By: Amy Grant
Citrus canker is a financially devastating disease that hasbeen eradicated from the citrus market a couple of times only to return again.During past eradication attempts, thousands of trees were destroyed. Today,mass eradication has been deemed unfeasible, but there is still a quarantineregarding shipping or taking citrus across state lines. So, what exactly iscitrus canker? Read on to learn about citrus canker symptoms and how to treatthe disease should it appear in the home garden.
Citrus canker goes way back to its discovery in Texas in1910 and into Florida in 1914. It was introduced on seedlings imported fromJapan. It is caused by the bacterium Xanthomonascitri and likely originated in southern Asia. The disease can now be foundin Japan, the Middle East, central and southern Africa, and Central and SouthAmerica.
This bacterium is extremely contagious and is fostered whenthere is steady rain combined with high temperatures. Both rainwater andoverhead irrigation spread the bacteria from plant to plant and is then furtherspread by wind, birds and animals, people and machinery.
Asian leaf miners also play a part in the spread of citruscanker. They do not act as vectors but rather cultivate infection and spread ofthe disease through damage caused in the foliage via feeding.
The initial symptoms of citrus canker are raised lesionsthat can be found on both sides of the leaf. They have a crater-like appearancesurrounded by concentric circles. They may have a water-soaked margin and acorky texture. As the disease progresses, the lesions may be surrounded by ayellow halo.
Further into the infection, these halos become shot holes.You may see fungi (white fuzz) and fruiting bodies (black dots) on olderlesions as well. The exact look of the disease varies depending upon the citrustree variety and the length of time the tree has been infected.
How to Treat CitrusCanker
During initial infections in the United States, the onlymethod available for treating citrus canker was to burn infected trees, aneffort first waged by growers and then taken over by the agricultural statedepartments. Rigorous citrus canker controls were instigated wherein infectedtrees were not only destroyed, but all green wood trees were removed within a 50-footradius of those infected. The disease was finally declared eradicated in 1933at a cost of $6.5 million!
Today, with regards to treating citrus canker via chemicals,worldwide the disease is managed with preventive copper-based bactericides.This is generally used in conjunction with cultural practices such as pruningand defoliation of diseased summer and fall shoots and the use of windbreaks.Pruning is also done in the dry season when conditions are less favorable forthe spread of the bacteria.
Other citrus canker control methods include the use ofresistant citrus varieties and the introduction of a USDA quarantine programwith restrictions on taking and bringing fruit into various states. Eradicationhas been deemed unfeasible due to a number of factors, primarily the cost andgeneral uproar by non-commercial growers.
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Read more about Citrus Trees
By Erin Marissa Russell and Matt Gibson
Citrus scab is a fungal disease caused by the pathogen Elsinoe fawcettii and Elsinoe australis which causes the unsightly brown to pale orange wart-like scabs on the leaves, stems, and outer rinds of citrus fruit and citrus trees. The Elsinoe australis pathogen differs from Elsinoe fawcettii in that it only causes symptoms on fruits and not on the foliage of affected trees. As opposed to Elsinoe fawcetti, Elsinoe australis mainly impacts orange and mandarin orange trees. Elsinoe fawcetti can also infect lemons, limes, tangelos, and grapefruits. The disease is sometimes called sour orange scab or lemon scab.
Although the disease is mainly spread by splashing water, it tends to enter the garden via infected nursery stock. When it affects seedlings, citrus scab can make the young plants develop to be especially bushy, and they may experience difficulty with budding.
Though the disease does not render affected fruit inedible, nor damage the interior of the fruit, it is very unsightly. Citrus scab is a particularly pressing issue for citrus sellers because it leads to crop losses and reduction in profit for those who sell their harvests, as infected fruit that is severely symptomatic may be scarred or deformed. In the beginning of spring, citrus fruit producers need to consider preventative methods for avoiding foliar fungal infections like citrus scab in order to keep their crops safe from losing their commercial appeal and value.
Citrus scab is an issue that affects many different types of citrus trees in humid climates, especially in wet subtropical and cooler tropical areas. Outside of these zones, it tends to occur when the time frames of new flush and fruit set overlap with especially warm, wet spells in the weather. Citrus scab can also be a problem wherever orchards are located in low-lying, shady areas, when trees are densely planted, and where the climate is particularly damp or wet.
Because the climate has such an impact on spread of citrus scab, the rates of infection can be extremely variable depending on the weather for that season overall and the local weather conditions. Zones with conditions that are unfavorable to citrus scab include those where annual rainfall is limited to fewer than 1300 mm, where seasons tend to have a temperature over 75 degrees Fahrenheit (25 degrees Celsius) are long-lasting, and where summers are especially dry. Because of these conditions, some of the areas where citrus is commonly grown, including the Mediterranean, California, and Arizona, do not need to be as vigilant against citrus scab as growers in climates that are more hospitable to the disease.
Names link to more information on identification and biology or management.
Ground squirrel girdling and burrows
Identification tip: California ground squirrels can chew bark and cambium virtually anywhere on trunks and limbs. Their burrow entrances (shown here) are open and about 4 inches in diameter, but openings vary considerably. Pocket gophers and moles also burrow in soil, but moles rarely if ever damage citrus.
Statewide IPM Program, Agriculture and Natural Resources, University of California
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Agriculture and Natural Resources, University of California
Mating disruption: A system based on mating disruption has been developed for citrus leafminer control. Thesystem works by emitting high quantities of sexual pheromones that disrupt mating and the males’ ability to find females. Monitoring leafminers is important to develop a program of mating disruption. A product called “SPLAT-CLM” that releases a high quantity of pheromones is commercially available and can be applied oncitrus. A reduction in mating causes a reduction of fertilized eggs and results in fewer larvae.
Attract and kill: Attract-and-kill systems do not eliminate the need for pesticides, but using these systemslowers necessary quantities and minimizes the risk of drift. Attract and kill for citrus leafminer (MalExTM) uses the sexual pheromone as an attractant and imidacloprid to kill males and prevent further mating.