The advent of increased virulent strains of citrus tristeza virus (CTV) in the San Joaquin Valley has prompted new measures to shield vital foundation budwood trees at the University of California’s Lindcove Research and Education Center near Exeter.
During a growers’ seminar at Exeter, Beth Grafton-Cardwell, director of the center and Extension specialist in entomology, described the steps being taken to protect the 175-acre site from the most serious viral disease of citrus.
The program features insecticide sprays for aphid vectors in the fall and spring and addition of screen house capacity to protect budwood stock from the insects.
For background on the incurable disease, Grafton-Cardwell noted that traditionally, the SJV has had less severe strains producing few or no symptoms. The severe strains, however, evident progressively from stem pitting and small fruit to unthrifty trees, are gradually increasing.
“And recently in this area, we’ve seen a lot of quick decline, in which a healthy tree becomes dead-looking in a matter of days,” she said, adding that the reason for this development is not known. Farm advisors, USDA researchers, and the Central California Tristeza Eradication Agency are searching for clues.
What is known, she added, is that trees grafted onto sour orange rootstock are susceptible, even to milder strains. When a susceptible root-scion combination, the virus, and its vector, the cotton, or melon, and aphid are all present, the disease can occur.
The reason for more infection may be a general build-up of the virus, or perhaps mutations of it into more lethal strains. Although the virus survives in its vector only about 24 hours, that can be enough time for transmission.
Grafton-Cardwell said more tristeza in the SJV has come about from fewer infected trees being removed, higher populations of the aphid, and changes in weather and crop distribution.
“We are looking for the more severe strains now more than we were 10 years ago because we now have improved molecular techniques to identify them,” she said. But she also warned that all strains need to be surveyed because the severe types may amount to only about 2 percent of the total.
The significance to the Lindcove facility is its location amidst the tristeza concentration in Tulare County. The practice has been to remove infected trees near the facility to create a buffer to aphid vectors around the foundation block.
Following findings of a 1998 eradication agency survey, an average of three infected trees were removed in the buffer each year. But in 2007, 52 trees, including four in the foundation block, were found to be infected and were removed, and the following year 84 trees identified with the disease were pulled.
“So we found that we could no longer protect the foundation block with tree removal by itself. The first impact to the station was that budwood from trees in the open was potentially infected and could no longer be distributed to the nursery industry. That limited the amount of available budwood to material from trees under the screen house,” she said.
Removal of trees interfered with replications of rootstock-scion experiments since a CTV infection cannot be distinguished from an unsuccessful graft combination.
Another casualty of the removals was the irradiated budwood field research of Mikeal Roose, UC Riverside citrus breeder. Prominent in his recent work has been development of the Tango seedless mandarin and related lines.
Grafton-Cardwell requested a survey by the Tulare County Pest Control District, which found a ten-fold increase in infected trees around the Lindcove center. The proliferation of pomegranate orchards nearby added to the reservoir of cotton aphid.
“That meant,” she explained, “that we could not ask growers to remove citrus trees adjacent to the station if those areas are highly infected and likely to get reinfected.”
The resulting plan was for the pest control district to launch during 2008 aphid vector control in a core area around the center, using insecticide treatments of Admire in the fall and Assail in the spring, during periods when aphid activity is greatest. Pomegranate trees in the area are also being treated to control the aphid.
The effects of the treatments cannot be determined until the spring of 2010 because of the time needed for symptoms of the virus to be detected in trees.
Removal of infected trees continued and amounted to 62 more this year.
The Citrus Research Board, she added, provided funds for a second screen house at the station to exclude disease-vectoring insects from key budwood varieties.
In a related presentation including methods of detecting strains of CTV, Raymond Yokomi, research plant pathologist at the USDA Station in Parlier, said the one most prevalent strain in the SJV is quick decline, found only on citrus with sour orange rootstock. It generally occurs in vigorous, mature trees and can be worsened by physiological stress caused by heat, drought, and other factors, possibly soil pathogens such as Phytophthora or Fusarium.
Signs of a tree collapsing from quick decline are the sour orange rootstock, CTV as determined by a lab test, and a simple field diagnosis with an iodine test to indicate the virus.
“One of the other things to look for is bud union pathology. The bud union can have a brownish stain, and the rootstock may have numerous stem pits forming a honeycomb pattern,” he said.
Briefly, the iodine test involves collection of branches and roots and their treatment with the chemical on cut ends. A positive sign of quick decline is the sour orange roots show no color, while the scion branches are discolored. This indicates a “choke point” in movement of starches at the bud union.
Yokomi leads a team of researchers probing into the numerous additional strains that have been found in the SJV during the past 40 years. The severe types have to be identified by molecular and other sophisticated laboratory testing.
When more is known about the severe strains and how to distinguish them, trees at highest risk can be selectively removed first, rather than all CTV-infected trees, he said.