Using leaf disc techniques, nine isolates of the mycoparasite Sphaerellopsis
filum were examined for pathogenicity to Melampsora
epitea, the most important species of rust in renewable energy willow
plantations. Five isolates were derived from Melampsora spp.
on willows and poplar, one from Puccinia coronata on couch grass,
two from Phragmidium violaceum on blackberry and one from
Triphragmiopsis laricinum on larch. Two inoculation experiments
were carried out. In the first, S. filum and rust were applied
simultaneously on to leaf discs of Salix burjatica cv. Korso.
In the second, leaf discs were inoculated with rust initially, then the
rust uredinial pustules were inoculated with S. filum. In both
experiments, all S. filum isolates from Melampsora spp.,
and that from P.
coronata, developed pycnidia on willow rust. No pycnidia were produced
from the other three isolates. Among isolates which
produced pycnidia, the frequency of rust pustules colonized by S. filum
and suppression of rust spore production differed markedly.
In the first experiment, 55–99% pustules were colonized and rust
spore production was suppressed by 64–98%. In the second, the
proportions were 33–97% and 53–73%, respectively. Suppression
of rust sporulation was closely correlated with the frequency of
rust pustules bearing S. filum pycnidia. The results suggest that
S. filum is composed of pathogenically specialized populations
differing widely in their virulence.