2009 Florida Citrus Pest Management Guide: Citrus Canker¹

M.M. Dewdney and J.H. Graham²

Citrus canker, caused by the bacterium Xanthomonas axonopodis pv. citri, is a leaf, fruit, and stem blemishing disease that affects most citrus. Grapefruit, Mexican lime, and some early oranges are highly susceptible to canker; Navel, Pineapple, and Hamlin oranges, as well as, lemons and limes are moderately susceptible; mid-season oranges, Valencias, tangors, tangelos, and other mandarin hybrids are less susceptible; and tangerines are tolerant.

Symptoms. Young lesions are raised on both surfaces of the leaf, particularly on the lower leaf surface. The pustules later become corky and crater-like with raised margins, sunken centers and surrounded by a yellow halo. Fruit lesions vary in size because the rind is susceptible for a longer period of time and more than one infection cycle can occur on fruit. Stem lesions can support long-term survival of the bacterium. Older lesions may darken when they become colonized by saprophytic fungi such as Colletotrichum spp.

Major outbreaks of citrus canker occur when new shoots are emerging or when fruit are in the early stages of development. Frequent rainfall in warm weather, especially storms, contributes to disease development. Citrus canker is mostly a cosmetic disease, but when conditions are highly favorable for infection, it causes defoliation, shoot die-back and fruit drop. The Asian leafminer wounds the leaves which can be contaminated by the bacterium. This allows the number and size of individual lesions to greatly increase and results in many times the inoculum compared to infections in the absence of leafminer.

Biology. The bacterium reproduces in lesions on leaves, stems and fruit. When there is free moisture on the lesions, the bacteria ooze out and can spread to new growth and other trees. Wind-driven rain is the main dispersal agent, and wind speeds >18 mph aid in the penetration of bacteria through the stomatal pores or wounds made by thorns, insects and blowing sand. Tissues become resistant to infection as they mature. Almost all leaf and stem infections occur within the first 6 weeks after initiation of growth. The most critical period for fruit infection is when the fruit reach 1/2 inch in diameter. That is the stage when the stomates on the fruit surface are opening and fruit are particularly susceptible to bacterial penetration. After petal fall, fruit remain susceptible during the first 60 to 90 days for oranges or tangerines and 120 days for grapefruit. Infection after this time may result in the formation of small and inconspicuous pustules.

Most spread of canker bacteria by wind and rain is over short distances, such as, within trees or to neighboring trees. Canker is more severe on the side of the tree exposed to wind-driven rain. Spread over longer distances, up to miles, can occur during severe tropical storms, hurricanes, and tornadoes. Long-distance spread occurs more commonly with the movement of diseased plant material, such as budwood, rootstock seedlings, budded trees, or less commonly fruit and leaves. Workers can carry bacteria from one location to another on hands, clothes and equipment. Grove equipment can spread the bacteria within and among plantings, especially when trees are wet.

Management. The 1900-ft rule was suspended in January 2006 and eradication of citrus canker- affected trees ended. The new Citrus Health Response Plan (CHRP) does not require removal of affected trees. Thus, growers should use their best judgment in management of citrus canker. The entire state of Florida is under quarantine, and fruit movement is subject to specific regulations based on market destination.

Although it is difficult to predict exactly how severe canker losses will be under Florida conditions, indications from outbreaks in the state are that it will be difficult to control on grapefruit and the most susceptible early season orange varieties. Areas that are currently canker-free should be protected to the extent possible.

Protecting Canker-Free Areas

Decontamination. Where canker is absent, decontamination protocols are still in place and should be followed. With more canker around the state, the likelihood of further spread is greater than ever. In moving equipment and personnel from grove to grove, every effort should be made to make sure that plant material is not moved inadvertently and that all equipment has been thoroughly decontaminated. Decontamination is especially important in harvesting operations, hedging and topping, and in any other practices involving extensive contact with foliage. Obviously, when equipment is moved from blocks where canker is endemic to other infected blocks, decontamination serves little purpose.

Tree removal. If canker is detected in areas previously free of the disease, removal and burning of trees on site may slow the establishment of the disease. For tree removal to be effective, canker has to be localized and limited to a small number of trees. Tree removal is not likely to be effective if canker is already present within a mile of the grove. Before tree removal is attempted as a control measure, blocks should be thoroughly inspected to be sure that canker is not more widespread than initially thought. All infected trees as well as some apparently healthy trees surrounding the infested area should be removed. More trees should be removed if the focus is large, but if the affected area is very large, tree removal may need to be reconsidered. This measure is unlikely to eradicate the disease, but can potentially slow disease development. Tree removal must be followed by monthly inspections and removal of any further trees found positive for the disease. At some point, tree removal will no longer be economically sustainable and should be discontinued.

