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.
2.3.5 Pathogen-induced perturbations of phenology
In addition to putative cases of pathogens operating as selective
influences on natural seasonal phenology, pathogens can exert more
direct phenotypic effects on features of the underlying seasonal
phenology of the hosts. Classically, foliage infection is a widespread
cause of premature leaf fall. Such cases generally entail hosts
responding to infection by abscission of foliage or foliage structures.
The responses can be adaptively significant, as defence mechanisms for
the hosts (Fraser et al. 2016). In some cases, shedding infected foliage
may remove inoculum, as well as possibly denying a pathogen further
sustenance. This appears to be so in the case (already mentioned) of
needle cast associated with Cyclaneusma minus in pine species. In
some other cases, prompt shedding of foliage can prevent dangerous
spread of the pathogen within the host. With white pine blister rust,
caused by Cronartium ribicola J.C.Fisch., prompt abscission of
infected fascicles can serve as an effective resistance mechanism in
both Pinus monticola Douglas ex D.Don (Hoff and McDonald
1971) and P. armandii Franch (Hoff and McDonald 1975). The effect
of a plant-pathogen interaction at the level of a plant may differ from
the impact on individual infected tissues; for example, powdery mildew
infection of pedunculate oak (Q. robur ) leaves resulted in
earlier senescence, but higher average infection promoted later autumn
phenology in those seedlings overall (Mutz et al. 2021). While
infection-related foliage abscission typically shows seasonality
governed by infection and/or sporulation cycles of the pathogens, along
with seasonal fluctuations in the host’s carbon economy, this does not
mean that the abscission is intrinsic to the host’s seasonal phenology.
In contrast to infection-triggered foliage abscission, infection of
larch (Larix ) foliage by Hypodermella laricis Tuboef can
stop natural abscission, thereby keeping the foliage as a continuing
source of inoculum (Cohen 1987).