2.3.3.1 Needle casts
Pinus radiata , when grown in the British Isles, is very subject to a foliage disorder called “yellows” (Lally and Thompson 2000; Fennessy et al. 2012). This is very severe needle cast, which was eventually linked to the fungus Cyclaneusma minus (Butin) DiCosmo after problems with false negative isolations. The disease occurs after long spells of mild, cloudy weather in winter and early spring which mean very low insolation at such latitudes. Similar needle casts have occurred in New Zealand, associated with prolonged very wet and mild winter weather. One of these has been attributed to C. minusemerging from latent pathogen status to cause “spring needle cast”. Another, called “physiological needle blight”, has been unexplained but is now suspected to have been caused by the oomycetePhytophthora kernoviae Brasier, Beales & S.A. Kirk (McDougal and Ganley 2021). Yet another, “red needle cast”, has been associated mainly with Phytophthora pluvialis Reeser, Sutton & Hansen and to a lesser extent with P. kernoviae (Fraser et al. 2020). Such weather, with temperatures allowing growth but with low insolation restricting photosynthesis, is likely to deplete the carbohydrate reserves that would help the host resist pathogens. By comparison, the more winter-dormant Guadalupe Island provenance ofPinus radiata is less prone to yellows (Fennessy et al. 2012) than cultivated P. radiata stocks of mainland California origin. Also, northern populations (Lat. >38°N in California) ofP. muricata , which is much more winter-dormant, are far less subject to such needle casts. While this comparison involves both species as exotics, rather than in natural habitats, it suggests that pathogen pressure can favour winter quiescence despite winter temperatures that permit active growth. That said, P. radiatamust suffer very severe needle cast before it is outgrown by P. muricata (Ades et al. 1992; Burdon and Low 2020). In the northern populations of P. muricata , however, selective pressures imposed by such foliage pathogens may have favoured winter dormancy. Winter cold in their coastal environments seems unlikely to be a strong selective force, especially as P. radiata (of origin 361/2−37°N) is notable for growing-season frost tolerance (ca - 6°C). This tolerance exceeds that of material derived from one such P. muricata population (Lat. 39−391/2°N) (Menzies & Holden 1981).