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