Defoliation and pruning. There are currently no registered defoliants, but it is possible to defoliate trees using high concentrations of legal copper or fertilizer products. However, no rates or spray volumes have been established for this practice. The results of chemical defoliation are highly variable depending on chemical rate, spray application method, tree age, water relations and environmental conditions at time of application. The results for the same rate and application method can vary from incomplete defoliation to severe dieback of brown wood. Defoliation may be useful in areas surrounding foci of infected trees that have been removed. These trees may appear healthy, but are likely to harbor undetectable canker lesions. Defoliation can eliminate this inoculum and save many trees. Severe pruning or buckhorning has met with some success as long as strict sanitation procedures are followed in the removal and disposal of the infected plant materials. Defoliation or pruning should only be attempted during dry times of the year and in conjunction with an intense inspection program. Removal of canker-infected trees and tissues is not likely to completely eliminate the disease. A strong flush of highly susceptible leaves will follow and is likely to become infected from residual inoculum in the tree or nearby infested groves. Following defoliation or pruning, the new growth flush should be treated with copper products once the growth is half expanded to protect it from new infections.

Endemic Canker. Where canker is already endemic, the primary means of control are: 1) planting of windbreaks, 2) protection of fruit and leaves with copper sprays, and 3) control of leafminer.

Windbreaks. Windbreaks are highly effective in reducing the spread of canker, but more importantly, they reduce the severity of the infection in endemic situations. When canker lesions are wetted, millions of bacteria ooze onto the leaf surface. While bacteria can swim very short distances, they have no active means to penetrate the tissues. The vast majority of the infection occurs by wind-blown rains. Winds of 18 to 20 mph are needed to actually force bacteria into the stomates on leaves and fruit.

Windbreaks are the single most effective means of dealing with canker. In our observations in Argentina, the number of canker lesions was ten times greater on the side of the tree exposed to the prevailing winds than on the protected side of the same tree. In tests in nursery situations, artificial windbreaks greatly diminished the distance of spread of canker down the nursery row and reduced disease to only a few scattered lesions.

Windbreaks reduce wind speed for a distance ten times the height of the windbreak. That is, a 30-ft tall windbreak will exert an effect for about 300 ft. To be effective for canker control, windbreaks do not need to be dense. All that is required is to reduce wind speed to less than 20 mph. The need for windbreaks and the distance between rows will depend on the destination of the fruit, fresh or processed, and cultivar susceptibility. Fresh market grapefruit in Florida will likely need a windbreak that surrounds each 5- to 10-acre block. In many groves of less susceptible cultivars, a windbreak down the row about every 300 ft may be sufficient. In situations where some protection exists and tolerant varieties are grown for processing, additional windbreaks may be unnecessary. Additionally, not topping outside rows of citrus can also serve as a viable, harvestable windbreak. Currently, the recommendation is that growers plant windbreaks along fence lines, ditches, around wetlands, or wherever they can plant without removing citrus trees. If it becomes obvious that more windbreak protection is needed, rows of citrus or end trees can be removed to accommodate more windbreaks.

For more information on selection of plant species and design, see the CREC Web site (www.crec.ifas.ufl.edu/extension/windbreaks/ index.htm).

Copper sprays. Over the last 30 years, IFAS has evaluated dozens of products for canker control in several projects in Argentina and Brazil. Products such as antibiotics, compounds that induce resistance in plants and disinfectants provide limited canker control, but no material has proven more effective than copper products.

Copper products are quite effective in preventing infection of fruit, but much less effective for reducing leaf infection. Application of copper to young leaves protects against infection, but protection is soon lost due to rapid expansion of the surface area. Also, copper has limited value in reducing spread of the disease. Fruit grows more slowly and is easier to protect. Orange fruit is susceptible to infection after the stomates open, when the fruit is about 1/2 inch in diameter, until they develop resistance in mid to late July. Grapefruit is susceptible from the 1/2-inch size to full expansion in late September to mid October. Infection through wounds can occur at any stage of fruit growth.

Programs needed for effective control of canker in Florida have not been fully determined. However, we believe that most of the infection of oranges will occur from April to July. With endemic canker, five copper sprays are recommended for early oranges grown for processing applied at 21-day intervals: one in early April (fruit at 1/2-inch stage); a second in late April, a third in mid-May, a fourth in early June and a fifth in late June to early July when the fruit is about 1/2-inch diameter. Three applications at a 21-day interval should be sufficient for Valencias and midseason varieties, in mid-April (fruit at 1/2-inch stage), and in early/mid May. Varieties of early oranges grown for higher color score (Early Gold, Westin, Ruby, Itaborai) are more susceptible than Hamlin and may require additional sprays before April and beyond July.

