[summer]

From FL's canker homepage -- I will post link:

Summary of the Justification for Removing Canker-Exposed Trees within 1900 Feet of Infected Trees

An epidemiological study is designed to track disease spread so that intelligent regulatory or other disease management options can be targeted to best advantage. Epidemiological studies conducted in both commercial and residential citrus in Florida and South America over the last 10 years have strongly reinforced the concept that removal of citrus exposed to citrus canker inoculum from infected trees is an essential component of any successful eradication program.

Inoculum of the canker pathogen is dispersed in two ways: via wind-blown rain, and by human activity that involves the transport of infected or contaminated plants, tools, clothing, etc. The removal of exposed plants is crucial for eradication because the best detection methods currently available for disease detection are always well behind the actual expression of the disease on host plants. Delays in detection are caused by slow expression of detectable disease symptoms after infection and the constraints on visual survey methods.

The most recent epidemiological study used mixed age and varieties of residential citrus, and was conducted in North Dade and South Broward Counties during 1998-99. A description of this study is being prepared for publication. The study was done in an area where canker was only recently established, where the citrus leafminer was present (a new factor in the epidemiological equation for the Western Hemisphere), and where many thousands of trees in four separate sites could be monitored to provide the data for the study. This scenario was made possible only because of the unfortunate continued spread of the disease into new areas in spite of various protocols that had been utilized previously in the program. Previous methods included hatracking exposed trees; removing all exposed trees within 125 feet; removing of all infected trees; and only infected trees as soon as possible after discovery.

Four study sites were selected based on their relative isolation from each other, the recent appearance of only a few infected trees in each area, and the absence of the disease in the surrounding citrus. At the beginning, all citrus (ca. 19,000) in the vicinity were identified and their location plotted using satellite-based global positioning technology. The disease status of each tree in the study area was then determined on a 30-day basis by a field plant pathologist. The trees infected at the outset were identified as focal trees, and presumed to be the direct or indirect source of inoculum for all subsequent disease development in the area. The data taken on each visit consisted of a determination of whether canker lesions were present or absent, host variety and age/size, lesion age, an estimate of disease severity based on percent of canopy exhibiting lesions, and location of the lesions within the canopy. Data was collected every 30 days at each of the study sites to monitor disease progress over time through the area. All trees remained in place throughout the course of the study.

The main conclusion that can be drawn from the composite data is that subsequent infections resulting from inoculum dispersal from focal trees lie within approximately 1200 feet 90% of the time, within 1900 feet 95% of the time, and within 2700 feet 99% of the time. In other words, in order to eliminate the next generation of canker infections (ones that are already established and not yet detected), the project will be successful nineteen times out of twenty if all citrus trees within 1900 feet of the infected tree(s) are removed. The program selected the 95% success level as striking a balance between taking too few and too many trees and still reaching the goal of eradication.

Prepared by Drs. Tim Schubert, Wayne Dixon, and Xiaoan Sun

Florida Department of Agriculture and Consumer Services
Division of Plant Industry
PO Box 147100
Gainesville, FL 32614

[nunya bidness]

SOURCE

Is the Cutting of Healthy Citrus Trees Necessary?

As citizens, we share the opinion of most South Floridians that the Citrus Canker Eradication Program is out of control. As scientists we are deeply concerned that scientific justification for this program is lacking. Scientific sources indicate that

(1) the Asian strain of the bacterium Xanthomonas campestris pv. citri is not devastating to citrus and

(2) that destruction of healthy citrus trees in attempt to stop the spread of this bacterium is ineffectual and unnecessary.

Consider, for example, the following:

1) Dr. Jack Whiteside, Citrus Research and Education Center, Institute of Food and Agricultural Sciences, University of Florida, writes 'One of the reasons given for continuing the canker scare is that no one has yet proven to the satisfaction of all concerned that canker would not be an economic problem in Florida. In reality, however, no one has yet provided any evidence that it could be a serious threat to the Florida citrus industry.' Whiteside explains that the disease of concern, canker A, is widely distributed in Argentina's citrus producing provinces, but is generally of minor importance. The Argentinean experience with canker A is of particular interest to the Florida citrus industry because of similarities in climate, production methods and commercial varieties grown. Whiteside further summarizes reasons why canker A. is likely to be even less troublesome in Florida than in Argentina. He also points out that previous history of canker in Florida suggests that the bacteria do not spread rapidly and cause heavy infection. These facts argue that the disease may in fact be largely self-eliminating. Whiteside concludes that "The economic threat of canker to the Florida citrus industry or other areas where the climatic conditions seem marginal or unfavorable for infection needs to be reexamined. Furthermore, there needs to be a more scientific, practical and realistic appraisal of the canker eradication efforts to consider if they are really needed and likely to achieve their stated goal of eradicating the pathogen." Citrus and Vegetable Magazine April 1988.

2) At an International Symposium on Citrus Canker held at the Institute of Food and Agricultural Sciences, University of Florida, 1985, scientists pointed out that there may be other host plants for the Asian strain of Xanthomonas besides citrus. The symposium concluded that unless infected trees were restricted and weather conditions unfavorable for spread of citrus canker, eradication would be waste of money and effort. We understand that a Science Review Panel convened in March 1998 by the USDA reached a similar conclusion; our efforts to obtain the 8-10 page report of the 1998 meeting have been unsuccessful.

3) Dr. Heinz K. Wutscher holds a Ph.D. in Pomology (the science of fruit growing) from Cornell University and has 32 years in the field of citriculture working for the Agricultural Research Service of the US Department of Agriculture. An affidavit sworn by Dr. Wutscher on August 21, 2000 states that it is his "professional, expert opinion that citrus canker caused by the Asian strain of Xanthomonas campestri pv citri cannot be eradicated by the methods now in use and that even if eradicated, it would be reintroduced quickly because of its world wide distribution. All of the major citrus growing areas with humid climates have citrus canker and co-exist with it without major problem." One does not need to be a plant pathologist to question the validity of destroying healthy citrus trees in attempt to eradicate the Asian strain of Xanthomonas. The current program is scientifically unjustified.

Professors, research scientists, and graduates from Rosenstiel School of Marine and Atmospheric Sciences, University of Miami:

Pamela Reid, Ph.D. Jack Fell, Ph.D. Rod Zika, Ph.D Peter Ortner, Ph.D. Greta Mackenzie, Ph.DH. Frederick Tappert, Ph.D.

.Samuel Snedaker, Ph.D .Barrie Taylor, Ph.D. Linda Farmer, Ph.D. Groschel-Becker Ph.D. Mitch Roffer, Ph.D. Terri Hood, Ph.D.

Peter Ortner, Ph.D. Greta Mackenzie, Ph.DH. Groschel-Becker Ph.D. Thomas Lee, Ph.D. Matthew Lynn, Ph.D. Arthur Myrberg, Ph.D.