CITRUS INDUSTRY • June 2006

The 1900-foot rule has been suspended and eradica-tion of citrus canker-affected trees has been essen-tially ended. The Citrus Health Response Plan(CHRP) is being developed, but it does not appear

that obligatory removal of affected trees will be a part of thatplan. Thus, growers probably have to use their best judgmentin management of citrus canker.

The Division of Plant Industry (DPI) has found canker inmost citrus areas of the state except the northwestern produc-tion areas. DPI will continue to monitor and report detectionsof the disease in commercial groves. Currently, 75 percent ofthe citrus acreage is within five miles of a canker find.

Although it is difficult to predict exactly how severecanker will be under Florida conditions, indications fromoutbreaks in the state are that it will be difficult to control.Areas that are currently canker-free should be protected tothe extent possible.PROTECTING CANKER-FREE AREASDECONTAMINATION

Rules for decontamination are still in place and should befollowed. With more cankeraround the state, the possi-bility of spread is greaterthan ever. In moving equip-ment and personnel fromgrove to grove, every effortshould be made to makesure that plant material isnot moved inadvertentlyand that all equipment has been thoroughly decontaminated.Decontamination is especially important in harvesting opera-tions and in any other practices involving extensive contactwith foliage. Obviously, when equipment is moved fromblocks where canker is endemic to other infected blocks, de-contamination serves little purpose.

INOCULUM SUPPRESSION

TREE REMOVALIf canker is detected in areas previously free of the dis-

ease, removal and burningof trees on site can slow theestablishment of the dis-ease. For tree removal to beeffective, canker has to belocalized and limited to asmall number of trees. Treeremoval is not likely to beeffective if canker is al-ready present within a mile of the grove.

A few apparently healthy trees surrounding the infestedarea should be removed as well or defoliated. More treesshould be removed if the focus is large, but if it is very large,tree removal may need to be reconsidered.

This measure is unlikely to eradicate the disease, but cansubstantially slow disease development. Tree removal mustbe followed by monthly inspections and removal of anymore trees found positive for the disease.

At some point, tree removal will no longer be economi-cally sustainable and should be discontinued.

DEFOLIATIONThere are currently no

registered defoliants.Some growers are usinghigh concentrations of ureaor soluble copper com-pounds on an experimentalbasis. However, no rates orspray volumes have beenestablished for this practice. Chemical defoliants may beavailable at some point in the near future.

Defoliation of known canker-infected trees is not likely toeliminate the disease. A strong flush of highly susceptibleleaves follows defoliation and that foliage is likely to be-come infected from residual inoculum in the tree or nearby.

Defoliation can be useful in areas surrounding foci of in-fected trees that have been removed. These trees may ap-pear healthy, but are likely to harbor undetectable cankerlesions. Defoliation can eliminate this inoculum and stillsave many trees.

ENDEMIC CANKERWhere canker is already endemic, the primary means of

control are:1) planting of windbreaks,2) protection of fruit and leaves with copper sprays,3) control of leafminer, and4) planting tolerant varieties.

WINDBREAKSWindbreaks are highly

effective in reducing thespread of canker, but moreimportantly, they reduce theseverity of the infection inendemic situations. Whencanker lesions are wetted,millions of bacteria oozeonto the leaf surface. While bacteria can swim very shortdistances, they have no active means to penetrate the fruit,leaves or twigs. The vast majority of the infection occurs bywind-blown rains. Winds of 18 to 20 mph are needed to actu-ally force bacteria into the stomates on leaves and fruit.

Windbreaks are the single most effective means of deal-ing with canker. In our observations in Argentina, the num-ber of canker lesions was 10 times greater on the side of thetree exposed to the prevailing winds than on the protectedside of the same tree. In tests in nursery situations, artificialwindbreaks greatly diminished the distance of spread ofcanker down the nursery row and reduced disease to only afew scattered lesions.

Windbreaks reduce wind speed for a distance 10 times theheight of the windbreak. That is, a 30-foot tall windbreakwill exert an effect for about 300 feet.

To be effective for canker control, windbreaks need not bedense. All that is required is to reduce wind speed to lessthan 20 mph.

The need for and the distance required between wind-break rows will depend on the destination of the fruit – freshor processed – and the susceptibility of the variety. Withgrapefruit for the fresh market in Florida, it is likely that

Fundamentals of citrus canker managementBy L.W. Timmer, J.H. Graham and H.L. Chamberlain

CITRUS INDUSTRY • June 2006

each 5 to 10-acre block will need to be surrounded by wind-breaks. In many groves of less susceptible varieties, a wind-break down the row about every 300 feet may be sufficient.

