the tree, a small lesion becomes visible which appears as a raised blemish affecting both leaf surfaces but mainly the leaf’s underside. The blemish further develops a yellow halo with a water-soaked margin then becoming corky. Leaf lesions are small (about ⅟₁₆  – ½ inch in diameter). Leaves and young green stems are decidedly susceptible to infection. Trees can survive for long periods with diseased plant tissues, which cause discoloration of the bark. Prevention is the best option to protect against citrus canker. Canker causes the tree to continually decline in health with reduced fruit production until all fruit production stops. Citrus canker is highly contagious and can be spread rapidly.

Preventative Measures:

• Do not relocate citrus trees (rootstock, seedlings, or budded trees from quarantined areas
• Purchase trees from growers certified by Texas Department of Agriculture (TDA)
• Impeccable plant hygiene requires equipment decontamination to reduce disease spread
• Remove/ destroy diseased trees eliminating bacteria for future infections
• Closely monitor citrus trees taking swift action if/ when infections appear
• State and Federal mandates disposal of infected trees and plant material
• Copper based chemicals may help prevent infection but will not stop the disease

The lemon tree borer can damage not only citrus trees but many tree types. Adults lay eggs in damaged areas of the tree bark. After eggs hatch, they bore into the ‘living’ wood causing damage and continue boring holes to the trees surface to expel waste (image: kiwicare.co.nz).

Citrus trees are host to several wood borer insects and controlling them can be challenging. Gardeners need to closely inspect citrus and fruit trees (trunks and limbs). Identifying the type of borer attacking the tree is a critical step. Their presence is witnessed by entry and exit holes (bores) in the tree bark or limb. Borers produce ‘sawdust-like’ material called frass (excrement) where they have bored. Witnessing adults flying about the tree is a sign there is an infestation!

If borers only appear on one area of the tree, remove the affected area until living tissue is exposed, and discard affected material by incineration to prevent further infestation. Never compost infected tree or plant materials! The appearance of borers is often associated with an unhealthy tree. Normally borers attack diseased, dead, or dying limbs on trees. Removing diseased, or dead wood quickly can aid in reducing borer infestation.

Spinosad insecticides are organic and are most often recommended for use on citrus trees to control borers. Using insecticides to eradicate borers can be challenging since the borers are shielded beneath the tree’s bark.

Common Borers:

• Citrus Fruit Borer (Citripestis sagittiferella)- moth’s larvae bore into the fruit causing damage and abnormalities
• Citrus Rind Borer (Prays endocarpa)- moth’s larvae ‘mine’ the rind of citrus fruits causing swelling and pocks that can deform the fruit
• Lime Tree Borer (Chelidonium cinctum)- serious pest of lime trees with larvae boring into the stems and branches
• Citrus Stem Borer (Zeuzera coffeae)- larvae bore into the stems and branches of citrus trees causing damage and often death
• Locust Borer (Megacyllene robiniae)- typically attack black locust trees but are known to damage citrus trees (in our area) causing weakened branches, dieback, and even death (if heavily infested)

If you suspect a citrus tree to have Asian Citrus Greening, Citrus Greening, or Invasive Fruit fly infestation report observations to the Texas Department of Agriculture (TDA) at (800) 835-5832 or online at http://www.citrusalert.com/report-a-tree for further assistance.

So long for now fellow gardeners, let us go out and create ourselves a greener, more sustainable world, one ‘citrus tree’ at a time! For answers to your gardening questions, send me an email: jongreene57@gmail.com.