Satsuma dwarf virus was first found to be transmissible by grafting in 1952 in Japan. An old-timer in the vicinity had noticed the disease and its gradual extension to the surrounding area since the 1920s. The foci are found in many citrus growing areas, but until recent years they were limited to small isolated areas. However, the number of affected trees is now increasing, due to the widespread use of topgrafting in addition to natural transmission. Citrus mosaic, Natsudaidai dwarf and navel orange infectious mottling are usually placed in the same category as Satsuma dwarf, but their incidence is still limited to small isolated areas of Japan.
A disease similar to Satsuma dwarf has been found in Cheju Island in Korea, Mainland China, and Turkey. In some cases, these were probably introduced from Japan in infected budlines. Because of their soilborne nature, these diseases have received considerable attention.
Affected trees of Satsuma mandarin become dwarfed, with short internodes and abnormal leaves showing a boat or spoon shape ( Fig. 1 and Fig. 2). Young leaves just after sprouting occasionally show flecking between the veinlets. These symptoms tend to appear at lower temperatures while new shoots are developing. Exposure to temperatures above 28°C for about 12 hours per day masks the symptoms.
In Japan, spring and autumn shoots develop typical symptoms but summer shoots are generally free of them. Double infection with a severe strain of tristeza virus results in extreme dwarfing ( Fig. 3).
A mildly affected tree bears normal fruit, but a severely affected tree has poor fruit-set and its fruit are small and irregularly shaped.
Early maturing varieties of Satsuma mandarin may also develop spoon-shaped leaves, especially on the lower branches, even if they are free of Satsuma dwarf virus (SDV). This is due to the inherent character of these varieties, coupled with tristeza virus. These trees have never shown any symptoms of decline.
When Iyo or navel orange are topgrafted onto affected trees, irregularly shaped fruit ( Fig. 4) and poor fruit-set are also observed.
The leaf symptoms in Satsuma mandarin of citrus mosaic virus are similar to those of Satsuma dwarf. It is the green blotches or ring-shaped spots on the rind of fruit at color break which characterize citrus mosaic ( Fig. 5). Fruit symptoms also develop on lemon trees, and trees of Marumera, Kinkoji, and Iyo.
Typical symptoms such as mottling, curling and crinkling of leaves develop on the spring shoots of Natsudaidai. Curling and crinkling are persistent, but mottling disappears with hardening of the leaves. No symptoms have ever been observed on the fruit itself, but there is sometimes poor fruit-set.
Navel Orange Infectious Mottling
Typical symptoms are mottling, and large diffused chlorotic blotches, on leaves at the spring flush. The most important damage is due to fruit dropping, which usually occurs after the June drop.
The Pathogen and Its Transmission
The causal viruses of Satsuma dwarf (SDV), citrus mosaic (CiMV), Natsudaidai dwarf (NDV), and navel orange infectious mottling (NIMV) are isometric, and approximately 26 nm in diameter ( Fig. 8). Some antigenic relationships are recognized among them.
These viruses can be mechanically transmitted to some herbaceous plants. However, mechanical transmission from citrus to citrus seems to be very rare in the field. In the case of grafting, successful transmission occurs when an affected bud has been attached to the receptor for at least four days.
Natural transmission through soil is more important in the field than mechanical transmission. Virus spread is in a compact circle across the path and soil bank, but not over ditches which contain flowing water. Replanted trees in orchards from which affected trees have been removed become infected within a few years. Recently, rapid transmission was observed in a field where China laurestine, also known as viburnum ( Viburnum odoratissimum Ker.), had been planted as a windbreak. China laurestine ( Fig. 6) appears to be a symptomless carrier which accelerates transmission through soil. An effective vector has not yet been found.
Enzyme-linked immunosorbent assay (ELISA) is the most useful means of diagnosing these virus diseases. Using anti-SDV serum can detect, not only SDV, but also CiMV, NDV and NIMV, even if activity varies with the virus. New sprouts or tender leaves of 0.5-1 g are used for indexing. Results can be obtained within two days.
Sap inoculation onto white sesame ( Sesamum indicum) can also be used. Sesame plants at the two- to four-leaf stage are inoculated and then kept at 20-25°C. Local lesions develop on the inoculated leaves, and subsequent necrosis occurs on the upper portion of the plants ( Fig. 7).
Identification of those viruses can be achieved by ELISA, using specific antisera, or by tissue-graft inoculation onto some citrus indicators.
The certification of budstock and prevention of contamination are the primary countermeasures to control the disease. Indexing of budstock by ELISA is also recommended. Heat-treatment or shoot-tip grafting can easily eliminate the viruses. Because the virus diseases can be soilborne, it is necessary to check nursery plants occasionally after they have been planted out in the field. The propagation of budwood by topgrafting should be avoided.
If affected trees are found, they should be immediately removed from the orchard. The soil should then be dug as deeply as possible and fumigated with chloropicrin.
China laurestine should never be planted in citrus orchards.
Index of Images
Figure 1 Satsuma Mandarin Tree Affected by Satsuma Dwarf Virus. Note the Boat Shaped Leaves and Poor Fruit-Set
Figure 2 A Seedling of Satsuma Mandarin Infected with SDV Only. Note That the Leaves Are Boat- or Spoon-Shaped, but the Tree Is Not Severely Dwarfed.
Figure 3 Severely Dwarfed Tree of Early Maturing Satsuma Mandarin (Right), Caused by SDV Plus a Severe Strain of Tristeza Virus (CTV). a Normal Tree Is Also Shown (Left) Which Is Infected with CTV Alone.
Figure 4 Irregularly Shaped Fruit of Iyo Tree Topgrafted Onto Satsuma Mandarin Tree Infected with SDV.
Figure 5 Typical Fruit Symptoms of Citrus Mosaic Caused by Cimv.
Figure 6 China Laurestine, or Viburnum, Is an Important Plant in Accelerating the Transmission of SDV through Soil.
Figure 7 Symptoms of White Sesame Inoculated Mechanically with SDV. Note the Local Lesions on the Inoculated Leaf and the Systemic Necrosis on the Upper Portion of the Plant.
Figure 8 Causal Virus of Satsuma Dwart: Isometeric Particles Are 26 NM in Diameter.
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