2.3.3.2 Western gall rust
A prominent shoot pathogen affecting pines within and beyond the natural
ranges of P. radiata and P. muricata isEndocronartium harknessii (J.P. Moore) Hirats. which causes
western gall rust (Old 1981; Ramsfield et al. 2007). It infects soft,
elongating shoots during spring to early summer. In certain years, when
cool, moist conditions persist during the infection season, abundant
‘wave year’ infection can occur. We suggest that the late flushing in
the northern populations of P. muricata , while it may reduce
growth potential, reduces exposure to infection hazard, in a trade-off.
By comparison P. radiata , with its earlier shoot flushing, is
vulnerable to infection over a longer season, although its dryer
habitats, all south of San Francisco Bay, mean that infection hazard is
lower. The fitness advantage of the greater growth potential of P.
radiata resulting from the longer growing season, maybe with some
enhanced genetic resistance, evidently outweighs the fitness cost of the
associated extension of the infection season in its habitat. The fitness
cost of susceptibility is likely mitigated by susceptibility being
largely confined to young trees (Old et al. 1986) which often arise at
high density from stand-replacing fires. Selection for resistance, with
gall rust infections affecting competitive ability, thereby contributing
to the natural self-thinning, would represent essentially ‘soft’
(density-dependent) selection.
2.3.3.3 Discussion
There is thus reason to suspect that both foliage pathogens and western
gall rust have imposed selective pressures contributing to the
comparative seasonal phenology of P. radiata and northern
populations of P. muricata . Problematically, however, this
interpretation does not account for the flushing and pollination dates
of the southern populations of P. muricata (<37°N).
Brown (1966), recording pollination dates, which are also a reliable
proxy for flushing dates, observed later pollination than in these
populations than in P. radiata , although those populations flush
earlier than the northern populations. These populations are also
acutely susceptible to foliage disease (Ades et al. 1992; Burdon and Low
2020) but are naturally exposed to low disease hazard. Brown’s
observations were made in an essentially common-garden situation near
Canberra, Australia. There is only one location, Monterey, where the two
species naturally co-occur, and different pollination dates are
putatively an adaptive crossing barrier there. Later pollination than inP. radiata in other southern populations of P. muricata ,
however, is not so readily explained, although it did not closely fit a
clinal pattern. Moreover, the coolness of the trial site, near Canberra,
may have distorted the comparative phenology of the southern
populations.
2.3.4