Confirmation of Fusarium solani as the causal agent of die-back of Dalbergia sissoo in Nepal

Sissoo (Dalbergia sissoo) is an important multipurpose timber tree, widely planted in South Asia. Die-back is considered an epidemic disease of sissoo in this region. The typical symptoms of die-back are characterised by yellowing of leaves, followed by browning and shedding of leaves from the top. In Nepal, this disease was noted in plantation sites in Kailali District in 1989 (Thapa, 1990). Since then the disease has been noted in almost all sissoo growing areas of Nepal and has now become an epidemic and is spreading steadily. Fungal species, such as Fusarium solani, F. oxysporum and Ganoderma lucidum have been reported as associated with this disease (Parajuli et al., 2000, Joshi & Baral, 2000), although pathogenicity tests have not been completed to confirm their role in die-back disease (Anonymous, 2000).

Samples of roots, twigs and pods were collected from die-back affected trees from breeding seed orchards in Dharan, Sauraha and Tilkane established by Tree Improvement and Silviculture Component (TISC), and Bishnu Devi Community forest in Saptari district. F. solani was consistently isolated from roots of affected trees. This fungus was also isolated from pods and living twigs of diseased trees. F. solani was identified on the basis of morphological and cultural characters. The identification of a single spore culture of F. solani TiRo7 has been confirmed by Global Plant Clinic, CABI Bioscience, UK and deposited in the Genetic Resource Collection (GRC), under IMI Ref. No. 394682.

Single spore isolates were inoculated into four-month-old sissoo saplings. On the stem surface at 5 cm above the soil level, a tangential cut, about 1 cm long, was made with a sterilised scalpel so that it slightly wounded the xylem. A small amount of one week-old F. solani culture was inserted into the slit and bound by grafting tape. The control saplings were also treated in the same way but without administering the fungus. All the saplings inoculated with F. solani isolates developed typical symptoms of die-back within two weeks and reisolation of F. solani was readily made from these seedlings, thus confirming pathogenicity. All saplings kept as controls remained healthy.

The consistent isolation of F. solani from roots of affected trees, the development of die-back symptoms on inoculated plants and reisolation of F. solani from these inoculated plants confirms this pathogen as the primary cause of sissoo die-back in Nepal.

Acknowledgments

The authors are grateful to HMGN/NARMSAP/DANIDA Tree Improvement and Silviculture Component (TISC) for providing partial funding.