Programs for fresh fruit are more complex, but many copper sprays are already used on these varieties. For fresh market grapefruit, a low rate of copper should be added to the spray of spring flush for scab. Subsequently, the copper spray program used for melanose control should also control canker, but additional applications will be required every 21 days when the fruit reach 3/4-inch size until fruit are fully grown in October. Copper may need to be added to applications of fungicides or petroleum oil.

Most tangerines are fairly tolerant to canker. Programs used for control of Alternaria should also protect against canker, but copper will have to be used in each spray. Navel oranges are more susceptible to canker and will probably need to be sprayed every 21 days from early April to mid-July. Fallglo is more tolerant and probably three sprays in April, May and June should suffice. Newly planted trees in canker exposed settings are more susceptible because they produce leaf flushes more often and the flush tissue represents a high proportion of the canopy volume. The recommendation for the more susceptible varieties (grapefruit and early oranges) is that the trees be sprayed every 3 to 4 weeks to coincide with vegetative flush cycles from spring though the fall. Sprays should be applied with a hoop sprayer that thoroughly covers the foliage on all sides of the canopy.

Spray programs for young and fruiting trees will have to be adjusted as more experience is gained. The rates of copper products depend on the length of protection expected and the weather. As little as 0.5 to 1.0 lb of metallic copper will protect spring flush growth or fruit during the dry spring season. However, in the rainy season, more than 1 lb of metallic copper may be required to protect fruit for 3-week periods.

To the extent possible, copper usage should be minimized since this metal accumulates in soil and may cause phytotoxicity to the fruit peel, or create environmental concerns.

Leafminer control. Leafminers do not spread canker, but extensive invasion of leafminer galleries by the bacterium greatly increases inoculum levels making the disease difficult to control. Leafminers are not usually a problem on the spring flush and no control is needed at that time. Leafminer control on the first summer flush can reduce disease pressure considerably. If properly timed, applications of petroleum oil, Agri-mek, Micromite, Spintor, or Assail will reduce damage by leafminer. Late summer flushes tend to be erratic and effective control at that time will probably be more difficult. (See ENY-604 section of this Guide on Soft-Bodied Insects Attacking Foliage and Fruit.)

The citrus canker situation and the rules and regulations involving canker are changing rapidly. For current information on disease status and regulations, see the website of the Florida Department of Agriculture and Consumer Services:

www.doacs.state.fl.us/pi/canker/index.html

or the CREC Web site:

canker.ifas.ufl.edu

Contact your county agent for additional information, training materials, and programs.

Recommended Chemical Controls

READ THE LABEL.

See Table 1.

Rates for pesticides are given as the maximum amount required to treat mature citrus trees unless otherwise noted. To treat smaller trees with commercial application equipment including handguns, mix the per acre rate for mature trees in 125 gallons of water. Calibrate and arrange nozzles to deliver thorough distribution and treat as many acres as this volume of spray allows.

Table 1. Recommended Chemical Controls for Citrus Canker

Pesticide	FRAC MOA1	Mature Trees Rate/Acre2
copper fungicide	M9	Use label rate.
1Mode of action class for citrus pesticides from the Fungicide Resistance Action Committee (FRAC) 2003. Refer to ENY624, Pesticide Resistance and Resistance Management, in the 2009 Florida Citrus Pest Management Guide for more details. 2Lower rates can be used on smaller trees. Do not use less than the minimum label rate.

---

Footnotes

1. This document is PP-182, one of a series of the Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Date printed: September 1999. Date revised: November 2008. This publication is included in SP-43, 2009 Florida Citrus Pest Management Guide. A copy of this publication may be found at http://edis.ifas.ufl.edu/CG040. Please visit the EDIS Web site at http://edis.ifas.ufl.edu. For a copy of this handbook, request information on its purchase at your county extension office.

2. M.M. Dewdney, assistant professor, Plant Pathology Department, and J.H. Graham, professor, Soil and Water Science Department, Citrus REC, Lake Alfred, Florida; Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611.

---

The use of trade names in this publication is solely for the purpose of providing specific information. It is not a guarantee or warranty of the products named, and does not signify that they are approved to the exclusion of others of suitable composition.

---

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Millie Ferrer, Interim Dean.