Detection and Identification of Asian Citrus Canker

 

 

 
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Contents:

This webpage provides supporting documents to the discussion of biological aspects of citrus canker, as provided in Chapter 3 of the book in the area of detection and identification.

An excellent website to aid in the identification of citrus canker and other diseases is ID Tools, as developed by UF/IFAS in conjunction with FDACS/IFAS.

Relevant Chapters

Chapters 1, 2 (see False Canker War), 3 and 9 (pages 172 to 174).

On the website:

SN 1.2: Dr. Whiteside's Contribution to Citrus Canker Research,

SN 1.5:Okeechobee Saga

SN 3.1 The case of the dirty scalpel

SN 3.3 Survey Errors and the Impact on the Eradication Program:

Images:

Yellow halos tend to coalesce over time, making it difficult to identify each lesion.

Links

IDTools.org: Fact Sheet on Citrus Canker

Diseases commonly mistaken for citrus canker

Citrus scab, University of Hawaii

The up close pictures are great, however consider an inspector within a residential area, with a 20-ft tree with branches extending over a fence, into a neighbor's yard. Inspectors had no ladders or flashlights, and worked until 7:00 pm at night. It is the equivalent of trying to identify a leaf with a lesion from the height of a house roof.

For this reason, it is believed FDACS wanted to simply clear-cut large swaths of citrus from residential homes, which they could not tell if they were healthy or diseased.

Identification problems plagued both epidemiology studies and eradication efforts. The articles by Dr. Whiteside focused mainly on difficulties in prior eradication efforts in commercial groves. The problems of identification in residential areas, are many times more difficult.

The presentation also shows the chlorotic halos fade with time, so the older lesions can be more difficult to reliably identify as citrus canker. The Florida field study relied on being able to identify the age of the oldest lesion on the tree and promoted the idea that the lesions expand at a uniform rate with time.

Wellington strain of citrus canker

X. Sun et al., Detection and Characterization of a New Strain of Citrus Canker Bacteria from Key/ Mexican Lime and Alemow in South Florida, Plant Disease. 88:1179-1188.

"Based on the preliminary results, a group of plant pathologists recommended in February 2001 that the Wellington group of strains of X. axonopodis pv. citri and all of its host plants, Key/Mexican lime and alemow be removed within 579 m (1,900 ft) of a disease plant." Page 1187.

What appears missing from the above quote is that the eradication program not remove other healthy citrus such as grapefruit, oranges, lemons, etc, until DNA testing is complete of an infected Key lime tree.

 

Articles by Dr. J. O. Whiteside and response from FDACS:

Dr. Jack .O. Whiteside cited numerous problems with the concept of full eradication of citrus canker from Florida. He wrote about Canker War 1, False Canker War and Canker War II.

Canker Threat, How Serious a Threat Canker is to Florida Citrus Production, Citrus Industry, 1985.

The History and Rediscovery of Citrus Canker in Florida, Citrus and Vegetable Magazine, April 1988.

Citrus Canker: Some Facts, Speculations and Myths about this Highly Dramatized Bacterial Disease, Citrus and Vegetable Magazine, Sept 1986.

Florida Department of Agriculture and Consumer Services review of articles by Dr. Whiteside as posted on their website, April 2001.

Difference between citrus canker (on right) and Citrus Bacterial Spot (left)

Fungal Disease misidentified as citrus canker in 1980's and USDA imposes a quarantine on Mexican Limes.

Stapleton, J.J, 1991, Citrus Leaf Spot in Mexico, A new Alternaria Disease South of the Boarder, Cooperative Extension, University of California, Kearney Plant Protection Group, Plant Protection Quarterly, April 1991.

 

Comments on Inspection and Detection of Citrus Canker during the CCEP

by David Lord

Inspections and identifications of canker are topics which entail some very simple aspects, such as the difficulties in accessing residential backyards in an urban setting or attempting to visually inspect the canopy of large citrus trees which may have multiple other foliar diseases such as citrus bacterial spot or greasy spot. Citrus trees can grow to over 20 ft, and branches may extend over to the neighbor's backyard or inaccessible areas. The care given to residential trees will vary widely.

The lowest cost security within urban areas is often chain link fences and dogs. This is very common in poorer neighborhoods.

