The Lindcove Research and Extension Center (LREC) in Exeter, Calif., will be forced to relocate its citrus research activities unless provisions are created to mandate the removal of citrus trees infected by the citrus tristeza virus (CTV) in areas near the center.
That statement from LREC Director Beth Grafton-Cardwell comes on the heels of findings in May that confirmed a high number of citrus trees at the research center were infected with the aphid-spread CTV. The infected trees were removed.
According to Grafton-Cardwell, testing found 46 CTV-infected trees in LREC research plots. Four infected trees were located in the Citrus Clonal Protection Program's (CCPP) foundation area located at the LREC.
Removing infected trees near the LREC is the next priority.
“If we can't clean up this problem then it threatens the viability of growing citrus at this station and having a research or foundation block program here,” Grafton-Cardwell said.
The Lindcove facility lies within the Tulare County Pest Control District where CTV-infected tree removal is currently voluntary. The future of the Lindcove citrus research center hinges on establishing a mandatory policy of CTV-infected tree removal in the LREC's surrounding area.
Two options exist to achieve the goal. One would create a division (including the LREC and the surrounding area) within the existing Tulare County district where infected tree removal would be mandatory. Grafton-Cardwell supports this option.
The second is creating a new pest control district to include the LREC and the surrounding area. The new district would require grower approval.
“The reason we want the option of splitting up the Tulare County Pest Control District is that it would be easier to split the district in two and have different tree removal requirements in each part. It would speed up the process.
Unless a mandatory CTV-related tree removal plan is created, LREC citrus research could be forced to move elsewhere. If that occurs, the current physical facility could be redesigned to focus on other crops. Moving citrus research elsewhere would cost several million dollars.
“The citrus industry has a lot invested in this station, and it would be silly to lose that because we're not conducting CTV-related tree removal.”
She said the board of supervisors and the Pest Control District Board of Directors in Tulare County would work with the agricultural commissioner to define an area for mandatory tree removal.
“That hasn't happened yet. The wheels are moving very slowly but they are turning.”
Several aphid species transmit the CTV virus in California: the cotton/melon aphid, Aphis gossypii; spirea aphid, Aphis spirea; and the black citrus aphid, Toxoptera aurantii.
Grafton-Cardwell characterized the heavy CTV finding at the LREC as an epidemic. CTV is the most destructive citrus virus disease in the world, yet the aphids found in California are not effective vectors (transmitters) of the virus.
“This is why growers are reluctant to pull trees because they don't see how destructive the virus can be,” Grafton-Cardwell said. “If we don't suppress the disease to extremely low levels to control it, California may eventually get one of the severe strains or the brown citrus aphid that efficiently moves the virus. Then we would become like the rest of the world where CTV is the most destructive citrus virus.”
Grafton-Cardwell believes CTV findings at the LREC would impact the California citrus industry two-fold.
First, the detection of CTV in the foundation area prevents budwood release to the nursery industry until trees can be retested over a period of time (one or two high virus titer periods) and shown to be free of disease. Titer is equivalent to the amount of virus particles in the plant.
Second, the high CTV outbreak would impact research programs since studies are heavily impacted by increased CTV-caused tree infection.
“Three of the blocks had significant numbers of infected trees — five, nine, and 21 trees. When five to 21 trees are removed from a 200-tree research plot, research is affected because replicates of the experiments are eliminated.” After 21 infected trees were removed from the one block, Grafton-Cardwell explained, “Now there is a patchwork quilt of a research block.”
From 1992 to 2005, no CTV-infected detections were found in the foundation block. Annually, the number of CTV detections in plots averaged three trees for the entire research center.
During 2006, the CCPP foundation area had two CTV findings signifying a breakdown of protection in the area. Budwood was not released from the foundation trees during the June period to ensure nurserymen would not receive CTV-infected budwood.
Infected trees cannot be cured and removal is the only way to halt CTV. The disease is spread by grafting infected plant tissue, or during aphid feedings and vectored to new trees.
The CCPP provides disease-free budwood to California nurserymen who utilize it to produce registered ‘mother’ trees, and increase trees that provide the buds to create field trees sold to growers.
The scion material (fruiting portion) of California (citrus) trees must come from periodically tested, registered trees. Rootstocks are grown from disease-free seed. This helps California growers produce trees with few diseases and maximum fruit yield and quality.
“The citrus industry will struggle to get sufficient budwood to make trees for them,” Grafton-Cardwell said of the 2007 CTV outbreak. “Growers are assessed a dollar amount based on boxes that fund the Citrus Research Board. A lot of the projects they are funding and supporting are located here at the research station. The CTV situation impacts whether that research can be successfully completed or not.”
CTV infections occur in younger blocks of trees. Aphids are attracted to young trees with heavy flushes of new growth. It is highly likely that other CTV-infected trees currently exist that don't have the sufficient virus titer this year to be detected.
“We can expect to find additional infected trees in research blocks in 2008. The LREC research program is compromised by the heavy CTV infection.”
CTV is non-persistent — aphids carry it inside the body for a short time. According to Grafton-Cardwell, the aphids' success in transmitting the virus is impacted by: the titer level of virus in the tree; the number of infected aphids flying to neighboring trees; the receptivity of the tree the aphids land on (tender flush availability); and the type (isolate) of the virus.
The citrus-growing region in the San Joaquin Valley (SJV) is divided into five pest control districts. Originally formed to eradicate California red scale, district efforts switched to CTV detection and tree removal in the 1960s. In the mid-1990s, citrus growers in two districts, Tulare and Western Fresno, voted to suspend mandatory CTV tree removal.
Researchers from the University of California (UC) Riverside, UC Davis, the U.S. Department of Agriculture, and Cooperative Extension farm advisors conduct over 30 various research projects on 125 acres at the LREC. Some projects examine how well new varieties of Navel, Valencia, and Mandarin strains perform on various rootstocks under SJV weather conditions.
Other projects study the effects of pre-harvest pesticides on nematodes, insects, mites, and post-harvest treatments for diseases. Each research project requires protection from diseases that may influence the results of the tests. California Citrus Research Board grant funding largely supports the studies.
SJV growers and UC Riverside established the LREC in 1959 to provide a research environment typical of the Central California citrus industry.