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Fundamentals of Citrus Canker Management by L.W Fundamentals of citrus canker management By L.W. Timmer, J.H. Graham and H.L. Chamberlain he 1900-foot rule has been suspended and eradica - DEFOLIATION tion of citrus canker-affected trees has been essen - There are currently no tially ended. The Citrus Health Response Plan registered defoliants. (CHRP) is being developed, but it does not appear Some growers are using Tthat obligatory removal of affected trees will be a part of that high concentrations of urea plan. Thus, growers probably have to use their best judgment or soluble copper com - in management of citrus canker. pounds on an experimental The Division of Plant Industry (DPI) has found canker in basis. However, no rates or most citrus areas of the state except the northwestern produc - spray volumes have been tion areas. DPI will continue to monitor and report detections established for this practice. Chemical defoliants may be of the disease in commercial groves. Currently, 75 percent of available at some point in the near future. the citrus acreage is within five miles of a canker find. Defoliation of known canker-infected trees is not likely to Although it is difficult to predict exactly how severe eliminate the disease. A strong flush of highly susceptible canker will be under Florida conditions, indications from leaves follows defoliation and that foliage is likely to be - outbreaks in the state are that it will be difficult to control. com e infected from residual inoculum in the tree or nearby. Areas that are currently canker-free should be protected to Defoliation can be useful in areas surrounding foci of in - the extent possible. fected trees that have been removed. These trees may ap - PROTECTING CANKER-FREE AREAS pear healthy, but are likely to harbor undetectable canker lesions. Defoliation can eliminate this inoculum and still DECONTAMINATION save many trees. Rules for decontamination are still in place and should be ENDEMIC CANKER followed. With more canker around the state, the possi - Where canker is already endemic, the primary means of bility of spread is greater control are: than ever. In moving equip - 1) planting of windbreaks, ment and personnel from 2) protection of fruit and leaves with copper sprays, grove to grove, every effort 3) control of leafminer, and should be made to make 4) planting tolerant varieties. sure that plant material is WINDBREAKS not moved inadvertently and that all equipment has been thoroughly decontaminated. Windbreaks are highly Decontamination is especially important in harvesting opera - effective in reducing the tions and in any other practices involving extensive contact spread of canker, but more with foliage. Obviously, when equipment is moved from importantly, they reduce the blocks where canker is endemic to other infected blocks, de - severity of the infection in contamination serves little purpose. endemic situations. When canker lesions are wetted, INOCULUM SUPPRESSION millions of bacteria ooze TREE REMOVAL onto the leaf surface. While bacteria can swim very short distances, they have no active means to penetrate the fruit, If canker is detected in areas previously free of the dis - leaves or twigs. The vast majority of the infection occurs by ease, removal and burning wind-blown rains. Winds of 18 to 20 mph are needed to actu - of trees on site can slow the ally force bacteria into the stomates on leaves and fruit. establishment of the dis - Windbreaks are the single most effective means of deal - ease. For tree removal to be ing with canker. In our observations in Argentina, the num - effective, canker has to be ber of canker lesions was 10 times greater on the side of the localized and limited to a tree exposed to the prevailing winds than on the protected small number of trees. Tree side of the same tree. In tests in nursery situations, artificial removal is not likely to be windbreaks greatly diminished the distance of spread of effective if canker is al - canker down the nursery row and reduced disease to only a ready present within a mile of the grove. few scattered lesions. A few apparently healthy trees surrounding the infested Windbreaks reduce wind speed for a distance 10 times the area should be removed as well or defoliated. More trees height of the windbreak. That is, a 30-foot tall windbreak should be removed if the focus is large, but if it is very large, will exert an effect for about 300 feet. tree removal may need to be reconsidered. To be effective for canker control, windbreaks need not be This measure is unlikely to eradicate the disease, but can dense. All that is required is to reduce wind speed to less substantially slow disease development. Tree removal must than 20 mph. be followed by monthly inspections and removal of any The need for and the distance required between wind - more trees found positive for the