Pg. 68-70 Litchi dieback: In vitro pathogenicity tests to establish the most prominent predisposing organisms

Fungal isolates obtained after a survey of litchi trees with dieback were inoculated into litchi trees to determine pathogenicity. Root rot symptoms were obtained after inoculation with Pythium groups HS, G and P, Pythium vexans and Phytophthora drechsleri/cryptogea complex. These pathogens could, however, not be re-isolated from infected tissue. Isolates of the genera Fusarium, Rhizoctonia, Pestalotia and Botryosphaeria were able to infect plant tissue and they were re-isolated from infected tissue.

Summary:

• The study aimed to determine the role of fungi in the decline and dieback of litchi trees in South Africa by testing fungal isolates obtained from affected trees.
• Root rot symptoms were induced after inoculation with several Pythium groups (HS, G, P), Pythium vexans, and the Phytophthora drechsleri/cryptogea complex, although these pathogens could not be re-isolated from the infected roots.
• Fungal isolates from Fusarium, Rhizoctonia, Pestalotia, and Botryosphaeria were able to infect litchi tissue and were successfully re-isolated from the infected tissue.
• The trial used containerized litchi trees and inoculated soil with various isolates of Pythium and Phytophthora to observe root rot symptoms and pathogenicity.
• Root rot symptoms included dry, cracked, corky cortex with light brown discoloration and reduced root density; some plants had nearly all roots destroyed.
• Pythium groups HS, P, and G were the most aggressive pathogens causing root rot, followed by the Phytophthora complex and Pythium vexans.
• No shoot decline or dieback symptoms were caused by the fungi using toothpick inoculation on shoots after three months, although some caused internal wood tissue discoloration.
• Fusarium was commonly isolated from affected roots, indicating a secondary invader role.
• The study suggested that these pathogens could cause more severe disease under stress conditions, and control of root rot pathogens and nematodes is critical for managing litchi decline.
• The inability to consistently re-isolate Pythium and Phytophthora from infected roots might be due to temporal or methodological factors.
• The study recommended further testing with more uniform and young plants like seedlings for more reliable data and fungicide screening.
• Overall, root rot caused by Pythium and Phytophthora species is a major factor in litchi decline and dieback, potentially exacerbated by nematode presence and environmental stress.
• The article acknowledges the South African Litchi Growers Association for funding and contributions from researchers in fungal identification.