Craig Kallsen

UCCE Farm Advisor, Kern County

Citrus, Subtropical Horticulture, Pistachios

May 2, 2002

Citrus tristeza virus (CTV) is, arguably, the most serious viral disease of citrus worldwide. In southern California, in the 1940's and 1950's, approximately 3 million citrus trees on sour orange rootstock were killed by CTV. Generally, infection by CTV may be symptomless or may be characterized by single or multiple symptoms including tree stunting, stress responses leading to eventual fruit size, quality and yield reduction and/ or quick decline leading to tree death. Citrus tristeza virus, like the common cold that plagues humanity, comes in many strains. Some strains are more severe than others. Often a tree will be infected with more than one strain. Certain strains prevent the infection, or the symptoms, of other strains. The affect on the tree of infection by several strains may be quite different than the effect of any single strain. The severity of the disease also varies with variety and rootstock. Mandarins are very tolerant of CTV regardless of rootstock. Grapefruit and oranges, which will decline quickly on sour orange rootstock, are tolerant of most California CTV strains if grown on commonly used citrange, trifoliate or lemon-type rootstocks.

Characteristics of the virus, how it is spread, and factors that affect its severity makes any management plan for this virus difficult and expensive, and no course of action can guarantee an acceptable outcome. Currently, in Kern County and in parts of several other counties in the San Joaquin Valley of California, the disease is managed through tree monitoring for CTV with mandatory removal of infected trees. This program, actively pursued since the mid 1990's, has resulted in a very low CTV infection in Kern County trees. However, this degree of control has come at a price in both fees paid by the growers for tree monitoring, and in costs associated with replacing CTV infected trees that are not covered by the existing reimbursement program. How best to manage CTV is a contentious issue within the citrus industry. Many trees that have been removed were on CTV-tolerant rootstocks and would have continued to produce normal or near-normal yields of high-quality fruit for some time into the future.

Tree monitoring in Kern County is extensive, and eventually, all orchards and most individual trees in each orchard in Kern County will be tested for CTV. This testing not only includes a test for the presence or absence of CTV, but also involves isolating strains that appear to be more severe. Unfortunately, much remains to be discovered about our ability to identify individual strains, especially if several strains infect the same tree, and to determine their potential for future damage. Constant monitoring is important in that, as happened in other citrus-growing areas of the world, new strains appear that may overcome the tolerance to CTV that many of the existing California rootstocks have. Recently, testing in Fresno County showed that a virulent strain of CTV had been illegally imported as budwood into California from Japan, and this had then been budded onto trees in a large commercial orchard. The CTV testing resulted in the early discovery of this new, severe strain and removal of the trees before it could spread throughout the industry.

In South Africa, CTV is endemic and no program of enforced tree removal is in effect. South Africa still has a viable citrus industry. Citrus tristeza virus is controlled by means of cross-protection. Trees are cross-protected by deliberately propagating them with infected budwood containing a mild-strain of CTV, which prevents a more harmful, severe strain from infecting or expressing itself in the tree. The process is somewhat comparable to vaccination in humans. Cross-protection does result in fruit yield and quality reductions in South African trees, but does allow the industry to continue. Tree longevity, also, appears to be reduced by cross-protection. CTV strain monitoring continues to be a very important part in the South African CTV control strategy. New strains can appear which can overcome existing cross-protection. Research efforts must be maintained to ensure that new cross-protective strains are available when needed. The CTV management program in South Africa remains an expensive program. Since CTV was endemic, tree removal was not an option for the South Africans.

The virulence of strains also appears to have a geographic component in South Africa in that different cross-protective strains have to be developed for effective control of CTV for different geographical areas. There is speculation that hot temperatures suppress CTV and virus levels are very low in trees in the San Joaquin Valley during summer. In the trees of the southern coastal areas of California, where CTV is widespread, the virus is readily detectable throughout the year. No mandatory tree removal program exists in the coastal or southern citrus growing of California and a viable citrus industry continues to exist. There is speculation, however, that continued problems with fruit size and quality may be related to CTV in many of these older orchards.

