Pg. 7-15 A study into the causes of litchi tree dieback and control strategies for the disease

Litchi dieback is becoming a serious problem in orchards where adult trees become sick and eventually die. Several organisms have been linked with the disease, but especially in the Nelspruit and Malelane areas a combination
of fungi were identified while none were directly linked with the disease and its symptoms. Also, factors like nematodes, drought stress and other abiotic factors were never really considered to be playing an important role. With an intensive survey, it might be possible to determine at least some factors responsible for the origin and spread of the disease. If this information is available, it might be easier to control the disease. 

Since new products have been developed, some of which have a wide range of control, it is important to test the efficacy of some of these products to control the disease. 

A survey was conducted in all litchi producing areas, including the Levubu, Tzaneen, Hazyview, Nelspruit, Malelane and Komatipoort areas. Sampling was done irrespective of presence of litchi tree dieback, but when present a representative sample of both healthy and sick plants were taken. The amount of samples was determined by the size of the area. Besides nematodes and fungi present on roots and in the soil, other data collected included irrigation schedule and type, soil nutrients, fertilisation, age, cultivar and size of trees.

In the trials in Nelspruit and Tzaneen, six products and an untreated control are being evaluated for control of litchi dieback. Products included were Enzone, Nemacur, Ridomil, Benomyl + Captan, Nemacur + AgriGold (root grower) and Nemacur + Ridomil. Because of extensive rainfall and high summer temperatures, it was difficult to estimate the effect of the treatments on the trees concerning better vigour and root growth. This will be determined in winter when symptoms are more pronounced. Nematode and fungal control varied in the two areas during the season and need to be investigated for another season.

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