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Publication #PP-181

2018–2019 Florida Citrus Production Guide: Tristeza Decline1

O. Batuman, A. Levy, M. E. Hilf, P. J. Sieburth, W. O. Dawson, and R. H. Brlansky2

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 a crop of very small fruit. Eventually, large limbs die back and the tree gradually declines. In extreme cases, trees may suffer from quick decline and wilt, and die in a matter of weeks. On sour orange rootstock, some isolates of CTV cause an incompatibility at the budunion, 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 budunion, 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 are not affected by tristeza decline. Furthermore, because huanglongbing (HLB) is endemic in Florida, now many trees most likely have a mixed infection with CTV and HLB and may show more profound decline regardless of the rootstock used.

Citrus tristeza virus has a wide range of additional isolates of varying symptom severities. Mild isolates have been wide- spread in Florida for many years and these have been widely disseminated by aphids and in budwood. Such isolates do not usually cause decline of scions on sour orange rootstocks. The incidence of decline has slowed due to the usage of tolerant (non-sour orange) rootstocks such as Swingle citrumelo and Carrizo citrange and potentialy because of aggressive chemical control of psyllids, which are assumed to be effective for Toxoptera citricida (the brown citrus aphid) too. However, many of these trees on the tolerant rootstocks are currently infected with CTV decline isolates but show no symptoms. Therefore, there is still a constant high risk for the reappearance of the decline CTV when switching to sour orange.

In some countries, CTV isolates occur that also cause stem-pitting in citrus scions (i.e., grapefruit) regardless of the root-stock used. Grooves and pits appear in the wood of the trunk, branches, twigs and rootstocks. Externally, branches may be twisted and ropy, and twigs may become brittle. Trees affected severely by stem-pitting grow poorly and have lower yields and smaller fruit. Limes and grapefruit are most commonly affected by stem-pitting, but sweet oranges also may be affected. Tangerines are generally tolerant of stem-pitting isolates, and show no symptoms. 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 closely related to severe stem-pitting isolates found in other countries recently have been found in Florida. Some of these isolates give a degree of sweet orange stem pitting, but it is not yet known whether they cause economic losses in field trees.

Virtually all isolates of CTV can be detected by graft inoculation of sensitive biological indicators such as Mexican lime but more modern techniques are preferred such as molecular tests including reverse transcriptase (RT)-PCR or ELISA using polyclonal antisera. The monoclonal antibody, MCA-13, detects most CTV strains which cause decline on sour orange rootstocks in Florida, and also reacts with most stem-pitting isolates, but does not react to mild Florida isolates. Private commercial and public research laboratories are certified by DPI to test samples for detection of CTV using RT-PCR, ELISA or tissue blotting techniques.

Budwood propagated for commercial distribution must be free of MCA-13 reactive CTV. As more registered trees become infected with MCA-13 reactive CTV, many nurseries are either screening their budwood source trees or are buying budwood from screened source trees. The key to preventing problems with stem-pitting in Florida is to avoid introduction, propagation, and distribution of stem-pitting isolates. Most stem-pitting isolates are MCA-13 reactive, but MCA-13 does not selectively identify stem-pitting isolates. Polymerase-chain reaction (PCR)-based methods identify isolates that are from groups that contain mostly 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.

Citrus tristeza virus is transmitted by aphids after minutes of feeding on an infected plant and transmit it to healthy plants within minutes after picking up the virus. Aphids lose the ability to transmit the virus within 24–48 hours after picking up the virus from an infected plant. The brown citrus aphid, which first appeared in Florida in 1995, is considered the most efficient vector of the virus, but experiments indicate that transmission efficiency may vary between isolates. 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 relatively inefficient vectors of CTV in Florida. The establishment of T. citricida in Florida is believed to have resulted in a more rapid spread of decline-inducing isolates of tristeza, as evidenced by an increased incidence of decline of trees on sour orange rootstock

Recommended Practices

  1. Budwood propagated for commercial distribution must be free of MCA-13-reactive CTV. Even in the presence of HLB, it is still recommended that farmers only use decline-resistant rootstocks. The key to preventing problems with stem-pitting in Florida is to avoid introduction, propagation, and distribution of stem-pitting isolates.

  2. Once tristeza-affected trees on sour orange rootstocks begin to decline, there is no treatment to reverse the decline. Generally, individual diseased trees should be replaced with certified trees on tolerant rootstocks as the yields of affected trees decline to uneconomical levels. However, if blocks on sour orange rootstock are affected with HLB decline too, then replace the entire block. (See Choose the Right Rootstocks UF/IFAS EDIS fact sheet HS-932 for rootstock recommendations.)

  3. Within groves with trees on sour orange rootstock that are currently unaffected by tristeza decline, it may be possible to inarch trees with seedlings of a tolerant rootstock. Inarching is most effective with relatively young groves that are still actively growing, and in order to be effective, the inarches must be well established before trees become infected. Given the high cost of the practice, it is probably advisable only in the case of high value crops in groves that are well separated from areas with severe strains of tristeza virus. It is more likely to be successful with younger than with older trees.

  4. Chemical or biological control of the aphid is unlikely to stop the spread of the virus in commercial groves, since acquisition and transmission of the virus by the aphid usually occurs before the aphid is killed by an insecticide. However, rigorous aphid control in nurseries and on budwood source trees could reduce infection rates (see ENY-604 Soft-bodied insects).

  5. Cross protection, which is the practice of inoculating trees with mild virus strains to protect them from the effects of severe strains, has been effective in South Africa and Australia in reducing losses in grapefruit due to stem-pitting, and against losses in sweet orange in Brazil. Cross protection against tristeza decline on sour orange rootstock has not yet been developed as an effective control measure.

Footnotes

1.

This document is PP-181, one of a series of the Plant Pathology Department, UF/IFAS Extension. Original publication date December 1995. Revised September 2013, April 2016, and May 2018. Visit the EDIS website at https://edis.ifas.ufl.edu for the currently supported version of this publication.

2.

O. Batuman, assistant professor, Plant Pathology Department, Southwest Florida Research and Education Center, UF/IFAS Extension, Immokalee, FL 34142; A. Levy, assistant professor, Plant Pathology Department, Citrus Research and Education Center, UF/IFAS Extension, Lake Alfred, FL 33850; M. E. Hilf, research plant pathologist, USDA-ARS-USHRL; P. J. Sieburth, biological scientist III (retired), Bureau of Citrus Budwood Registration, DPI-FDACS; W. O. Dawson, eminent scholar, Plant Pathology Department; and R. H. Brlansky, professor emeritus, Plant Pathology Department, Citrus Research and Education Center, UF/IFAS Extension, Lake Alfred, FL 33850.


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