In some situations where some protection exists and tol-erant varieties are grown for processing, additional wind-breaks may be unnecessary. Currently, we recommend thatgrowers plant windbreaks along fence lines, ditches, aroundwetlands, or wherever they can plant without removingcitrus trees. If it becomes obvious that more windbreakprotection is needed, rows of citrus or end trees can beremoved to accommodate windbreaks.

For more information on selection of plant species anddesign, see the CRECWeb site (www.crec.ifas.ufl.edu).COPPER SPRAYS

Over the last 30 years,University of Florida-IFAShas evaluated dozens ofproducts for canker controlin several projects in Ar-gentina and Brazil. Productssuch as antibiotics, com-pounds that induce resistance in plants, and disinfectantsoften provide limited canker control, but no product hasproven more effective than copper products.

Copper products are quite effective in preventing infec-tion of fruit, less effective for reducing leaf infection, andhave limited value in reducing spread of the disease.

Application of copper to young leaves protects against in-fection, but protection is soon lost due to rapid expansion of thesurface area. Fruit grows more slowly and is easier to protect.

Fruit is susceptible to infection after the stomates openwhen the fruit is about one-half inch to 1 inch in diameteruntil they develop resistance in mid to late July. Infectionthrough wounds can occur at any stage.

Programs needed for effective control of canker in Floridahave not been determined. However, we believe that most ofthe infection will occur during June and July here. With en-demic canker, we suggest that three copper sprays be usedfor early oranges grown for processing — one in mid-May, asecond in early to mid-June, and the final one in early July.Two applications should be sufficient for Valencias, in earlyJune and in early July.

Programs for fresh fruit are more complex, but many cop-per sprays are already used on these varieties. For fresh mar-ket grapefruit, a low rate of copper should be added to thespray of spring flush for scab. Subsequently, the copperspray program used for melanose control should also controlcanker, but additional applications may be needed in lateJune and July. Copper may need to be added to applicationsof fungicides or petroleum oil.

Most tangerines are fairly tolerant to canker. Programsused for control of Alternaria should also protect againstcanker, but copper will have to be used in each spray. Naveloranges are highly susceptible to canker and will probablyneed to be sprayed every three weeks from late April to mid-July. Fallglo is more tolerant and probably three sprays in

May, June and July should suffice.Spray programs will have to be adjusted as we develop

experience. The rates needed depend on the length of protec-tion expected and the weather. As little as 0.5 to 1.0 lb. ofmetallic copper will protect spring flush growth or fruit dur-ing the dry spring season. However, in the rainy season, upto 2 to 3 lbs. of metallic copper will be required to protectfruit for three to four weeks. To the extent possible, we aretrying to minimize copper usage since this metal accumu-lates in soil and may cause phytotoxicity and creates envi-ronmental concerns.LEAFMINER CONTROL

Leafminers do not spread canker, but extensive invasion ofleafminer tunnels by the bacterium greatly increases inocu-lum levels, making the disease difficult to control. Leafminersare not usually a problem onthe spring flush and no con-trol is needed at that time.Leafminer control on the firstsummer flush can reduce dis-ease pressure considerably. Ifproperly timed, applicationsof petroleum oil, Agri-mek,Micromite, Spintor, or Assailwill reduce damage byleafminer. Late summerflushes tend to be erratic andeffective control at that time will probably be difficult.VARIETAL RESISTANCE

Of the varieties grown in Florida at present, grapefruit,navel oranges, and some early oranges like Early Gold arehighly susceptible. Hamlins and tangelos are somewhat lesssusceptible, but still can be severelydamaged. Tangerines and some hy-brids like Murcotts are more toler-ant, as are Valencia oranges.

Since nursery trees are rela-tively unavailable currently, it willbe difficult to change the varietiesgrown in Florida in the near future.Canker may greatly limit shipmentof fresh fruit to many areas. Theability to market fruit, consumerpreference, and our ability to dealwith canker will ultimately dictatethe varieties grown in Florida. Our industry will be greatlychanged by endemic canker, but we will survive this chal-lenge as we have others.ACKNOWLEDGEMENT

We gratefully acknowledge contribution of figures byMichael Rogers and Gary England.L. W. Timmer is a plant pathologist; J. H. Graham is a soil microbi-ologist, and H. L. Chamberlain is the Citrus Health Response Planimplementation coordinator — all with the University of Florida-IFAS Citrus Research and Education Center at Lake Alfred.