Chet Roistacher's submission

Dear friend,

These are times of great changes for perhaps all of us but especially for the citrus industries of the world. Huanglongbing (greening disease) now threatens the two largest citrus industries in the world and as its vector Diaphorina citri moves into new areas along with Toxoptera citricida the prime vector of tristeza - new and serious problems will arise.I wish to share with you a new slide show on the history of indexing for citrus. This is more or less a review of my lifes work in this field. I was asked to present this work as an invited lecture at the 8th International Congress of Citrus Nurserymen in October, 2008 in Chongqing, China. I have now converted my Power Point lecture to an EcoPort slide show number 209 and I share this with you. You can access this slide show with the URL below or go into http://www.ecoport.org/ww.ecoport.org/ - Click on pictures, then on slide shows and enter 209. Click on 209 on the far left to bring up this slide show and you can also download this show as a permanent file.What I would appreciate is for you to suggest any errors or updates, or you can send me a picture or two with text for inclusion into this, or any of the various slide shows. These can readily be included into the slide shows as well as any editing you might suggest.Here is the URL for this slide show titled Indexing for Graft-Transmissible Diseases of Citrus: Its History, Importance, Current status and use in Certification Programs. http://ecoport.org/ep?SearchType=slideshowList&ID=209I also enclose this URL as a direct link to all of my EcoPort slide shows and eArticles: http://ecoport.org/ep?SearchType=domainContents&id=9&type=groupSearchType=domainContents&id=9&type=group



The picture shows the planting of the first citrus tree in the Foundation Block at Lindcove, California in 1961 (yes, it is Chet). This tree was the result of five years of indexing, growth and preparations.






Abstract: Prior to 1945 indexing for the detection of citrus graft transmissible pathogens (CGTPs) was limited or non-existing since virus and virus-like diseases of citrus were poorly understood or little known. The common practice was for growers and nurserymen to select their budwood for propagation from their best and most productive trees. However, in later years it was discovered that even their best trees were often symptomless carriers of one or more dangerous CGTPs. Topworking was, and still is, a common practice which perpetuates the spread of infectious diseases. Following up on the seminal finding by Fawcett (1933) on the virus nature of psorosis, the first registration program for citrus was initiated in California providing growers and nurserymen with psorosis-tested budwood. Between 1937 and 1961, field inspections of mother trees resulted in over 2,400 trees registered as psorosis inspected. The discovery of a seedling index for psorosis by Wallace (1945) was a major breakthrough in citrus virus detection and a new era of discovery of short term biological indexing for citrus pathogens began. Indexing was reduced from years to weeks for the expression of definitive symptoms of diseases. The worldwide tristeza epidemics in the 1930's initiated a conversion to new CTV-tolerant rootstocks to replace the popular and predominant sour orange. However, with indexing, hidden and symptomless viruses and viroids emerged which led to the recognition that most old clone citrus budwood sources were infected with CGTPs. The virus nature of tristeza was proven by Fawcett and Wallace (1947) and a second major breakthrough in indexing occurred with the discovery that seedlings of the small fruited lime could be used as a rapid indicator for tristeza. The destructive nature of this devastating disease was a major factor for bringing scientists together in 1957 for the first conference and the formation of International Organization of Citrus Virologists (IOCV). Scientists throughout the world could now formally meet and communicate and publish their research on indexing. Certification for citrus took on new meaning and possibilities. In the early 1950's, there were very few greenhouse or laboratory methods available for detection of most CGTPs. However, in relatively rapid succession, new technology for indexing and identifying these graft-transmissible pathogens evolved: the Orlando tangelo for cachexia (1952); the lemon and sour orange for seedling yellows (1952); Mexican lime for vein enation (1953); the trifoliate and Rangpur lime field index (1954) and the laboratory color test for exocortis (1957). In the beginning there were four or five major diseases of citrus for which indexing was proposed and needed for a certification program. These were tristeza, psorosis, exocortis, cachexia and stubborn. Indexing was the key for the identification of these diseases. As indexing methods continued to improved, nearly all existing old line source trees were found to be infected with one or more CGTPs. The nucellars which were found to be free of most CGTPs gained dominance. However, there were many problems associated with the nucellar. The subsequent development of thermotherapy (1957-1972) and shoot tip grafting (1975) virtually ended the interest and the need for the nucellar. With new indexing technologies, certification programs were initiated in Texas (1948), Florida (1953) California (1954) and in Arizona (1955). These programs are briefly reviewed. Detailed histories are presented for indexing for psorosis, tristeza, exocortis, cachexia, and stubborn diseases. Diagnosis by field symptoms plus biological and laboratory tests are presented. A brief look at other CGTPs and their indexing procedures are also presented for the oak leaf pattern diseases of blight, concave gum, cristacortis and impietratura and for citrus chlorotic dwarf, citrus variegated chlorosis, gummy bark, infectious variegation, leprosis, satsuma dwarf, tatter leaf, vein enation and witches broom disease of limes. Technology in virus purification, molecular biology and immunology opened a new era for assays such as ELISA, sPAGE and PCR which utilized the sequence specificities of ribosomal spacer regions. These assays are rapid but though they are both costly and labor intensive they are now important components for indexing in certification programs. The current worldwide destruction by huanglongbing (greening disease) has placed new emphasis on the importance of detection of this disease before it becomes epidemic. Older and newer indexing procedures for HLB are reviewed and an important new technology using RT-PCR for showing the presence of the bacteria inside the insect vector is reviewed. This new technology can show the presence of HLB/greening disease one to two years before field symptoms are seen and verified by PCR.