October 2017. A request for information on a problem in longan in Cambodia. The flowers and fruit abort, and die off. The problem is widespread on farms and nurseries in the region.
A member (moderator) wrote: In 2009, Pestnet was sent images of a condition in Vietnam. The images were rather small, but we were fortunate in having members who had seen similar symptoms and were able to suggest thrips or mites, and how to detect them. Go to the Summaries section on the website Crops/Fruit snuts/Longan/Yellowteathripsor Eriophyidaemite,Vietnam.aspx
There was also a suggestion that the problem was caused by a witches’ broom, and increasing problem of longan in Vietnam and throughout South East Asia. It has also been reported as a problem on rambutan. It has been attributed by different workers to phytoplamsa and to eriophyid mites. The most convincing evidence that it is caused by an eriophyid mite comes from a recent PhD study by Dr Hanh of the Southern Fruit Research Institute (SOFRI) in Vietnam. She was able to generate the symptoms on healthy seedlings by infesting them with mites. As part of that study, EM observations of samples of young, deformed leaves and stems by microscopists at Plant and Food Research Auckland failed to detect any evidence of phytoplasma.
A member from Vietnam also wrote saying that the problem is caused by mites. He said that there had been 3-years’ work on the problem in vietnam on longan and rambutan. The cause is Eriophyes dimocarpi, an eriophyid mite. It is even suggested that it may transfer a phytoplasma to the shoots and flower and cause witches’ broom, but this is still controversial. One longan variety called “tieu-da-bo” is very sensitive to the disease. It produces a delicious fruit, but farmers have to change to other longan varieties, such as Ido-Ido, thanh-kiet or xuong-com-vang which are not infected or seldom infected. At present, farmers graft the other longan species on tieu da bo.
In Thailand, (another moderator) the eriophyid mite is Abacarus euphoriae, but there it is considered a minor pest of longan. “Soft” pesticide options could be the use of sulfur wp or dust, or kaolin clay. Other chemicals that are possible are abamectin, kelthane (dicofol), or even malathion or pyrethrin. In Cook Islands, mancozeb has been used with effect against citrus rust mite; it was as good as sulfur and predatory mites recovered faster.
CABI too has been involved with this problem in Vietnam, working with SOFRI to find the cause. The collaboration started some 20 years ago as part of the Global Plant Clinic enterprise. CABI tested at least four samples for phytoplasma but none were found. However, it is well known that some phytoplasmas are difficult to identify because of their unequal distribution in the plant. Perhaps comparisons should be made between the condition on longan and that in the UK on ash which is associated with the mite, Aceria fraxinovora, which feeds on the flowers.
Another example of witches’ broom-like symptoms that failed to yield phytoplasma was given by a member in Australia. Pometia pinnata has been found with symptoms in the Pacific, but tested negative in a standard P1/P7 PCR test.
And it’s the same with cocoa.
These examples generated a discussion of many other examples where witches’ broom (WB) diseases symptoms have been found to be caused by pathogens other than phytoplasma. WB on cocoa is, unusually, a fungal disease: Moniliophthora perniciosa (previously Crinipellis perniciosa).
Taphrina is perhaps the best known example of a fungal pathogen which induces WBs (on birch) – at least in Europe, North America and other places. Where there are fewer scientific resources to investigate unusual symptoms, things start to get messy and all sorts of odd ideas have been generated. Some gain common currency, even when scientific evidence clearly indicates a precise cause.
Take mango proliferation, for example. There are countless people who insist that this is due to ‘a virus’, based on a loose association with strange and unexplained growths. A similar association occurs for unexplained ailments that people suffer, often described as ‘a virus’.
Mango proliferation (‘witches’ broom’) is caused by a number of Fusarium species. But some are not convinced, still favouring a virus aetiology.
As modern diagnostics help to explain more, so we should become more savvy about what is a phytoplasma and what is not. But we still hold on to cherished ideas – such as, longan WB being caused by a phytoplasma. It doesn’t help that phytoplasma symptoms induce a range of expressions, from ‘yellows’ to diminished growth. Even with sophisticated molecular tools, identifying a phytoplasma in a lab is still a skilled and challenging job.
For those of you who have Melia azedarach (Persian lilac, paraiso, white cedar: a relative of neem), look out for branch death, drying up of seeds and general yellowing which can resemble seasonal casting of leaves. But it’s almost certainly not. Melia yellows is widespread, only occasionally bursting out into gorgeous WBs in places such as Bolivia, Brazil and Argentina. I saw melia yellows last week in southern Italy, mild but distinctive symptoms. I’ve been looking at melia yellows for a long, long, long time …
Keep an open mind about WBs. Check the literature. There’s a good paper by Bertaccini and Duduk which recently reviewed phytoplasma diseases (http://fupress.net/index.php/.