Pg. 61-67 Investigations into the cause of litchi decline and dieback in South Africa

Litchi decline and dieback was observed during the past season. Isolations were made from affected and healthy trees from Malelane, Nelspruit, White River, Hazyview, Ofcolaco, Tzaneen and Levubu, South Africa. The nematodes Criconematidae, Xiphinema and Paratrichodrorus were often found associated with affected trees. The Oomycetes Phythophthora and Phytium were always found associated, alone or in combination with each other. Fusarium, Pestalotia, Rhizoctonia and Botryosphaeria spp. were often isolated.

Summary:

• Litchi decline and dieback were observed in South Africa with symptoms including reduced vigor, yellowing leaves, dying twigs, and sudden tree death within about two weeks.
• Samples were collected from seven production areas and analyzed for biotic factors in roots and soil.
• Nematodes (Criconematidae, Xiphinema, Paratrichodrorus) were frequently associated with affected trees and cause damage to the root cortex.
• Oomycetes Phytophthora and Pythium species were consistently isolated from diseased trees, alone or together.
• Other fungi such as Fusarium, Pestalotia, Rhizoctonia, and Botryosphaeria were often isolated, especially in advanced stages of dieback.
• Symptoms included a corky cortex on feeder roots, root cortex sloughing off, and root system deterioration affecting tree nutrient uptake.
• Historical attributions of decline included Armillaria fungus and nematodes, but Armillaria was not isolated in this study.
• Environmental stress, including drought and waterlogged soils, likely predisposes trees to infections.
• The nematode feeding may facilitate fungal infections by creating entry points.
• Different Phytophthora and Pythium species groups were identified in different geographic areas.
• Other fungi like Chaetomonium and Paecilomyces were found but their pathogenic role is unclear.
• Management recommendations emphasize an integrated approach controlling both nematodes and soilborne fungal pathogens.
• Further research is needed on pathogenicity, specific species involved, and treatment effects over multiple seasons.
• The findings correspond with similar reports from other countries about fungal and nematode associations with litchi decline.