|Longan witches’ broom (aetiological agent unconfirmed)|
Common name(s): Longan witches’ broom
Host(s): Dimocarpus longana (longan) (DOA, 2003b; Qui, 1941); Litchi chinensis (lychee) (Chen I., 1996) but questioned by AQSIQ (2003).
Plant part(s) affected: Flower, leaf, seed, shoot (Chen et al., 2001; Menzel et al., 1989).
Distribution: Brazil, China (Guangdong, Guangxi, Hainan, Hong Kong), Taiwan, Thailand (Kitijima et al., 1986; Koizumi et al., 1995; Menzel et al., 1989; So and Zee, 1972; Zhu et al., 1994).
Longans: The earliest description of this disease on longans is by Qui (1941). Young leaves of infected shoots are small and light green in colour, with curved margins. They appear stunted and deformed, and tend to roll up rather than expand (Zhang and Zhang, 1999). Adult leaves are light yellow-green with marbled yellow spots and brown veins. Leaves form blisters and become distorted and dry before falling off (Menzel et al., 1989; Zhang and Zhang, 1999). Shoots on infected branches become compacted clusters and the inflorescences are unable to extend. The flower organs develop abnormally, and consequently, the flowers either fail to produce fruits or develop into small and empty fruits. A characteristic symptom of the disease is the loss of flowers from panicles, resulting in a ‘broom-like’ appearance of inflorescences (Menzel et al., 1989).
In Thailand there are reports of fine light green hairs forming an erinium on both sides of affected leaves. Aceri dimocarpi mites reside inside the erinium mass (Visitpanich et al., 1996).
Different cultivars of longan vary in their sensitivity to damage by this disease (So and Zee, 1972). Although the causal organism appears to be systemic, not all branches of an infected tree show symptoms of the disease (Vera and Zee, 1972). Symptoms of longan witches’ broom resemble those described for lychee witches’ broom (Chen et al., 1996; Koizumi, 1995). A study conducted in Hong Kong revealed that disease symptoms were more frequent on younger trees (10-25 years) than on older trees (30 years) (So and Zee, 1972). However, there is disagreement among the available literature as to whether longan witches’ broom is caused by a virus (Chen et al., 1996; Chen et al., 2001; So and Zee, 1972; Ye et al., 1990), a mycoplasma (MLO) (Menzel et al., 1989), or a mite (He et al., 2001).
Several studies indicate that the causal agent of the disease is viral. So and Zee (1972) carried out electron microscopy of ultrathin sections from diseased leaves and found filamentous particles that measured about 12 nm in diameter and about 1000 nm in length. The virus particles seemed to be restricted to the sieve tubes, and in the mature sieve tubes appeared to be closely associated with the plasma lemma and the cell wall. They were rarely present in the lumen of the sieve tubes and have never been seen in the non-infected tissues. These virus particles seldom occurred singly, but usually in a cluster. Ye et al. (1990) partially purified a filamentous virus from the leaves and bark of infected longan trees and reported filamentous virions with a diameter of about 15 nm and a length of 300-2,500 nm, with most 700-1300 nm in length.
Details of longan witches’ broom virions from diseased trees were detected by means of an enzyme linked immuno sorbent assay (ELISA). Chen et al. (1996) also found filamentous virus particles in leaf phloem cells of infected plants. Using immuno sorbent electron microscopy (ISEM) technique, filamentous viral particles were trapped from the extract preparations of diseased plant materials and the salivary glands of Corngenasylla sinica (longan psylla) and Tessaratoma papillosa (litchi stink bug) (Chen et al., 1994). From these results, Chen et al. (2000) concluded that the disease is caused by a filamentous virus.
Since no photos of the virus were available and the experimental results were not replicated, the existence of a virus pathogen of the disease remained controversial. In order to clarify the cause, a series of research projects have been conducted since 1986 in the Fujian Academy of Agricultural Sciences. Other organisms such as a phytoplasm (= MLO) and twig borer insects were suspected of being the causal agent (Li, 1983), although administration of anti-biotic treatments to seedlings failed to suppress the disease, indicating that a phytoplasm was unlikely to be the cause (Chen et al., 1989).
