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Toru Iwanami
NARO Fruit Tree Institute
Tsukuba, Ibaraki 305-8605, Japan

ABSTRACT

Incidence of some citrus diseases has been obviously affected by climate change in Japan. Tropical and sub-tropical diseases that had been rarely seen are now commonly found in the southern part of Japan. Among others, prevalence of citrus greening is mostly feared. The disease is apparently introduced from neighboring countries to Japan, and it occurs in most of the subtropical islands. Warmer climates promote expansion of areas where the vector insect, Diaophorina citri inhabits. Some coastal locations of Kyushu, one of the main islands of Japan, are now infested with D. citri, and outbreak of greening in Kyushu is greatly feared. Citrus phytoplasma disease is another possible transboundary disease under warmer climate, but extensive survey in subtropical islands in Japan failed to detect the disease so far.

Key words: citrus greening, Huanglongbing, Diaphorina citri, climate change

INTRODUCTION

Invasion and spread of citrus greening in Japan

Citrus greening (Huanglongbing or HLB) is a serious disease of citrus which limits production in many parts of Asia and Africa (da Graca, 1991). Recently, the disease is also found in Florida and Sao Paolo. The Asian causal agent is designated "Candidatus Liberibacter asiaticus" (Las, Jagoueix et al., 1994). It is non-culturable phloem-limited bacterium, and is transmitted by citrus psyllid, Diaphorina citri. The adult and nymph of citrus psyllid require Las by phloem-feeding on infected citrus trees, and is transmitted to the pathogen in a persistent manner. The Asian greening vector, D. citri is common in subtropical islands in Japan (Miyatake, 1965) rendering trees in those regions susceptible once the pathogen is introduced. The disease was found for the first time in the southernmost islands of Japan in 1988 (Miyakawa and Tsuno, 1989). Since then, the disease has spread northward to the main production areas, posing a great threat to domestic citrus cultivation.

The disease is currently spread northward near to Amami island (Fig. 1). There is growing concern about the damage in local citrus production. Presumably, due to warmer climate, the citrus psyllid has recently invaded the Kyushu main island (Fig. 1). Fortunately, the disease is not yet found in this main island, which is a major citrus production area. However, it is very likely that the outbreak of greening follows the invasion of citrus psyllid in a new area, as observed in Florida recently, and local citrus growers are greatly concerned.

DISCUSSION

Possible establishment of citrus psyllid populations in the southern coastal areas of the Kyushu main island

Experimental data obtained by rearing adults citrus psyllids in temperature-controlled chambers indicated that adult citrus psyllids survive winter when the daily coldest temperatures averaged above 5oC in the coldest months (Ashihara 2007). Establishment of citrus psyllids populations in Ibusuki city, when the average daily coldest temperatures in January and February are about 3.5oC, indicated that citrus psyllids have even more cold-hardiness. The average daily coldest temperature in January and February in the southern coastal areas in the Kyushu main island range 2.5oC-4.5oC, and more regions will be as warm as these coastal areas due to climate change. It is expected that citrus psyllid might spread and establish in the southern coastal areas of the Kyushu main island in the near future, if aggressive control measures for enclosure or elimination were not taken.

Northbound spread of citrus greening indicated by DNA markers

Citrus greening in the southern chains of islands in Japan has been reported from southernmost islands up to the northern islands. However it was not clear if these first-records correspond to the first occurrences in each island. Some arguments say that the disease was found from south to north, just because investigation was made from south to north.

Recently, genome sequences of Las (citrus greening bacterium) has been reported

Subsequently, it was found that the genome contains several simple sequence repeats (SSR) which can be applied as DNA markers to estimate population structures of Las (Katoh et al., 2011). About 100 isolates of Las from the southern chains of islands in Japan as well as those from Taiwan were differentiated using the SSR markers. The results showed that Las populations are complex in the southern-most islands of Japan, as well as of Taiwan, but they have been found to be very simple in the northern islands. Generally, it is considered that complex population structure of pathogen indicate long establishment, whereas simple population structure reflect new invasion. SSR analysis indicated that Las has spread northbound in the southern chains of islands in Japan, presumably after introduction from Taiwan.

Current research topic on citrus greening in Japan

National Institute of Fruit Tree Science, NARO and the research institutes of local governments have started a concerted effort to develop emergent control measures of citrus greening. Current research topics include detection of the pathogen utilizing Loop mediated Isothermal Amplification (LAMP), a molecular differentiation of field isolates using SSR in the bacterial genome, and analysis on the mode of transmission by psyllid (Okuda et al., 2005, Katoh et al., 2001, Inoue et al., 2009). These newly developed methods support eradication and rehabilitation , which are now practiced by the Ministry of Agriculture, Fishery and Forestry and local governments in the subtropical islands. The goal of future research projects should be the establishment of sustainable control measures including introduction of tolerant citrus cultivars, reduction of the psyllid population with minimum use of pesticides, and development of genetically modified citrus that is highly tolerant to greening.

