CITRUS TRISTEZA VIRUS OR QUICK DECLINE OF CITRUS (CTV): Group: Closterovirus

DISTRIBUTION : CTV is found in all citrus growing areas in the United Sates and elsewhere. The most recent survey of commercial citrus in AZ was negative for CTV. Because of suppression efforts, CA has some areas they consider free of CTV.

DESCRIPTION: CTV is only visible using an electron microscope after positive staining (Fig 1 bar indicates 55 nm). It has the shape of a long flexuous (bending or winding alternately from side to side) rod about 11 X 2,000 nm in size. The genetic material of the virus consists of a single strand of ribonucleic acid (RNA) which is enclosed by a protein coat. There is more than one strain of CTV and recent molecular studies have revealed considerable differences among various isolates (pure stains).

HOSTS: Almost all citrus species, hybrids, and relatives.

DAMAGE: The most economically important symptom is the quick decline or death of orange, grapefruit and mandarin trees on sour orange rootstock. Virus infection in the scion (the slip that is graphed onto rootstock) causes dieback of the phloem (food-conducting tissue) in the sour orange rootstock below the bud union (scar at base of tree where the top was grafted to the root stock). This results in the rootstock being girdled and therefore starved as the starch reserves are depleted. A second symptom of the disease is a 'slow decline' where the trees decline in health over a period of years. This decline is accompanied by a loss in productivity but the tree does not necessarily die. A third common symptom is stunting where the virus does not kill the tree, but the tree does not grow. One strain causes stem pitting (fig 4) of grapefruit and sweet orange.

INSPECTION TIPS: Leaf samples can be collected for testing (ELISA test), preferably in the spring or fall. Signs of diseased trees include: overgrowth of the scion at the bud union, scarcity of feeder roots, stunting (Fig 2), yellowing leaves (Fig 2), reduced fruit size, poor growth, dieback (Fig 3), wilting, and death. Citrus on sour orange rootstock may exhibit honeycombing on the inner surface of the bark. Stem-pitting (Fig 4) may not be apparent until the bark is pealed back, but rope-like symptoms (Fig 5) may be apparent. Twigs are brittle and break easily in the wind and under fruit load.

LIFE CYCLE: CTV spreads through use of CTV-infected budwood and by aphids. It is most abundant (highest titer) at times when the aphid is most abundant (times of growth flush). The titer is at its lowest in the summer. Viruses are made up of a protein coat (capsid) protecting genetic material (RNA). The protein coat recognizes and fits with the protein on the target cell membrane. It attaches and dumps the virus’s genetic material (RNA) into the cell. The RNA is used as a template to produce more viruses. The cells’ resources are used up, disrupting phloem cell function.

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