Identification of Rosellinia taxa relies primarily on morphology
of stromata and ascospores, less frequently on ascal apical apparatus morphology.
The subgeneric classification adopted by Petrini (1993) is based on the morphological
characters of the stroma. According to Petrini (1993), the stroma, covered
and/or surrounded by the subiculum, comprises the ectostroma and the entostroma,
encasing the perithecium.
The shape of stromata is usually subglobose. Whether the top of the stroma is
may be diagnostic between some species.
The ectostroma may be
hard and carbonaceous,
up to 75-100 µm thick in subgenus Rosellinia,
associated with a persistent subiculum.
It is thinner, 25-50 µm thick, in subgenus Calomastia, associated with
a non persistent subiculum.
In both above cases, the stromatal surface is smooth or finely roughened. In contrast,
the ectostroma in subgenus Corrugata is thin, rarely over 25 thick, and
deeply folded in lower part.
This last feature is conspicuous on dried material, but easily overlooked in
the field on fresh material. Members of subgenus Corrugata are likewise
associated with a persistent subiculum.
features a whitish to brown tissue enclosing the perithecium, progressively reducing and usually
inconspicuous in mature stromata. Its characters are not diagnostic, neither
are those of the perithecia which are as a rule papery and fragile.
The subiculum in Rosellinia, that Læssøe & Spooner (1994) name
"a false subiculum" on account of its situation around but not beneath
the stromata, may be
or can bear
in subgenus Rosellinia. Its is usually
felty and persistent
in subgenus Corrugata, felty and evanescent
in subgenus Calomastia. The colour of the subiculum may vary from light to dark purplish
brow, rarely whitish, but this character is rarely diagnostic. The persistence
of the subiculum is sometimes difficult to evaluate in case of old stromata
of subgenus Rosellinia. Remnants of subiculum are usually present around
the stromatal base and visible when carefully checked.
Ascospore morphology is the more discriminating criterion at the specific
level. Ascospore size, as reported by Petrini (1993) from statistical studies,
usually is a very reliable character for separating related species. Their
colour, from light to dark brown, is rarely a distinctive feature. Their
shape can be diagnostic in some species,
but the morphology of the germ slit is more informative. The germ slit may be
on the more convex side
but most frequently
on the flattened side.
It may be
short (1/2 spore length or less) and straight,
short and sigmoid,
less than spore length,
as long as spore length,
Ascospores of most of Rosellinia species bear
cellular appendages or slimy sheaths.
Cellular appendages may be
pointed and conspicuous
but are often
short, blunt and inconspicuous.
Their presence is frequently highly
diagnostic and a major difficulty in identifying Rosellinia taxa is to
assess whether they are present or not. They are refractive structures, and
probably best seen in water, but in most cases finding them is unrewarding.
Unfortunately, we failed to illustrate these appendages with photographs taken
from a bright field microscope.
When a slimy sheath is present at the ends of ascospores as
it can be seen in water, but is more conspicuous when ascospores are observed
in diluted Indian ink (ink:1, water:1), or heated in Phloxine or
Toluidine blue and then observed in water. Slimy sheaths are visible on freshly
discharged ascospores, but are difficult to observe on ascospores still in the
ascus, and soon disappear in aged material.
Asci usually have stipes shorter than the spore-bearing
part, but they are fragile and rarely found in good condition. Ascal apical
apparati usually are higher than broad and from rectangular to urn-shaped. Their
dimensions are diagnostic in only a few species for which better discriminating
characters can be found.
Anamorphs in nature or obtained from cultures
lack characters that are distinctive enough to be used for identification, or
better than morphological characters (Petrini, 1993).