Mycelium initially hyaline, becoming olive-buff to deep olive-buff, branched, septate, 2-7 µm wide. Conidiophores similar to mycelium in colour, septate, unbranched or occasionally branched, erect, broader towards the distal end, on the host single or fasciculate, emerging through stomata, amphigenous, geniculate or straight, length variable, between septa 17-28 x 3-6 µm. Conidia acrogenous, borne singly or in chains of 2-4, smooth, irregularly ovoid, both ends rounded, or ellipsoid, or conical-ellipsoid, gradually tapering into a beak;beak (a secondary conidiophore) concolorous with the main conidial body, straight, 20-37 x 3-7 µm. Spore body pale-brown to dark olive-buff, becoming darker with age, verrucose, transverse septa 1-10, longitudinal septa 0-5, constricted at septa, varying in size;length including beak 15-89 µm, width 7-30 µm (for additional details, see Prasada and Prabhu, 1962;Prabhu and Prasada, 1970;Anahosur, 1978;Simmons, 1994;2007;Dugan and Peever, 2002).
The disease can be detected in the field on the basis of symptoms on the leaf, leaf sheath, awns and glumes. Lesions will lack the dark pycnidia or perithecia produced by some other leaf-spotting fungi (Wiese, 1987). Under severe conditions the heavily infected fields present a distinct burnt appearance that can be seen from a distance (Prabhu and Prasada, 1966).
The agar plate method is recommended for detection in seed (Mathur and Kongsdal, 2003).
Related invasive species
- Alternaria triticina
Related Farm Practice
- Hosts
A. triticina is one of several species in the genus that have been isolated from wheat leaves;it is demonstrated to be pathogenic, whereas others appear to be primarily saprophytes. The leaf blight disease it causes has been a serious problem on susceptible cultivars of durum [ Triticum turgidum subsp. durum ] and bread wheat [ Triticum aestivum ] in India. The species has been reported from other hosts and other countries on several continents, but recent taxonomic examinations (Mercado Vergnes et al., 2006;Simmons, 2007) have only supported its presence in southern and southwestern Asia. Nevertheless, it is definitely seedborne, so that the possibility of accidental introduction by that means is a threat for growers using imported seed.
The disease appears when wheat plants are 7-8 weeks old and becomes severe when the crop is mature. Infection is first evident as small, oval, discoloured lesions, irregularly scattered on the leaves. As the lesions enlarge they become dark-brown to grey and irregular in shape. Some are surrounded by a bright-yellow marginal zone. The lesions vary in size, reaching a diameter of 1 cm or more.
As the disease progresses, several lesions coalesce to cover large areas, resulting in the death of the entire leaf. In some cases the leaf starts drying up from the tip, prematurely, when lesions appear. Black powdery conidia may cover the lesions at this stage under moist conditions. The lowermost leaves are the first to show the signs of infection;the fungus gradually spreads to the upper leaves. In severe cases, similar symptoms are produced on the leaf sheath and stem, as well as the awns and glumes if spikes are infected at the pre-anthesis stage. If the spike is infected this early, seeds do not form. Infection at the dough stage of seed development results in glume infection, ear infection and seed infection. Heavily infected fields present a burnt appearance and can be identified from a distance (Prasada and Prabhu, 1962;Prabhu and Prasada, 1966;Singh, 1990).
The main hosts of A. triticina are bread wheat (Triticum aestivum) and durum wheat (Triticum turgidum subsp. durum). Durum wheats are more severely attacked by the pathogen than are bread wheats (Prasada and Prabhu 1962;Prabhu and Prasada, 1966;Singh et al., 1990). Nevertheless, the results of inoculations by Vergnes et al. (2006) lead them to consider that this fungus is a weak pathogen of wheat with little virulence on modern wheat cultivars.
The pathogen has also been reported to infect Triticum dicoccum (Kulshrestha and Rao, 1976), Triticum sphaerococcum (Kumar et al., 1974a), triticale (Chaudhuri et al., 1976;Wiese, 1987), barley [ Hordeum vulgare ] (Mehiar et al., 1976), oats [ Avena sativa ] (Logrieco et al., 1990) and rye [ Secale cereale ] (Logrieco et al., 1990). Prabhu and Prasada (1966), on the other hand, were not able to obtain infection by inoculation of barley, oats or ten species of wild grasses. Isolation from banana [ Musa paradisiaca ] was reported, although the fungus was probably a secondary invader (Jones, 1991).