He et al. (2001) carried out investigations in orchards in Guangdong Province between 1995 and 1998, and reported that longan witches’ broom is caused by the mite ^ Kuang, and not by a virus or a twig borer. They observed that witches’ broom diseased shoots could occur both in the presence and absence of twig borer tunnel damage. However, when longan seedlings were inoculated with mites, 50% developed symptoms of witches’ broom disease and hosted mites, whilst no mites were found on the leaves of the symptomless plants. The mite was always found to exist on diseased shoots and spikes, and the number of mites was positively correlated to the severity of the disease. Integrated management of pruning and spraying with a mitecide on diseased shoots restored blossoming and reduced the average incidence of diseased spikes from 80% to 9% in three orchard trials. Further evidence of a mite being implicated in the witches’ broom disease has been reported in Chiang Mai and Lam Phun provinces of Thailand, where the aetiology is thought to be the mite Aceria dimocarpi (Kuang) and a transmitted phytoplasma (Chantrasri et al., 1999; Visitpanich et al., 1999). After one month, feeding by the mites caused witches’broom symptoms along the shoot of seedlings. Electron micrographs revealed phytoplasma cells in the cytoplasm of infected sieve tube elements and were confirmed by PCR techniques (Chantrasri et al., 1999). However, Sdoodee et al. (1999) were not able to confirm the presence of phytoplasmasa in infected longan tissue with PCR despite the DNA indicated the presence of a prokaryote.
Studies relating to the transmission of the ‘virus’ were undertaken from 1985-89 by Chen et al. (1992). It was found that longan witches’ broom was transmitted from one longan tree to another and from longan to lychee trees by the vectors Tessaratoma papillosa Drury (litchi stinkbug) and a longan psyllid, Cornegenasylla sinica (Koizumi, 1995). The transmission rate by adults and nymphs of the litchi stink bug was 18.8-36.7% and 26.7-45%, respectively, with the latent periods ranging from 53-72 days up to one year. The transmission success rate by the longan psylla was 23.3-36.7% with a latent period of the disease from 80-88 days up to one year.
Transmission has also been demonstrated by inarching or marcotting from diseased parent trees (Li L.R., 1955; Menzel et al., 1989).
Another possible vector of longan witches’ broom is dodder weeds. A study of transmission by Cuscuta campestris (dodder) conducted in 1987 and 1988 in China, found that infectivity caused by the dodder weeds was 20-40% with a latent period of 130-136 days (Chen et al., 1990b). Dodder feeding on infected longan shoots was able to transfer the phytoplasma and produce symptoms in periwinkle plants (Catharanthus rosea) (Chantrasri et al., 1999).
A preliminary survey of the incidence of the disease in Hong Kong indicated that witches’ broom of longan was most likely to have originated from Kwantung, China (Li L.R., 1955), where the proprietors of the local orchards obtained planting materials. A study that followed the discovery of the disease in Hong Kong, indicated transmission of the disease via seeds and grafting prompting Li L.R. (1955) to suggest that the cause of longan witches’ broom may be viral. So and Zee (1972) grafted seriously infected longan trees onto two-year old disease-free trees. Seven months later, typical symptoms were evident on the young foliage of all test plants, with the exception of one that failed to graft. The controls did not show virus symptoms. These results agreed with preliminary findings by Li L.R. (1955) on the transmission of the disease in China.
Chen and Ke (1994) reported that the incidence of the disease on seedlings in Fujian Province was 5-30%, while the incidence after grafting onto three different longan varieties was 4.3, 14.0 and 19.4% respectively. Longan witches’ broom has spread quickly in Guangdong Province in China with 11% of trees infected in 1995 rising to 50% by 1997.
Results obtained in a grafting test indicated that scions may have caused 4.26-19.44% morbidity of the graftlings, which showed symptoms of the disease within 3-10 months (Chen et al., 1990b). An investigation revealed that the morbidities of seedlings, aerial layerings and tongue graftings in the field were 0.1-45.2%, 21-32% and 5-20%, respectively (Chen and Ke, 1994). The extremely high morbidity of seedlings in the field was most likely to be caused by repeated infection by insect vectors.
Seedlings grown from the seeds of infected trees cultivars ‘Youtanben’ and ‘Dongbi’, showed an average morbidity of 2.17% (0.19-4.41%) (Chen et al., 1990b), suggesting that seed of the fruit was one of the factors spreading the virus (Chen et al., 1992) supporting the work of Li (1955). In another test, pollen from diseased flowers of longan were aseptically cultured and typical symptoms of longan witches’ broom were present on some of the anther-derived plantlets, indicating that the pathogen may have been transmitted by pollen (Chen et al., 1990b). It remains uncertain whether pollen of the infected longan flowers carried the virus, however, the healthy leaves smeared with the juice of young leaves from diseased trees did not develop any symptoms of the disease (Chen et al., 1990b) excluding the possibility of virus transmission by sap smearing.
Chen et al. (2001) reported after conducting further transmission tests suggesting that the seeds and budwoods of longan; insects, litchi stink bug (^ ) and longan psylla (Cornegenapsylla sinica); and dodder plants (Cuscuta campestris) were positive in transmitting this virus.
Witches’ broom has variously been described as ‘the only significant disease affecting longan in Asia’ (Menzel et al., 1989), as ‘a widely spread and most important longan disease in China’ and ‘most serious disease to the crop’ (Chen et al., 1992).
