Shot-Hole Reaction of Trichilia emetica in Response to Infection by Cocconia concentrica in South Africa

Abstract

Natal mahogany (Trichilia emetica Vahl) is native to South, Central, and East Africa. In South Africa, it is widely planted as an ornamental and street tree; its wood is used for furniture and the seeds used for medicinal oil. In June 2010, during the Southern Hemisphere winter, severe leaf damage resembling insect herbivory was observed on the leaves of a T. emetica specimen in a subtropical garden in White River, Mpumalanga Province, South Africa (25°19'37.98″S, 33°00'19.47″E, elevation 959 m). However, closer inspection revealed that the damage was not insect related but the result of a pronounced shot-hole reaction of the leaves in response to a fungal infection. Concentric patterns on infected leaves facilitated easy macroscopic identification of the pathogen as Cocconia concentrica (Syd.) Syd. Microscopic characteristics of the fungus associated with these patterns also corresponded to descriptions of C. concentrica by Inácio and Minter (1) and Theissen and Sydow (3): asci were cylindrical-clavate, mostly eight spored, and 62 to 65 × 12 to 15 μm; ascospores were light brown, elliptic to oblong, mostly one-septate, usually wider at one end, somewhat constricted at septum, and 16 to 18.5 × 6 to 7.5 μm; paraphyses were longer than asci, septate, 2.5 μm in diameter, and hyaline but brownish at the rounded, often swollen tips. The occurrence of C. concentrica on Trichilia spp. and other hosts has been well documented, but only as a leaf-spot parasite without any reference to the prominent shot-hole reaction, which to our knowledge, is reported here for the first time. After infection, which is presumably by airborne ascospores (1), scattered or coalescing, relatively round black spots, up to 7.5 mm in diameter, developed on the adaxial leaf surface. Each spot is composed of many circular to elliptical, black, coalescing stromatic ascomata conspicuously arranged in centrifugally expanding concentric rings, later appearing to be covered by an ascoma-bearing crust. Eventually, the formation of an abscission layer, generally along the outer edge of the outer ring or some distance away, may be triggered in the leaf tissue along the periphery of each spot, separating and isolating the infected leaf area from the rest of the lamina. Infected areas eventually slough off from the rest of the leaf (but may remain attached more intimately and longer along veins) and fall out, leaving gaping circular to irregular shot-holes edged by a prominent corky rim. A similar phenomenon has been reported and illustrated for T. connaroides infected by Phloeospora trichiliae in India (2) and T. tuberculata infected by an unidentified ascomycete in Panama ( http://researchwatch.net/nsf_grants/9902346 ). Shot-hole formation as a self-defense mechanism against fungal infection thus appears to be a widely occurring generic character in Trichilia, and it is remarkable that this symptom, subsequently witnessed elsewhere in Mpumalanga, has apparently gone unnoticed in Africa until now and that it has received no mention in the abundant literature on the trees of the region. Voucher material (PREM 60718) has been deposited at the National Collection of Fungi, Pretoria, South Africa.