Selection of natural tolerant citrus varieties are also in progress. Although Las apparently infects most of the citrus and citrus relatives, some lines of citrus varieties do show some tolerance at least under experimental conditions in greenhouses. Several local mandarins including Shikuwasha (Citrus depressa Hayata) are grown in the subtropical islands of Japan. Recently, Shikuwasha has become very profitable cash crop, and local growers tried to produce more fruit. On this backdrop, the occurrence of greening on these local mandarins is greatly feared. Seedlings of over 100 lines of Shikuwasa were graftly inoculated with Las. Severe mottling, yellowing and intervenial chlorosis appeared on leaves of all but for four lines of Shikuwash. The tolerance of these lines is yet to be co firmed by further experiments including field trials.

Phytoplasmas related to citrus greening

Phytonplasmas are bacteria without cell walls, that just like Las, inhabit phloem sieve elements in infected plants. Phytoplasmas are transmitted by sap-sucking insects, typically leafhoppers. There are two reports showing that two species of phytoplasma are related to citrus greening-like symptoms in People's Republic of China and Brazil (Chen et al., 2008, Teixeira et al., 2008). Niether of the two reports identified insect vectors. Putative insect vectors of these phytoplasmas may favor warmer climate, and thus climate change may promote spread of citrus greening-like symptoms over borders. So far, extensive survey in subtropical islands in Japan failed to detect any greening-related phytoplasmas. Occurrence of such phytoplasma-related disease should be carefully watched, and in case of outbreak, should be contained and eliminated immediately. As shown, Las SSR markers would be a powerful tool to trace movement of greening-related phytoplasma over boundaries.

Outbreak of pear psyllids in Japanese pear orchards in the Kyushu main island

Some pear psyllids are serious pests in pear orchards of temperate and subtropical regions. The infested leaves are severely damaged by necrosis and they drop prematurely. Furthermore, some of these psyllids transmit phytoplasmas which induce pear decline. Outbreak of one pear psyllid, Cacopsylla chinesis is reported in Taiwan (Lee et al. 2008). Recently, there was an outbreak of C. chinesis in several Japanese pear orchards in the Kyushu main island of Japan. The occurrence of the pest is still contained in one city, and it is likely that invasion took place recently. It is feared that warmer climate might promote further spread of C. chinesis in the Kyushu main island.

CONCLUSION

Warmer climate promotes spread of pests introduced from tropical and subtropical regions. Recently, outbreak and spread of citrus and pear psyllids, which prefer warmer temperatures, in some areas of Japan are very evident. These insects are not only serious pests, but they transmit pathogens of citrus greening bacterium and pear decline phytoplasma. Citrus-infecting phytoplasma is also a potential threat to citriculture in Japan. One of the important steps in the establishment of control measures is to trace movement of these pests and diseases over boundaries. DNA markers including SSR of genomic DNA would be very useful for this purpose.

REFERENCES

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  • Lee, H.C., Yang, M.M., and Yeh, W.B. 2008. Identification of Two Invasive Cacopsylla chinensis (Hemiptera: Psyllidae) Lineages Based on Two Mitochondrial Sequences and Restriction Fragment Length Polymorphism of Cytochrome Oxidase I Amplicon. Journal of Economic Entomology 101(4):1152-1157.2008.
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  • Su, S.J. and Iwanami, T. 2011. Differentiation of Strains of "Candidatus Liberibacter asiaticus" Strains by Variable Number of Tandem Repeat Analysis. Applied and Environmental Microbiology 77: 1910-1917.
  • Inoue, H., Ohnishi, J., Ito, T., Tomimura, K., Miyata, S.., Iwanami, T. 2009. Enhanced proliferation and efficient transmission of Candidatus Liberibacter asiaticus by adult Diaphorina citri after acquisition feeding in the nymphas stage. Annals of Applied Biology 155:29-36.
  • Okuda, M., Matsumoto, M., Tanaka, Y., Subandiyah, S., and Iwanami, T. 2005. Characterization of the yufB-secE-nusG-rplKAJL-rpoB Gene Cluster of the Citrus Greening Organism and
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Index of Images

  • Fig. 1 Spread of citrus greening in Japanese sub-tropical islands between Taiwan and Kyushu. Numbers and arrows correspond to years and locations, when and where greening was found.

    Fig. 1 Spread of citrus greening in Japanese sub-tropical islands between Taiwan and Kyushu. Numbers and arrows correspond to years and locations, when and where greening was found.

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