Wheat yield as affected by diseases

Amongst 350,000 botanical plant species identified in the world, only 24 (less than 0.008% of them) are used as crops to satisfy most human requirements for food and fiber (Wittwer, 1980).

Wheat
Bread wheat (Triticum aestivum L.) plays a major role among the few species widely grown around the world as food sources and was central in the beginning of agriculture (Harlan, 1981). Nowadays, bread wheat is the most widely grown crop in the world, with one sixth of the arable land in the world.

Pathogens
In this crop, fungal pathogens like Pyrenophora tritici-repentis (Died.) Drechs. (anamorph Drechslera tritici-repentis (Died.) Shoem) “Tan spot”, Septoria tritici Roberge ex Desm.(teleomorph Mycosphaerella graminicola (Fuckel) Schrot. in Cohn) “Septoria leaf blotch” and Puccinia recondita Roberge ex Desmaz. f. sp. tritici “leaf rust” are the main biotic causes of yield reduction in Argentina as well as in other regions (Annone, 2001; Serrago et al., 2009; Schierenbeck et al., 2014).

Fungal pathogens can be classified as necrothrophic, biotrophic and hemibiotrophic according to the nutritional relationship with the plant cells; in all cases the infected plant shows alteration in its physiology and morphology, which leads to damage and yield reduction.

Necrotrophs
Tan spot (classified as a necrotroph) kills tissue cells during the progress of colonization of fungal structures through the secretion of toxins and cell wall degrading enzymes. The control of this disease by genetic resistance strategies has been found to be difficult because the pathogens do not show a high degree of specialization. Hence, integrated disease management including crop rotation, seed treatment and chemical control has been used by growers (Reis et al, 1992).

Biotrophs
Leaf rust is a biotrophic pathogen which derives its resources for growth and sporulation from living host cells. Wheat rusts have been controlled specially through resistant cultivars but chemical control has been recommended in those cases where resistance is broken.

Hemi-biotrophs
Hemi-biotrophs like Septoria leaf blotch behave initially as biotrophs, but then change to a necrothrophic mode of nutrition (Ney et al., 2013).