disease. break rows will depend on the destination of the fruit – fresh At some point, tree removal will no longer be economi - or processed – and the susceptibility of the variety. With cally sustainable and should be discontinued. grapefruit for the fresh market in Florida, it is likely that CITRUS INDUSTRY • June 2006 each 5 to 10-acre block will need to be surrounded by wind - May, June and July should suffice. breaks. In many groves of less susceptible varieties, a wind - Spray programs will have to be adjusted as we develop break down the row about every 300 feet may be sufficient. experience. The rates needed depend on the length of protec - In some situations where some protection exists and tol - tion expected and the weather. As little as 0.5 to 1.0 lb. of erant varieties are grown for processing, additional wind - metallic copper will protect spring flush growth or fruit dur - breaks may be unnecessary. Currently, we recommend that ing the dry spring season. However, in the rainy season, up growers plant windbreaks along fence lines, ditches, around to 2 to 3 lbs. of metallic copper will be required to protect wetlands, or wherever they can plant without removing fruit for three to four weeks. To the extent possible, we are citrus trees. If it becomes obvious that more windbreak trying to minimize copper usage since this metal accumu - protection is needed, rows of citrus or end trees can be lates in soil and may cause phytotoxicity and creates envi - removed to accommodate windbreaks. ronmental concerns. For more information on selection of plant species and LEAFMINER CONTROL design, see the CREC Web site (www.crec.ifas.ufl.edu). COPPER SPRAYS Leafminers do not spread canker, but extensive invasion of leafminer tunnels by the bacterium greatly increases inocu - Over the last 30 years, lum levels, making the disease difficult to control. Leafminers University of Florida-IFAS are not usually a problem on has evaluated dozens of the spring flush and no con - products for canker control trol is needed at that time. in several projects in Ar - Leafminer control on the first gentina and Brazil. Products summer flush can reduce dis - such as antibiotics, com - ease pressure considerably. If pounds that induce resistance in plants, and disinfectants properly timed, applications often provide limited canker control, but no product has of petroleum oil, Agri-mek, proven more effective than copper products. Micromite, Spintor, or Assail Copper products are quite effective in preventing infec - will reduce damage by tion of fruit, less effective for reducing leaf infection, and leafminer. Late summer have limited value in reducing spread of the disease. flushes tend to be erratic and Application of copper to young leaves protects against in - effective control at that time will probably be difficult. fection, but protection is soon lost due to rapid expansion of the VARIETAL RESISTANCE surface area. Fruit grows more slowly and is easier to protect. Of the varieties grown in Florida at present, grapefruit, Fruit is susceptible to infection after the stomates open navel oranges, and some early oranges like Early Gold are when the fruit is about one-half inch to 1 inch in diameter highly susceptible. Hamlins and tangelos are somewhat less until they develop resistance in mid to late July. Infection susceptible, but still can be severely through wounds can occur at any stage. damaged. Tangerines and some hy - Programs needed for effective control of canker in Florida brids like Murcotts are more toler - have not been determined. However, we believe that most of ant, as are Valencia oranges. the infection will occur during June and July here. With en - Since nursery trees are rela - demic canker, we suggest that three copper sprays be used tively unavailable currently, it will for early oranges grown for processing — one in mid-May, a be difficult to change the varieties second in early to mid-June, and the final one in early July. grown in Florida in the near future. Two applications should be sufficient for Valencias, in early Canker may greatly limit shipment June and in early July. of fresh fruit to many areas. The Programs for fresh fruit are more complex, but many cop - ability to market fruit, consumer per sprays are already used on these varieties. For fresh mar - preference, and our ability to deal ket grapefruit, a low rate of copper should be added to the with canker will ultimately dictate spray of spring flush for scab. Subsequently, the copper the varieties grown in Florida. Our industry will be greatly spray program used for melanose control should also control changed by endemic canker, but we will survive this chal - canker, but additional applications may be needed in late lenge as we have others. June and July. Copper may need to be added to applications ACKNOWLEDGEMENT of fungicides or petroleum oil.
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