First occurrence of teak leaf rust caused by Olivea tectonae in Cote d'Ivoire and Africa

Teak (Tectona grandis) is one of the most valuable hardwoods in the world. It is native to India and Southeast Asia but it is grown as a plantation species worldwide in the tropics. In recent years several African countries have become increasingly important exporters of teak logs and sawn wood in the world market (Kollert & Walotek, 2015). In Ivory Coast about 40% of the forest plantations consist of teak, covering around 67,300 ha situated in the savannah and forest biomes. Teak has become the first forest product to be exported from Ivory Coast (Harmand et al., 2015).

Leaf rust was found in September 2017 on two-eight year old teak in the region of Haut Sassandra (6'54'49.65N; 6'21'54.74W) close to the city of Daloa. The disease was then recorded from another teak plantation (Haut Sassandra, 6'54'32.47N; 6'19'18.02W) on trees that were more than 20 years old, in the vicinity of the city of Gonaté. Some teak plants were severely infected (Fig. 1). In both plantations disease prevalence was assessed using a "zigzag-monitoring approach" investigating approximately 540 teak trees and reached around 60%.

Infected leaves initially had small, angular, brown to grey, necrotic areas on the upper leaf surfaces. As the disease progressed the lesions enlarged and coalesced to form large necrotic areas (cf. Daly et al., 2006). The lesions corresponded to numerous subepidermally erumpent uredinia on the lower leaf surfaces (Fig. 1). Rarely, uredinia were detected on the upper leaf surfaces (Fig. 2).

Microscopic investigations of the fungus revealed uredinia with hyaline, incurved paraphyses measuring 31- 39 × 15- 20 µm (Fig. 3). Urediniospores were produced singly on a short pedicel, subglobose, ovoid to ellipsoid, echinulate, orange-yellow or hyaline measuring 25- 36 × 18- 21 µm. Teliospores were not observed. The observed morphological characters are in accordance with those given for Olivea tectonae, the causal agent of teak leaf rust, by Mulder & Gibson (1973), which is the only rust known from teak.

To fulfil Koch's postulates urediniospores from infected leaves were used to inoculate healthy teak seedlings at the eight-leaved stage under greenhouse conditions by rubbing the infected leaves on the seedlings' lower leaf surfaces. Three weeks after inoculation symptoms identical to those on field samples developed on all fifteen inoculated plants (Fig. 4). Urediniospores were re-isolated from these plants and their identity with O. tectonae was microscopically confirmed.

Olivea tectonae causing teak leaf rust disease is a widely distributed pathogen in Asia and more recently has been reported from Australia, and Central and South America (Daly 2006; Cabral 2010). However, as far as could be established, teak rust disease has never been reported in Africa (Doilom et al., 2016). Thus, this is the first report of teak leaf rust disease in Côte d'Ivoire and Africa. The pathogen might have been introduced with planting material for commercial teak plantations.