Citrus Trees: Recognizing Greening and Canker Diseases
Last updated 5/2/2023 at 8:28pm
Gardeners, this week we are discussing Asian Citrus Greening and Citrus Canker and how we can recognize each of the two citrus diseases. There are numerous plant diseases that may attack citrus trees, but in our area of SETX most notably are Asian citrus greening and citrus canker. The Texas Department of Agriculture (TDA) and US Department of Agriculture (USDA) have allotted sizable resources and undertaken massive efforts to control the spread of both these devastating diseases. It's important to note that Texas counties where the diseases have appeared are quarantined, including counties near to us: Harris, Montgomery, Galveston, Brazoria, and Fort Bend. Home-grown citrus located within quarantine 'zones' must be consumed within the zone and home-grown citrus trees cannot be transported outside of the quarantined area.
Commercial citrus nurseries and retailers within the State of Texas must be able to demonstrate they are following Federal and State quarantine guidelines. Before buying citrus trees, gardeners should ask questions of the seller, to validate the seller is following Federal and State guidelines before purchasing citrus trees.
Citrus Greening is a bacterial disease (Candidatus Liberibacter Asiaticus), spread by the Asian citrus psyllid (ACP). "Yellow dragon disease" as it's called, is found worldwide with confirmed cases discovered in Texas (2014). There is no cure for this disease! It has devastated millions of acres of citrus production around the world.
Citrus trees infected by citrus greening often have the Asian citrus psyllid. The adult insect is small (about 3mm or ⅛ inch) with a distinctive 45-degree angled posture when present on leaves. The ACP nymphs are yellow orange in color, smaller, 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 will become yellowed, blotchy, or 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.
Citrus Canker is another incurable bacterial disease which causes lesions on citrus fruit, stems, and leaves. While the disease isn't harmful to humans, but it is highly contagious spread by wind-driven rain, through unsanitary equipment, and by people who can transmit the infection on their hands, clothing or by handling infected plants or plant parts (fruit, leaves, or stems). Citrus canker is spread long distances by movement of infected plants or plant parts including nursery trees and propagation material (budwood, rootstock seedlings and budded trees). Severe weather events, such as hurricanes, easily spread Citrus Canker long distances. Unfortunately, the bacteria can survive up to 10 months in lesions on living citrus plants. The bacteria thrive in our warm, moist environment entering the citrus tree (host) through leaf pores or wounds, caused by insects, such as those caused by leaf miners. In a short amount of time (7 to 10 days) after entering the tree, a pinpoint lesion becomes visible, and appears as a raised blemish, which affects both leaf surfaces, but mainly the leaves underside. The blemish develops a yellow halo and a water-soaked margin, which becomes corky, with a raised margin and submerged center. Leaf lesions are usually about ⅟16 – ½ inch across. Leaves and young green stems are extremely susceptible to infection. Trees can survive for long periods with diseased plant tissues, which cause discoloration of the bark.
As there are no cures for either disease, as gardeners we must rely on prevention by following the steps below:
• Do not purchase or transport citrus trees, budwood, rootstock, seedlings, or budded trees from quarantined areas.
• Purchase certified plants from Texas Department of Agriculture (TDA)-certified citrus growers and nurseries only!
• Practice overall general cleanliness and decontaminate equipment by using alcohol-based sanitizers to reduce disease spread.
• Remove & destroy diseased trees eliminating potential bacteria for future infections. In my opinion, this is by far the most difficult step, as I have removed 7 citrus trees due to these diseases.
• Monitor nearby citrus trees and act swiftly if new infections appear. TDA regulations mandate disposal of infected trees and plant material by incineration or bagging, burying material at least 2 feet deep at a municipal landfill.
• Copper based chemicals may help prevent infection but will not stop the disease from occurring. Application timing is critical to provide protection since new growth tissues are the most susceptible to infection. Multiple applications will be necessary to ensure proper coverage of the tree. Read application directions and follow instructions before applying any chemical control agent.
If you suspect that a citrus tree has Citrus Canker or Citrus Greening, report the tree to 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's go out and grow ourselves a greener, more sustainable world, one plant at a time!
Have gardening questions, send questions by email to: [email protected] or [email protected]. Or phone the Orange County Master Gardeners (OCMG) Helpline: (409) 882-7010. Interested in becoming an OCMG, visit our website: https://txmg.org/orange or see us on Facebook: Orange County Texas Master Gardeners Association.