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.