MSc Huynh Kim Ngoc

 “Witches Broom “disease also known as "shrinkled top", "ruffled top", "unicorn head"... has just appeared (about 2003), at first the disease appeared and caused scattered damage in the Southeast, however, Recently in the Mekong Delta, the disease has spread on the largest scale in Dong Thap, Tien Giang, Vinh Long, and Tra Vinh. The disease causes severe damage to productivity, there is no specific treatment and prevention is the main thing.

Disease symptoms:

Witches Broom disease causes damage on young shoots and inflorescences causing these parts not to develop, reducing the ability to set flowers, fruit, poor quality fruit, and loss of yield. Symptoms of witches broom disease are easy to identify: The diseased young shoots grow in clusters with many small, short, deformed,twisted branches, The diseased branch shrinks to look like a bunch of brooms, so people call it witches broom" Diseased inflorescences also have similar symptoms, underdeveloped inflorescences, short inflorescence, petals do not unfold but shrink, bright color, very low fruit setting rate, fruit if beans do not develop, dry gradually and die. Witches broom disease ease can appear anywhere on the longan tree, quickly spreads throughout the garden, the diseased tree develops normally, but the diseased buds do not develop but gradually degenerate. The disease causes damage all year round, but it is most severe in the dry season, the disease is common in thick planted gardens, dense canopy, unbalanced fertilizer application, high nitrogen fertilization, garden lacks water in the dry season, poor hygiene and garden care.

Agent:

So far, the causative agent of "witches broom" is not unified, some people think it is Phycoplasma, another author thinks it is a virus, recently some researchers think the cause is bacteria, however. The disease is closely related to spiders and sucking insects. According to research by many domestic and foreign authors, the disease is closely related to the villous spider (Eriophyes litchii) and this is the vector that transmits the witches broom disease. Eriophyes are difficult to identify due to their very small size, yellow-white body, 2 pairs of legs on the thorax, and hairy tail. Life cycle 8 - 15 days, a year with 13 - 15 litters, spiders appear and cause damage in the dry season. Spider stings suck mainly on young parts such as buds, shoots, newly bloomed flower clusters, but when there are no buds, spiders suck old parts.

How to prevent:

- Planting resistant varieties, if the area is under high disease pressure,it is recommended to graft

 scion of xuong longan with yellow pulp into the rootstock of cowhide colored small seeded longan  (strong vitality, but seriously infected disease) or long longan, ...xuong longan with white pulp

- Using disease-free plants as propagating materials. Avoid transporting planting material from an infected area to another.

- Sufficient watering, reasonable fertilization, avoiding excessive nitrogen fertilization to make the leaves develop and produce leaves is not concentrated, creating favorable conditions for the disease to develop..

- Pruning branches to create canopy to create conditions for concentrated flowering, uniform, favorable for disease management.

- Pruning branches after harvest. Should be pruned deeply (pruning below the diseased location over 50 cm to remove diseased branches, harmful spiders... should cut off branches close to the ground).

- Visiting the garden regularly, when detecting diseases, should be removed and destroyed immediately. Care should be taken to clean pruning tools to avoid the spread of disease.

Combine pruning to handle flowering longan with control of villous spiders by spraying when the plants just sprouting and flowering such as: Sulox 80WP, Saromite 57EC, SK 99EC mineral oil. Pay attention to spray a lot of water, focus on the young parts of the plant, spray once every 7-10 days, in addition, because harmful spiders are resistant to products quickly, so it is recommended to rotate the product.