citrus packing machine

Citrus varieties having natural resistance to or toler­ance for CTV

A first-management strategy to employ is to plant CTV-tolerant or -resistant citrus varieties (27, 54). For example, tristeza, which is a disease of the graft union when sour orange rootstocks are used, can be avoided by using nonsusceptible rootstocks. Sour orange was once a popular citrus rootstock  citrus packing machine due to its tolerance to other plant diseases such as foot rot, caused by Phytophthora spp. Thus, these diseases often once again become a problem when non-sour orange, tristeza-resistant rootstocks are used.

Rigorous testing of various tristeza-resistant root­stocks under local conditions is the best way to determine which rootstocks to use. In Hawai‘i, such testing has revealed that ‘Cleopatra’ and ‘Sunki’ mandarins,

 ‘Heen Naran’ tangerine, and ‘Rangpur’ lime rootstocks grow best under our local conditions (39). However, the use of resistant rootstocks will not effectively manage the more  severe, stem-pitting strains of CTV, as the symptoms manifest in the scion. Most economically important citrus varieties, however, do not show resistance to stem pitting. Yet, some varieties of pummelo appear to be immune or highly resistant to CTV, including the stem-pitting strains (20, 31). A large pummelo growing in Waiäkea, Hawai‘i, surrounded by citrus trees harboring tristeza and stem-pitting strains, has remained  citrus packing machine CTV-free for many years, despite numerous natural and artificial attempts at inoculation (28, 31). ‘Persian’ limes, white grapefruits, ‘Valencia’ oranges, and most mandarin va­rieties are the varieties most resistant to stem pitting (29).

Various citrus relatives such as Poncirus trifoliata were also thought to be immune to CTV (27), although it has been recently demonstrated that some strains of CTV can overcome this resistance (9).

Mild-strain cross protection  citrus packing machine

Mild-strain cross protection (MSCP) is another strategy for controlling diseases caused by CTV where virus incidence is high. In this approach, healthy plants are inoculated with a mild strain of CTV, which thereafter confers resistance against subsequent infections by more severe strains of CTV. The more widespread such cross-protection programs are in a geographic region, the more effective they are (81). Several different strategies have  citrus packing machine been employed to find and test potential mild strains for use in cross protection (8, 47, 59, 67, 80), often with posi­tive results (8, 43, 55, 63). With the recent development of infectious clones of both CTV (68) and CTV defective RNAs (dRNAs) (77), however, future MSCP programs may utilize strains of CTV or dRNAs that are engi­neered for optimal protection (29). Some plant nurseries in Hawai‘i sell cross-protected citrus plants to growers.

However, there are some risks associated with mild-strain cross protection. First, mild strains are plant pathogens and may reduce the productivity of infected plants. This strategy is only used in those regions such as Australia, Brazil, Florida, New Zealand, and South Africa where the potential losses from CTV are so high that growers  citrus packing machine are willing to accept some losses in fruit yields to remain in production. Second, the resistance provided by cross protection is not complete and gener­ally does not last for the life of a tree. Depending on the effectiveness of the mild strain, the effects of MSCP may last for two to ten growing seasons (43). There is also a concern that mild strains may spread to other hosts where their pathogenic effects might be more severe (26). In addition, a synergistic reaction may occur between the mild strain and an invading unrelated virus (26), caus­ing severe disease. Finally, a mild strain may mutate to a more virulent form, resulting in severe disease.

Developing citrus varieties resistant to CTV

The most promising and useful strategy for controlling CTV where epidemics are caused by severe strains is the development of resistant citrus varieties. There  citrus packing machine are three approaches used to develop such varieties: 1) conventional breeding, 2) somatic hybridization, and 3) genetic engineering.

Conventional plant breeding usually involves cross­ing a susceptible, desired variety with a closely related, resistant variety (54). Unfortunately, conventional breed­ing in citrus is difficult due to many factors, including  citrus packing machine complex reproductive biology, compatibility factors, weak zygotic embryos, partial or complete pollen/ovule sterility in important cultivars, and long juvenility pe­riods (58).

Somatic hybridization of protoplasts derived from citrus and related species can overcome the compatibility barriers encountered in conventional plant breeding. This method is being used to develop plants that are poten­tially tolerant to CTV diseases as well as other important diseases such as citrus blight (53).

Genetic engineering is arguably the most promis­ing method for incorporating resistance to CTV into host plants. The first transgenic citrus were developed by inserting DNA sequences directly into citrus proto­plasts (40, 46, 75). This approach, however, was largely abandoned when more efficient  citrus packing machine Agrobacterium-based transformation protocols were developed (57, 60). Transgenic plants have since been reported for species of the genera Citrus (C. aurantifolia, C. aurantium, C. grandis, C. limon, C. paradisi, C. reticulata, C. sinensis); Poncirus (P. trifoliata and its hybrids); and Fortunella (F. crassifolia) (61, 78).