Saprolegnia - The iLumina Digital Library

According to Dick (1969a), the delicate, flexuous mycelium and the “gritty”
granular aspect of the hyphae were characteristic of the three species assigned to
Scoliolegnia. He remarked that this mycelial texture contrasted with the stout, straight
hyphae prominent in species of Saprolegnia and Isoachlya. These characters alone,
however, did not form the sole basis on which Scoliolegnia was established.
Dick (1969a) concluded that the genera of Saprolegniaceae were defined on too
few characters. Accordingly, he compared and contrasted the species of Scoliolegnia on
twenty structural criteria. The descriptive matter provided by Dick (particularly the
characterization in Table 1 of his 1969a account), and the accompanying illustrations
permit but one conclusion: there are essentially only two features that could possibly be
used to distinguish Scoliolegnia species from members of Saprolegnia. These are, for
Scoliolegnia, the flexuous hyphae and the flared aspect of the rather prominent exit
papilla after it has opened terminally at spore emergence. The first characteristic
(mycelial texture) is, in fact, one by which de Bary’s Saprolegnia asterophora (one of the
species assigned to Scoliolegnia) usually can be recognized in water culture; Dick’s two
new species of Scoliolegnia also at times may be made out in gross culture on this
feature. The prominently flared, opened exit papilla is not a consistent characteristic of
Saprolegnia asterophora, and other species of Saprolegnia sometimes produce similar ones.
Other structural features that Dick used to circumscribe his species, such as mean
diameters of the exit papillae and of the oogonial stalks, are in our judgment too
variable to be of value at the species level, to say nothing of the generic level. In any
event, Dick did not mention the conspicuous nature of the exit papilla remnants in the
formal description of Scoliolegnia.
We find commendable Dick’s (1969a) argument that genera should be defined on
the basis of several characters, but his contention falls short with respect to Scoliolegnia,
since that genus rests in our view on only two criteria. In what we regard as a
fundamental morphological characteristic essential for generic delimitation -- spore
discharge pattern and sporangial renewal -- Scoliolegnia species cannot be distinguished
from those of Saprolegnia. We interpret Dick’s argument favoring the recognition of
Scoliolegnia basically to be a proposition that generic determinations in the
Saprolegniaceae may be made independent of reliance on sporangial renewal method.
Admitting to Saprolegnia species with flexuous hyphae does not at all distort that
genus into an unrecognizable complex. The same may be said for including species
with subeccentric oospores. Indeed the concept of Saprolegnia is more firmly and
competently established by the admission of Dick’s Scoliolegnia species.
As with Achlya, Saprolegnia has been variously divided into infrageneric units.
Schröter (1893) accepted three subgenera: Eusaprolegnia (the S. ferax group), Desmolegnia
(the S. monoica complex), and Astrolegnia. Coker (1923) recognized Eusaprolegnia with
four “Groups” -- Diclina, Ferax, Hypogyna, and Torulosa -- and Pseudosaprolegnia, with the
single species, S. asterophora. In 1959(a), Cejp raised Coker’s Hypogyna and Diclina
groups to section status (in the subgenus Eusaprolegnia, and did likewise with the Ferax
group, which he attributed to de Bary. In addition, Cejp (1959a:251) proposed a fourth
taxon in Eusaprolegnia, the Section Terrestres. Three subgenera of Saprolegnia --
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