The laboratory testing protocols for proper identification are a more complex area of study. The DNA based testing is outside the scope of my book. I believe testing during the CCEP rarely went beyond visual identification by inspectors.

The Department had never revealed how many properties could not be accessed. It is likely residents are generally not at home when inspectors were making their surveys.

In discussions with inspectors with the CCEP, I learned that many of yards could not be accessed, simply because the homeowners were not at home. Locked fences are very common, particularly in the more urban areas of Miami-Dade and Broward counties. Inspectors did not bring ladders, flashlights or binoculars with them to make inspections.

Short Notes and Chapters in my book

On this website, SN 3.3 discusses further the survey error problems and how they likely affected the selection of 1900-ft eradication zones. Other relevant notes include: SN 1.5 Okeechobee Saga, SN 1.7 Eradication Delays and Related Statistics and 3.1 The Case of the Dirty Scalpel. The latter short note discussed the final legal challenge to the eradication program in Broward County, where the testing and 1900-ft measurements were disputed by residents. Broward Case 3 is discussed on page 99 of my book. Chapters 1, 3 and 9 (pages 172-174) in my book discuss identification problems. Historical problems in proper identification is discussed in Chapter 2.

Department was complacent

The Department seemed content with both its inspection and testing procedures. Prior to a positive identification, the properties were to be inspected two times, first with a possible identification of citrus canker, and later to confirm citrus canker with an inspector with more experience. I had my doubts this actually was occurring, as every follow-up visit confirmed the initial diagnosis.

Also, the scientists with the Department suggested at times, that access and inspection problems with new technology, including aerial surveys, and instruments which would identify a unique spectrum of reflected light.

False Negatives

Errors in identification can occur as false negatives, when canker is present on a citrus tree, but is either thought to be another foliar disease, or is not detected. False positives occur, when a diagnosis of canker is made, and the foliar disease is not citrus canker.

False negatives can occur because the chloriotic halo, which makes the lesions distinct tend to fade with age. and appear more like scab.

Click to see image

It is the false negatives which seemed to be most concerning to regulators as this would result in incomplete removal of infected trees. Dr. Gottwald suggested that a "fudge factor" needs to be applied to eradication radii to overcome the potential of false negatives (2002 International Workshop, page 319). Also see pages 172 to 174 in my book.

Inspection Problems

It is normal for an owner to try to fight citrus canker by cutting the branches with noticeable lesions. The younger lesions may not be identifiable as citrus canker. This is be a problem on trees with only older lesions. Symptoms first appear on new flushes, and may be too high to be seen by inspectors. Lesions in wounds may be difficult to clearly identify because the normal round shape with a yellow halo is not present.

Residential inspections can be very time consuming, as typically inspectors can only survey approximately 20 to 30 yards a day and only half of them are likely to have citrus on them.

Legal aspects of inspections

My book discusses the difficulties in entry onto property, both as a practical problem for the Department and as a legal one.

It is against both our Florida Constitution and the US Bill of Rights to enter property without reasonable cause. The Department argued in court that an agricultural emergency existed, and with the rapid spread of canker, search warrants should not be required. However, Judge Fleet on May 24, 2002 in Broward Court, ruled that the provision of "blanket warrants" which would cover all properties in a county was a violation of the constitution.

The Beagle Story

The means by which citrus canker can be discovered is inspectors physically entering the area where citrus is grown, and inspecting the citrus tree. The USDA/ARS supported research by which trained beagle dogs could sniff out citrus canker. The Director of the USDA/ARS, Charles Arnold, stated in an interview that dogs could even sniff out exposed infections of citrus canker. This seemed absurd, as these are infections which exist only in the mesophyll tissues, and have not yet broke the rind of the fruit.

Concluding Remarks

The CCEP is one for the history books. As I stated in Chapter 1, the officials at the USDA and FDACS knew well how elusive citrus canker could be, even in an easy to inspect grove setting. Dr. Whiteside had clearly described the problems, which occurred during Canker War II.

Both the USDA and FDACS/DPI likely know how difficult it is to conduct inspections in residential areas. What is true for the CCEP is true for the 1998 Florida field study, access to properties and visual determinations of citrus canker would be significant problems. These problems are further explained in my book.