An early survey in China revealed that 80-100% of longan trees in an old orchard, and 5-10% in newly established orchards were attacked by witches’ broom disease (So and Zee, 1972). According to an investigation conducted into longan production areas in 17 counties or cities in Fujian Province of China, the percentage of diseased trees varied from 20-100% with higher infestation in mature groves. The disease causes crop losses of 10-20% in average years, whilst crop losses of over 50% have been recorded in some severe cases (Chen et al., 1990a).
Lychee: Chen et al. (1992) report that witches’ broom symptoms have been observed on lychee in Fujian Province for 10 years. The disease is transmitted by seedling, inarching and by the vector, ^ , and is also associated with the presence of filamentous virus particles in leaf phloem cells. This suggests that lychee and longan witches’ disease are caused by the same virus (Chen et al., 1996). Lychee witches’ broom is known to infect seedlings, juvenile and adult trees. Young leaves on the shoots of infected plants become rolled and reduced in size, with excessive proliferation of shoots that become broom-like in appearance. The flowering panicles become considerably aggregated in clumps and resemble those described for longan witches’ broom. Chen et al. (1992) reported that longan witches’ broom disease is closely related to that of lychee, because Tessaratoma papillosa can successfully transmit the pathogen of longan witches’ broom to lychee.
However, other Chinese technical experts reported a lack of adequate evidence to prove that witches’ broom disease infects lychee fruit or that the disease exists in lychee (AQSIQ, 2003).
Witches’ broom disease has never been recorded on lychee in Thailand (DOA, 2003).
Control: The pathogen may be controlled by integrated methods, including strict quarantine of longan material from infected areas; use of resistant varieties; careful selection of propagating material and virus-free seedlings; and chemical control of the vectors (Coates et al., 2003). The best strategy for disease management appears to be controlling the vectors (Chen et al., 2001; Zhang and Zhang, 1999). It was found that spraying with chlorophos (trichlorfon) or with Sumicidin gave good control of the vector (Chen et al., 1999b).
In Thailand, sucking insects were controlled with carbaryl and infected trees injected with the anti-biotic Pyrrodinimethyl tetracycline (PMT) near the affected tip. The tip was then cut and in 1-2 months the disease allegedly disappeared (Ungasit et al., 1999).
Experiments to eliminate the virus from planting material showed that alternative heat treatments at 40C in daytime and 30C at night for 40-90 days gave a disinfection rate of 10-20%. Shoot-tip culture gave a rate of 18.5%, and the combination of alternate heat treatment and shoot-tip culture, gave 47.3%. Virus-free plantlets were obtained by heat treatment and used as scions (Chen et al., 1999a). Biological and timely chemical control of insect vectors, and removal of the infected branches and inflorescences were also important measures for the management of the disease (Chen, 1990; Chen et al., 1990).
The close relationship between different varieties of longan and the incidence of disease was first observed in China in the mid-1980’s (Chen et al., 1990a), but few further investigations have been made since. Chen et al. (1988) found great differences in susceptibility to the disease among longan varieties, and suggested careful selection and breeding as an important means of control. Varieties such as ‘Lidongben’ and ‘Shuinan No. 1’ were found to be highly resistant, whilst ‘Pumingyan’, ‘Youtanben’, ‘Dongbi’, and ‘Honghezgi’ were more susceptible. Top grafting with scions of resistant varieties effectively reduced the morbidity caused by the disease in severely infected orchards. However, none of the longan cultivars from China, Hong Kong or Thailand can be guaranteed to be free of the virus. Consequently, Menzel et al. (1989) advised that all longan [nursery] material introduced into Australia should be closely examined for symptoms of the mycoplasma.
In Thailand, the popular longan cultivar ‘Biew Kiew’ and ‘Deang Klom’ and ‘Ma Teen Klong’ are the most prone to witches broom and develop severe symptoms (Ungasit et al., 1999; Visitpanich et al., 1996); however, cultivars ‘Daw’ and Heaw’ are only mildly affected (Visitpanich et al., 1996). The longan cultivar of choice for export is ‘Daw’ which is considered resistant (DOA, pers. comm., 2003).
Based on the knowledge of the pathogen, its transmission sources and vectors and the principles of pest control (Chen et al., 1999b), six measures have been proposed for an integrated pest management program: strict quarantine inspection; selection and use of disease-resistant varieties (e.g. ‘Lidongben’ and ‘Shuinan No. 1’); establishment of virus-free nurseries; timely control of vectors; removal of infected branches, inflorescences and trees from nurseries and orchards; and judicious fertilisation, irrigation and soil management to promote tree vigour and enhance resistance to the disease (Chen et al., 2001).
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