ABSTRACT
On average, about 20% of the global wheat production is lost due to diseases and pests every year. In 2012, these losses amounted to about 140 million tons, equivalent to about $35 billion (Anonymous, 2014). Fungal pathogens like rusts (Puccinia ssp.), Septoria leaf blotch (Septoria spp.), powdery mildew (Blumeria graminis) and Fusarium species are ranked among the top ten of the most important fungal pathogens (Dean et al., 2012). Historical and current sources report epidemics leading to sometimes devastating yield losses in wheat caused by these pathogens. In regions with low productivity where no seed dressing is conducted, smuts and bunts are also important pathogens (Oerke and Dehne, 2004). Furthermore, in specific wheat-growing areas, fungal pathogens such as Pyrenophora tritici-repentis causing tan spot, Oculimacula spp. causing eyespot of wheat or Cochliobolus sativus are of importance. One option to avoid yield losses caused by these pathogens is the application of fungicides. However, the repeated use of fungicides induces a considerable selection pressure on respective pathogens resulting in fungicide resistance or tolerance, which has been detected already in B. graminis, Septoria spp. or Fusarium spp. (Becher and Wirsel, 2012; Cools and Fraaije, 2013) to azoles mediated by mutations of the sterol 14-demethylase P450 (CYP51) or to strobilurins due to mutations in the cytochrome b gene (Torriani et al., 2009). The application of fungicides also depends to some extent on grain prices, fungicide costs and the possibility of applying these at the right time (Lopez et al., 2015). Hence, cultivars carrying resistances are the
most environment-friendly and cost-effective way of preventing yield losses in wheat. In particular, resistances against leaf rust, stripe rust, stem rust and powdery mildew infections result in complete resistance, but are at risk to be overcome by virulent isolates due to the extensive spore production of these pathogens. At a moderate infection level, 1 trillion stem rust spores per acre are generated (Fetch et al., 2011), and therefore, the occurrence of a virulent pathotype is very likely. In contrast, quantitative, race non-specific resistances encoded by genes like Lr34 or quantitative trait loci (QTLs) are more durable and today molecular markers facilitate their effective combination (Ellis et al., 2014). Besides fungal pathogens, viruses are important pathogens of wheat, as Triticum species are natural hosts of more than 40 different viruses of which some are important pathogens. As viruses cannot be combated by chemicals directly, the only way of avoiding yield losses are taking chemical measures against their vectors, which in case of soil-borne vectors, are inefficient, and growing of resistant cultivars is the only way of ensuring wheat cultivation in the growing area of infested fields. In the following sections, the most important fungal and viral diseases of wheat and the possibilities of reducing yield losses caused by these are briefly described (for details cf. Bockus et al., 2010).
