Introduction to Fungi, Third Edition

MELAMPSORACEAE
635
Fig 22.15 Section through a telial lesion of
Melampsora euphorbiae on Euphorbia peplus.The
teliospores are formed in a crust-like layer beneath
the epidermis.
plants. A similar disease is caused by the purely
aecial (anamorphic) Peridermium pini. Both species
are genetically identical, and Hantula et al. (2002)
have recommended that they be considered as
one species.
22.7.2 Melampsora and Melampsoridium
In Melampsora spp. the unicellular teliospores
are sessile and often form a subepidermal crust
(Fig. 22.15). Germination is by an external
metabasidium of the usual type. The aecia lack
peridia so that they are diffuse instead of cupshaped.
Such diffuse aecia are called caeomata
(singular caeoma). Melampsora lini var. lini is an
autoecious rust common on Linum catharticum,
and M. lini var. liniperda infects cultivated flax.
It was this fungus which Flor (1955, 1971) used
for his pioneering work on the gene-for-gene
hypothesis. Several Melampsora spp. (M. populnea,
M. larici-populina, M. medusae, M. allii-populina)
produce brightly coloured and extremely abundant
uredinia and, later in the season, telia on
Populus (poplar) trees (Plate 12g). Their alternate
hosts are Larix, Abies, Picea, Pinus and Allium spp.
(Smith et al., 1988). Some of these rusts seem to
differ very little other than in their aecial host,
and much more work is required before an
acceptable species concept is in place. Further,
hybridization between different rusts can occur
(Spiers & Hopcroft, 1994). There is an even more
bewildering complex of Melampsora rusts on
willows (Salix spp.), alternating with Allium,
Larix, Ribes or orchid species (Smith et al., 1988;
Pei et al., 1993). The most important species is
M. epitea, which alternates with Larix and exists
as numerous races. Since willow and poplar trees
can be important crops in certain forest situations,
efforts at resistance breeding are being
made (Ramstedt, 1999).
Melampsoridium betulinum causes a rust on
birch as its uredinial and telial host, on which it
is extremely common everywhere. Larix is the
alternate host. In contrast to Melampsora spp.,
aecia of Melampsoridium are surrounded by a
peridium, and phylogenetic studies have also
shown that the genera Melampsora and
Melampsoridium are not particularly closely
related (Maier et al., 2003).