If the mandatory infected-tree removal program is eliminated in Kern County, and if severe strains appear with frequency in citrus trees, the California industry will probably have to develop a program similar to that of South Africa to ensure its survival. This program would not only require the level or higher levels of CTV monitoring than that which currently exists, but would, also, require an expensive research component for development and maintenance of effective cross-protective budwood.
Since CTV is not endemic in the San Joaquin Valley of California, removal of CTV-infected trees remains a valid control option. The continuation of the mandatory CTV-infected tree removal program in Kern County is based primarily on the concern that not enough is known about strain severity, or the severity of combinations of CTV strains, to adequately evaluate the risk that a given tree may pose to the industry. Mandatory tree removal is based on the assumption that eventually 1) severe strains already present in California or which may appear in the future will continue to spread and 2) that even if new severe strains or combinations of strains do not appear, existing mild strains in trees on tolerant rootstocks will produce smaller yields of less marketable fruit. On-going research work by Louis Whitendale, at the University of California Citrus Research Center in Lindcove, California, suggests that even mild strains of CTV adversely affect citrus fruit yield and quality with time. Louis Whitendale has shown that in areas of the San Joaquin Valley where CTV infected trees are no longer removed, rates of CTV infection in orchards are increasing. His results show that recently infected young trees exhibit symptoms of a typical stress response, which include reduced vegetative growth and production of greater yields of poorer quality fruit. Mature trees, which have been infected for a relatively short time (5 years), show similar stress responses. Yields are higher but fruit quality may be reduced. Results from most parts of the world suggest that the longer the tree is infected (including cross-protected trees) the greater is the deleterious effect on fruit yield, quality and size.

An important component of the current CTV eradication program in Kern County is the lack of an efficient vector of CTV. currently, the principle vector of CTV in California is the cotton aphid (also called melon aphid), Aphis gossypii. The cotton aphid is not as efficient at spreading CTV as is the brown citrus aphid (BrCA), Toxoptera citricida. In areas of the world where the BrCA exists, the rate of spread of CTV greatly increases. CTV infections increased from less than 5% to greater than 95% in 2 to 5 years in the Dominican Republic after the introduction of BrCA. Unlike the cotton aphid, BrCA can rapidly build to high populations in citrus. Fortunately, the BrCA is not present in California, but it has arrived recently in Florida. One reason frequently given for lowering our CTV infection rate through tree removal in California is to lower our inoculum level prior to the expected arrival of the BrCA. However, the more likely scenario is that once the BrCA arrives even a very low rate of CTV will quickly translate to the rapid spread of CTV. Should BrCA arrive in Kern County, the development of a cross-protection strategy may be our only long-term CTV control option.
Currently, a California Department of Food and Agriculture regulated quarantine is in effect preventing importation of trees from certain areas of California, especially the southern coastal areas, into Kern County. Not only does this quarantine protect our area from CTV, but also potentially from other pests, vectors (like BrCA) and diseases that may enter this part of California illegally from other parts of the world. Should the CTV eradication program be abandoned, the associated broad-spectrum umbrella of a protective quarantine program would also end.

As a farm advisor working in Kern County, the CTV eradication effort has made my job easier. As a result of the trouble and expense that growers have endured to clean their groves of CTV, this disease can normally be eliminated early as a causal factor in diagnosing a sick tree. Additionally, the absence of CTV, undoubtedly, reduces the number of farm calls that I have to make related to tree decline. Obviously, the purpose of controlling CTV is not to make my job easier. Citrus growers are the ones who have been paying the high costs associated with controlling this disease and there appears to be no cheap option over the long term. The good news is that the San Joaquin Valley citrus industry has been very effective in keeping CTV under effective economic control since the early 1960's and, with patience, money and effort, I fully expect this trend to continue for the next forty years.