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.

Summary:

• Litchi dieback is a serious and growing problem where adult litchi trees become sick and eventually die, with symptoms including dull leaves and branch death over months or years.
• The disease involves a complex of fungi (e.g., Armillaria, Fusarium, Botryosphaeria, Phomopsis), but no single pathogen is directly linked as the sole cause; physiological stress and secondary invaders are likely important.
• Nematodes (especially Hemicriconemoides mangiferae and Xiphinema spp.) damage roots, stressing trees by reducing nutrient and water uptake, possibly contributing to disease development and spread.
• An intensive survey in South African litchi-producing areas examined fungi, nematodes, soil conditions, irrigation, cultivars, and tree health to identify factors linked to dieback.
• The survey found no single factor clearly differentiating healthy and diseased trees, but sheath nematodes, Pythium fungus, and soil chemical balance (CaMgK) were significantly associated with diseased trees.
• Nematodes were found to play a role by causing root damage that facilitates fungal invasion and tree stress; controlling nematodes could help reduce disease incidence.
• Field trials tested six chemical treatments (including nematicides and fungicides such as Enzone, Nemacur, Ridomil) to control dieback; results showed varying effects on nematode and fungal levels and some improvement in tree vigor and root growth.
• Enzone showed potential nematicidal and fungicidal effects but required further evaluation, especially as environmental factors like rainfall influenced outcomes.
• Litchi dieback is linked to tree stress caused by a combination of nematodes, fungi (Pythium, Phytophthora, Cylindrocladium), irrigation methods, and nutrient imbalances.
• Future research is recommended to extend surveys to more varied orchards, include additional parameters like soil compaction and leaf analysis, and perform longer-term trials to confirm effective control strategies.