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Mekong Delta fruit crops hit as disease spreads ———————————————– With the Mekong Delta province of Vinh Long failing to control witches’ broom when it affected longan trees [ProMED-mail post 20110818.2503], the disease has now spread to the rambutan crop, causing farmers severe losses.
In recent years the disease, which causes an abnormal brush-like cluster of drafted, weak shoots arising from or near the same point on branches and affects fruiting, has infected large areas of longan in the delta. Vinh Long was among the worst hit during the peak outbreaks between September 2011 and March 2012, with 9000 ha [22 240 acres] out of its 9840 ha [24 315 acres] of longan affected.
[Where] the disease hit longan trees, [some farmers] cut them down and replaced [them] with rambutan. [But when] the rambutan began to fruit they have been affected by the disease. [An official said that] more than 126 ha [311 acres] out of 550 ha [1359 acres] of rambutan are diseased in Hoa Ninh Commune [Long Ho district, Vinh Long province].
Authorities have warned farmers to apply measures to stop the disease from affecting rambutan, including cutting infected branches and using chemicals. Vinh Long province Department of Agriculture and Rural Development, said [they are taking] measures to treat and prevent the spread of the disease. In the province, a large area of longan crop remained affected by the disease, which has the potential to spread to more rambutan trees.
[Longan (_Dimocarpus longan_), rambutan (_Nephelium lappaceum_), and lychee(_Litchi chinensis_) are members of the family _Sapindaceae_. They are important fruit crop in parts of Asia. Witches’ broom (or rosette shoot) of longan (LWB) is a serious disease of the crop with reported yield losses of up to 50 percent. It was first described from China in 1941 where in some areas up to 100 percent of trees were infected, with higher incidence in mature trees. Since then it has also been reported from other countries in the region, for example Thailand and Taiwan.
Symptoms appear on branches and leaves but not on fruit. They may include distorted mature leaves, unexpanded young leaves, dense clusters of shoots, poorly developed flowers, as well as abnormal development of flowers and panicles resulting in “broom-like” appearance of the inflorescences. Similar symptoms have been reported from lychee and a close relationship between the 2 diseases is indicated.
The term “witches’ broom” is descriptive of symptoms only and is associated with a range of plant diseases caused by different pathogens. A co-infection of at least 2 phytoplasmas (elm yellows 16SrV and stolbur 16SrXII groups) has been reported to be associated with LWB, and a filamentous virus has also been suspected. However, a causal relationship (Koch’s postulates) has not yet been established for any of these pathogens. The disease was shown to be transmitted by vectors such as litchi stink bug (_Tessaratoma papillosa_), longan psyllid (_Cornegenapsylla sinica_) and a new species of gall mite (_Eriophyes dimocarpi_). LWB is also spread through grafting, and preliminary evidence suggests that it may be seed transmitted. Since spread between longan and lychee has been reported, a spread of the pathogen(s) to rambutan (and perhaps other related species) appears feasible.
LWB management may include orchard sanitation (removal of potential pathogen reservoirs), phytosanitary measures (disinfecting cutting tools), and use of clean planting and grafting material. Agrochemicals to control suspected vectors (as mentioned above) may also be helpful. Resistant longan varieties are available for some areas.