Diplodia theobromae associated with sudden decline of mango in the Sultanate of Oman

Mango (Mangifera indica) is an important crop in the Sultanate of Oman, producing 12,000t from 1,500 ha annually. Grafted and non-grafted trees are cultivated, the former using exotic scions on local rootstocks; non-grafted trees are selections from trees with good fruit qualities. Since 1999 a new disease has appeared with up to 60% of trees affected in parts the Al Batinah region. Trees show gummosis from the trunk, wilting, and eventual browning of leaves on a single branch (Fig. 1). As the infection progresses tree death occurs, perhaps 6 months from first appearance of symptoms. The wood is stained dark brown, spreading from a point of infection. Damage caused by a bark beetle (Cryphalus scabrecollis), is frequently associated with the disease.

Pycnidia are observed in lesions on infected bark and the pathogen Diplodia theobromae (IMI Reference W6341) has been isolated from affected trees. When inoculated onto healthy seedlings, the pathogen fulfills Koch's postulates. The pathogen has been isolated from bark beetles, suggesting the insect may act as a vector for dispersal. Non-grafted (Omani) trees show higher levels of disease severity than exotic scions (65% and 14% respectively). Insect damage is significantly less on exotic scions than on local, non-grafted trees. Insect log traps protected with polyester fleece to exclude the beetle showed no symptom development, while logs left unprotected were quickly infested with beetles and showed wood-staining. D. theobromae causing citrus die back has been reported from Oman (Waller & Bridge, 1978). Research to evaluate varietal resistance to the disease and vector, potential management techniques, and relationship with Ceratocystis fimbriata, a pathogen that has occasionally been isolated from trees showing mango decline symptoms, continues.

Reference

Waller JM Bridge J, 1978. Plant diseases and nematodes in the Sultanate of Oman. PANS 24, 313-326.