Alternaria triticina Pras. & Prab. has been reported as a foliar pathogen in most of the non- traditional wheat growing areas of the Gangetic plains and also in some regions of Mexico and Italy (Pellegrineschi et al., 2001; Chaurasia et al. 2000; Casulli, 1990; Sinha et al. 1991; Raut et al. 1983; Prasada and Prabhu 1966). Symptoms of A. triticina leaf blight can be first seen in the field on 7 to 8 week-old plants (Prasada and Prabhu, 1966). The lowermost leaves are always the first to become infected and the disease gradually spreads to the upper ones. In severe cases, symptoms can be observed on leaf sheaths, ears, awns and glumes. The symptoms appear as small, oval, discoloured lesions, irregularly scattered on the leaves. As the lesions enlarge, they become irregular, dark brown to grey and surrounded by a bright yellow margin. Later, several lesions coalesce and cover large areas, sometimes associated with the death of the entire plant. In some cases the leaves may dry from the tip. Under moist conditions in lesions show black powdery masses of conidia. One week-old wheat seedlings are relatively resistant to infection by A. triticina. Susceptibility increases with age and reaches its peak in mature plants, especially those with decreased vigour (Prasada and Prabhu, 1966). The pathogen overseasons in debris left on the soil surface (Vjiaya Kumar et al, 1979) which contrasts with the overseasoning mechanisms in other Alternaria diseases (Rotem, 1994). There are controversies concerning the host-range of A. triticina. Sivanesan (1981) reported it from leaves, leaf sheaths, glumes and grains of Triticum spp., triticale (Triticosecale Wilt.) and barley (Hordeum vulgare L.), while Prasada & Prabhu (1966) had been unable to detect susceptibility among 8 different grass species including barley and oat (Avena sativa L.). Prasada & Prabhu ( 1966) also reported the occurrence of resistant as well as of susceptible cultivars within same Triticum spp. species pointing of the risk to draw conclusions concerning the host range on base of limited number of cultivars. The Indian dumm were found to be highly susceptible while some of the highly rust resistant genotypes were found to be also resistant to Alternaria leaf b light. Casulli (1990)confirmed susceptibility to A. triticina in durum wheat cultivars from southern Italy. Recent pathogenicity test with a reference strain of A. triticina (IMI 289962) showed that some of the elite helminthosporium leaf blight resistant wheat genotypes and some susceptible checks were resistant to Alternaria leaf blight (Mercado, 2002). From 15 genotypes tested, only the Indian durum wheat cultivar Bansi was found to be susceptible. On wheat, the A. triticina germ tubes form appressoria-like structures and penetrate the epidermis directly or through stomata (Prasada and Prabhu, 1966). Infection requires a minimum wetting period of 12 h, with 24 to 48-h wetting at an optimum temperature of 25°C leading to severe infection. As other Alternaria species, A. triticina produces toxic metabolites. A culture filtrate of A. triticina has been reported to inhibit the germination of seeds and induces symptoms on leaves (Rotem, 1994). The toxic principle is thermostable and non-specific (Vijaya Kumar et al., 1979). Fabrega et al. (2002) reported that A. triticina strain IMI 289962 produced tentoxin, a non-host specific toxin, which could be just another pathogenecity factor but is not believed to be of primary importance during penetration of the parasite (Rotem, 1994). Sivanesan (1981) provide a detailed description of the leafblight pathogen A. triticina. Mycelium is initially hyaline, turning later to olive buff to deep olive buff, branched septate, 2- µm broad. Conidiophores are similar to mycelium in colour, septate, usually unbranched but occasionally branched, straight, erect, single or fasciculated, 17-28 x 3-6 µm. Conidia are acrogenous, borne single or in chains of 2-4, smooth, irregularly oval, both ends rounded, ellipsoid or ellipsoid conical, gradually tapering into beak. Simmons (1994) considered the beak as a secondary conidiophore produced from the blunt or broadly tapered apex of initially erostratre conidia. On field specimens primary conidia of A. triticina are 50-80 x 20-25 µm, sometimes producing a secondary conidiophore 20-40 x 2-3 µm. They are yellowish brown, essentially smooth, have up to 8-9 complete or partial transverse septa and 1- 3 longitudinal septa in each of the broadest transverse segments. The spores of A. triticina are similar in shape and dimensions to those of A. alternata and A. tenuissima (Rotem, 1994) and the correct identification remains complex despite distinctive characteristics such as a very restricted number of conidia per sporulation unit. The Alternaria spp. associated with blight lesions on wheat in the Indian subcontinent appeared to be very heterogeneous (Maraite et al., 1998). The same authors reported difficulties to precisely identify A. triticina because of its similarity to A. tenuissima and didn't see any pathogenic isolate among a world-wide selection of 12 strains, including strains considered as A. triticina, after inoculating them on four different wheat varieties (Bansi, HD2329, Sonalika and UP 262) at the three-leaf and heading stage. A voluminous literature on the subject of wheat foliar blight and it relation to A. triticina has been published during the past few years. Essentially all of this work is dealing only with a set of symptoms on Triticum sp. with which various kinds of Alternaria have been found associated, however, there is no published evidence that these Alternaria inhabitants were correctly identify as A. triticina. In order to clarify the real importance of this pathogen on wheat, pathogenicity tests on different wheat genotypes were carried under controlled conditions with CABI Bioscience references strains of A. triticina and Alternaria spp. strains isolated from wheat leaves showing blight lesions collected in warm area and not colonized by the tan spot nor spot blotch pathogens.

Research and Partnership Program

English

0407|AGRIS 0401|AL-Wheat Program

Juan Carlos Mendieta

INT1237

CIMMYT Staff Publications Collection