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).