Pg. 2-4 Armillaria Rot of Litchi Trees (Litchi chinensis) in South Africa

Armillaria rot is causing considerable losses to litchi (Litchi chinensis) plantings in the high rainfall area of the Northern Transvaal in South Africa. Initial symptoms of the disease are a loss of vigour and the absence of new growth, which is followed by defoliation and the death of the trees. Aboveground symptoms characteristically develop on sections of the trees first. Young plants often collapse suddenly during the warm rainy summer period, when the pathogen is most active and rapid girdling of the trunk occurs. Infected large trees may deteriorate for several years showing symptoms of a slow decline. The pathogen enters and kills major roots first and later invades the trunk. The disease typically spreads in a circle in the orchard and is more severe in stressed trees, such as plantings in lands with a high water table. No fruiting bodies of the fungus have been observed on any of the infected trees, but abundant white mycelial sheet production under the bark and rhizomorph development on the roots have been recorded. Most isolates in this study have produced rhizomorphs in axenic culture.

Summary:

• Armillaria rot is causing significant losses to litchi (Litchi chinensis) plantings in the high rainfall area of Northern Transvaal, South Africa.
• Initial disease symptoms include loss of vigor, absence of new growth, followed by defoliation and tree death.
• Symptoms often develop on sections of the trees first; young plants can collapse suddenly during the warm rainy summer when the pathogen is most active.
• The pathogen infects and kills major roots first, then invades the trunk, spreading in a circular pattern in orchards.
• Disease severity is higher in stressed trees, such as those in areas with high water tables.
• White mycelial sheets under the bark and rhizomorph development on roots are key diagnostic features; no fruiting bodies were observed.
• Armillaria mellea was isolated from infected trees, showing pathogenicity in artificial inoculation tests on young seedlings.
• Other soil-borne pathogens such as Pythium spp., Fusarium oxysporum, and Rhizoctonia sp. were also frequently isolated but not confirmed as primary causes.
• Environmental stresses like high water tables and root pruning wounds may predispose trees to infection.
• Older trees tend to show slow decline symptoms, while younger trees experience sudden wilt and death.
• There is a possible correlation between nematode damage and susceptibility to Armillaria infection, though not confirmed in this study.
• The disease is a problem of increasing importance, and further studies on control measures are recommended for South African litchi growers.