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<i>Lr34 and Lr46 </i>are adult plant resistance genes providing durable resistance against biotrophic pathogens like rusts and powdery mildews in wheat. In seedlings, however, these genes increase susceptibility to Septoria tritici blotch (STB), caused by the fungus <i>Zymoseptoria tritici</i>. <i>Lr34</i> and <i>Lr46</i> also favoured the necrotrophic pathogens <i>Magnoporthe grisea</i> and <i>Ramularia collo-cygnii</i> in seedlings. A similar effect was observed in adult plants tested in polytunnels. The hypothesis that leaf age has a role in <i>Lr34</i> and <i>Lr46</i> resistance to mildew and susceptibility to <i>Z.tritici</i> was tested.  In particular, it was hypothesised that enhanced senescence in plant leaves can make them resistant to biotrophs while making them more susceptible to necrotrophic pathogens.  Leaf age altered significantly the effect of <i>Lr34</i> and <i>Lr46</i> on STB.  The plant material tested included the Indian spring wheat Lal Bahadur (LB) and near-isogenic and mutant lines developed from LB.  In young leaves, LB was less susceptible to STB than near-isogenic lines carrying <i>Lr34</i> or <i>Lr46</i>.  The opposite pattern was observed, however, in older leaves. Gene expression analysis of <i>Lr34</i> lines revealed a higher expression of senescence and cell-death associated genes indicating a possible role of <i>Lr34</i> in enhancing senescence. The results indicate that there may be significant consequences for the use of <i>Lr34 and Lr46 </i>to control rust and mildew in areas where necrotrophic diseases are prevalent.