2024–2025 Florida Citrus Production Guide: Tristeza Decline

Citrus tristeza virus (CTV) is a major cause of the decline and eventual death of trees on sour orange rootstocks. Initially, affected trees have small leaves and twig dieback. Diseased trees often produce small fruit, and the yield declines. Eventually, large limbs die back and the tree gradually declines. In extreme cases, trees may suffer from quick decline and wilt, dying in a matter of weeks. On sour orange rootstock, some isolates of CTV cause graft incompatibility at the bud union, which results in the loss of fibrous roots and reduced ability for water uptake. Bark flaps cut from across the graft union of declining trees often show pitting consisting of small holes (honeycombing) on the inside face of the bark flap from the rootstock side of the union. Quick-decline trees may only have a yellow-brown stain at the bud union and not show the honeycombing. Only trees on sour orange rootstock are affected by tristeza decline. Sweet oranges usually are more affected than grapefruit, whereas lemons on sour orange rootstock may not be affected. Losses from CTV-induced decline have been offset by the use of nonsusceptible rootstocks, primarily trifoliate hybrids. Because huanglongbing (HLB) is endemic in Florida, many trees are now mixed infected with CTV and HLB and may show more profound decline regardless of the rootstock used.

Citrus tristeza virus is transmitted by aphids. They acquire it within minutes of feeding on the plant and transmit the virus to healthy plants equally rapidly. The brown citrus aphid (Toxoptera citricida), which first appeared in Florida in 1995, is considered the most efficient vector of the virus. The cotton or melon aphid (Aphis gossypii) is a less efficient but still effective vector, whereas the green citrus or spirea aphid (Aphis spiraecola) and the black citrus aphid (Toxoptera aurantii) are considered to be less efficient vectors of CTV in Florida. The establishment of brown citrus aphid in Florida is believed to have resulted in a more rapid spread of decline-inducing isolates of tristeza.

Mild isolates, inducing no stem-pitting nor decline, have been widely disseminated in Florida by aphids and in infected budwood. The incidence of decline has been reduced due to the use of tolerant (non–sour orange) rootstocks such as Swingle citrumelo and Carrizo citrange. This also can be attributed to reduced brown citrus aphid populations and CTV reinfection rates probably due to recent more aggressive pest management practices for Asian citrus psyllid in the groves. However, many of the trees on these tolerant rootstocks are shown to be infected with CTV decline isolates without showing symptoms. Therefore, there is still a high risk for reemergence of tristeza decline disease if the sour orange rootstock is used in Florida.

In some countries, certain CTV isolates also cause stem-pitting (see EDIS PP-227, "Citrus Diseases Exotic to Florida: Citrus Tristeza Virus-Stem Pitting [CTV-SP]") in citrus scions including grapefruit and sweet orange, regardless of the rootstock used. Isolates that cause decline of sweet orange on sour orange also may cause stem-pitting in trees on other rootstocks, but many decline isolates produce no stem-pitting in grapefruit or oranges. Isolates that are genetically similar to severe stem-pitting isolates found in other countries have been found in Florida. Some of these isolates give a degree of stem-pitting on sweet orange, but it is difficult to assess whether they cause economic losses in field trees due to the widespread presence of HLB.

Virtually all isolates of CTV can be detected by graft inoculation onto sensitive biological indicators, such as Mexican lime. The virus can be detected in young leaves, or in psyllids nymphs feeding on the leaves. More modern and much faster techniques, such as molecular tests including reverse transcriptase (RT) polymerase chain reaction (PCR) or ELISA using polyclonal antisera, are nowadays preferred. The monoclonal antibody MCA-13 detects most CTV isolates that cause decline on sour orange rootstocks in Florida and also reacts with most stem-pitting isolates, but it does not react with mild Florida isolates. If an appropriate and reliable laboratory test for CTV is needed, contact your local Extension agents, Extension specialists, or FDACS-DPI (FDACS-DPI Helpline Number: 352-395-4600).

Budwood propagated for commercial distribution must be free of CTV. Most stem-pitting isolates are MCA-13-reactive, but not all. PCR-based methods can identify stem-pitting isolates, but the only absolute means to detect stem-pitting isolates is by graft inoculation of grapefruit and sweet orange seedlings and examination of symptoms after 6–15 months (a.k.a. indexing).