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Yield reduction

Yield reduction due to fungal contamination of barley grains (see introduction) can result via different mechanisms. Fungi such as Microdochium nivale, Fusarium graminearum and other seed-borne and soil-borne fungal plant diseases can result in conditions such as seedling blight or damping off of seedlings, leading to retarded germination of the seedling, which is eventually killed prematurely. This effect will decrease the number of plants growing per unit area, leading to a yield reduction as compared to a reference area with healthy plants. Aside from being seed-borne, the above-mentioned pathogens, as well as a variety of others, can also have a devastating effect when present in the soil to which sound barley has been sown (soil-borne).

Once germinated without microbial attack, the young barley plant develops into the flowering state where it becomes vulnerable to fungal attacking mechanisms that aim at either the glumes or anthers and gynoecium of individual spikelets or at the spike's rachis. Glume infection is mainly by entering of the germination tubes of airborne spores into the stomata with subsequent dispersal of the fungus into the glume tissue and, later on, into the developing endosperm (Pritsch et al., 2000; Xu, 2003). Fungal infection via anthers and the gynoecium apparatus depends strongly on the presence of appropriate climatic conditions during flowering, i.e. a period of high humidity and medium temperatures in order to establish infection. Provided the prevalence of optimum conditions over 24-48 hours post infection, growth of cereal pathogens through the anther tissue into the ovaries of the developing seed can lead to deep seated infection of the endosperm, eventually killing the young embryo or establishing seed-borne infection. Growth in species such as F. graminearum was found to be highly stimulated by the presence of choline and betaine, which are typically present in cereal anthers in high concentrations (Strange et al., 1974). Deep-seated infections will strongly reduce grain development and grain filling, resulting in small and often shrivelled grain with a strong diminution in grain yield. Spread of fungal infection from single spikelets to the rachis may result in cutting off the water supply for spikelets above the point of the primary infection. Affected spikelets will ripen early and show symptoms of head blight with grains showing strong reduction in size and filling. All symptoms described cause a reduction in thousand-kernel weight, i.e. reduced yield.

A third mechanism responsible for yield reduction due to fungal infection is the lowering of assimilative leaf area. Fungal pathogens such as Ery- siphe graminis (powdery mildew), Puccinia hordei (brown rust, Scholes and Farrar, 1986), Septoria tritici and Rhynchosporium secalis (leaf blotch of wheat and barley, respectively, Fowler and Owen, 1971) or Pyrenophora teres (net blotch of barley, Evans, 1969) strongly reduce the assimilative capacity of cereal plants, resulting in reduction of grain filling and overall yield.

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