Pg. 3-4 Litchi Dieback

There are two distinct disease syndromes associated with litchi death. One is attributable to Armillaria on its own. This is found in the valleys north and west of Tzaneen. The second is associated with Fusarium, Botryosphaeria [sensu larga], Phomopsis and perhaps Armillaria. This is causing tree death in the Nelspruit and Malelane regions. Five to six thousand mature trees have been lost to the disease, which represents an annual crop loss of around three million rand, although only a score of farms have the disease. A field trial has been initiated.

Summary:

• There are two distinct disease syndromes associated with litchi tree dieback:
  • One is caused by Armillaria alone, found in valleys north and west of Tzaneen.
  • The other involves a complex of Fusarium, Botryosphaeria sensu larga, Phomopsis, and possibly Armillaria, causing tree death in the Nelspruit and Malelane regions.
• The disease has led to the loss of 5,000–6,000 mature trees, with estimated annual crop losses of about three million rand, although only about 20 farms are affected.
• Trees between 4 and 40 years old are susceptible, with death occurring one to several years after initial symptoms.
• Common symptoms:
  • Loss of leaf gloss and dull olivaceous color on branches
  • Brown, dried leaves remain attached as branches die
  • Disease progresses mainly during summer months; slower during winter
  • Presence of insect borers (pecan stem borer and bark borers) in dead or dying wood
• Armillaria syndrome (Tzaneen area):
  • Dead, cracked bark at the soil line under dying branches
  • Bark flakes off easily, spongy texture, interwoven with white/yellow leathery mycelium
  • Fungus found beneath bark, replacing the cambial layer
  • Wood discolored in affected area, rarely extends more than 10 cm above soil line but can ring-bark and kill tree
  • Bark borers found, sometimes stem borers
  • Identification confirmed by symptoms; Lab cultures from local researchers
• Armillaria Plus complex (Southern Lowveld):
  • Darker, cracked bark at base
  • Bark is hard and dry, tightly attached in younger lesion areas
  • White powdery fungal mycelium (Fusarium species) often seen below bark
  • Wood may have red/yellow-brown streaks, hard even when discolored, spreading deeply into the trunk
  • Several pathogenic fungi identified (mainly Fusarium, Botryosphaeria, and Phomopsis)
• Extent and impact:
  • Severe losses at some farms: up to 3,500 mature trees lost at worst sites
  • Annual loss valued at R2.7 million
  • No practical control measures currently recommended; fungicides show some lab efficacy but application is limited by fungus location underground
  • Field trials and further investigations currently underway to improve management options.