Quercus petraea
Flushing date can also involve a trade-off in defences between climatic and biotic factors. An example has been documented for sessile oak (Quercus petraea (Matt.) Liebl.) in the Pyrenees mountains (Desprez-Loustau 2010; Dantec et al. 2015). The abiotic factor is late spring frosts, and the biotic factor is oak powdery mildew (caused byErysiphe quercicola Takam. et al.). Late flushing is a defence against the frosts, whereas early flushing helps protect against the mildew. At higher altitudes, where late frost represents the main adaptive hazard, late-flushing genotypes are favoured. At lower altitudes, which are more conducive to the powdery mildew but less subject to late frosts, early-flushing genotypes are favoured. Indeed, the greatest incidence and severity of disease occur at intermediate altitudes. With the trade-off meaning no closely defined or geographically broad optimum for flushing date, the large tree-to-tree variability in flushing date, especially at lower altitudes (Alberto et al. 2011), is not surprising. Some validation of the interpretation could in principle be obtained by studying the impact of chemical control of the pathogen, but such a measure faces prohibitive practical difficulties.