Variation in the pine stem rust fungus Peridermium pini in the United Kingdom

Peridermium pini aeciospores from lesions on Scots pine ( Pinus sylvestris ) in Scotland and in Thetford Forest, East Anglia, differed in terms of germ tube morphology and cytology. The former produced long aseptate germ tubes containing a pair of nuclei. The latter produced short septate germ tubes, each cell containing a single nucleus. One third of the 'Scottish' germ tubes ended in a vesicle. Nuclei could be counted in only a small proportion of ungerminated aeciospores. On average 83% of Scottish spores had two nuclei and 4% had one. The equivalent figures for Thetford spores were 66 and 20%. A small number of aeciospore samples from other parts of England resembled Scottish spores rather than Thetford spores in germ tube morphology and cytology.
Both Scottish and Thetford spores proved capable of causing infection of wounded Scots pine shoots. In addition, some Thetford spores caused infection on leaf discs taken from certain Paeonia cultivars. From these results it is concluded that two distinct forms of Peridermium pini exist in the UK. The implications of this finding are discussed in relation to the history of pine stem rust in the country and the genetics and taxonomy of P. pini.