Introduction to Fungi, Third Edition

PHYLLACTINIA AND LEVEILLULA
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Fig13.13 Phyllactinia guttata.
(a) Conidiophores showing the
single terminal conidium. (b) T.S.
leaf of Corylus avellana showing
penetration of stoma in lower
epidermis, formation of a
substomatal vesicle (arrow) and
extension of the mycelium into
the mesophyll.
collected and kept dry until required for classes
(Weber & Webster, 2001a). Chasmothecia are
formed only on the lower leaf surface. They bear
two types of appendage, an equatorial group of
radiating unbranched appendages with bulbous
bases, and a crown of highly branched appendages
which secrete mucilage (Fig. 13.14). The base
of the bulbous appendage is thin-walled in the
region facing the leaf surface, but thick-walled
facing away from the leaf. On drying, the
appendage bends towards the leaf surface as
the thin part buckles inwards, and the pressure
of the appendage tip levers the chasmothecium
free from the superficial mycelium (for a video
sequence, see Webster, 2006b). The bulbous
appendages now function as ‘flights’ or vanes,
and the chasmothecium plummets downwards
like a shuttlecock, orientated during its fall with
the sticky mucilage on the lower side. The blob of
mucilage between the apical crown of branched
appendages helps to stick the chasmothecium
onto twigs and leaves. The asci usually contain
only two spores (Fig. 13.14e). Overwintered ascocarps
open by means of an equatorial line of
dehiscence, and the base of the ascocarp hinges
back to place the asci in a suitable position for
ascus discharge to occur (Cullum & Webster,
1977; Weber & Webster, 2001a).
For historical interest, we may note that in
1861 the Tulasne brothers, working on P. guttata
and other powdery mildews, were among the first
to demonstrate and illustrate in timeless beauty
the connection between a conidial state and a
morphologically very different-looking sexual
state (see Tulasne & Tulasne, 1931).
13.7.2 Leveillula
This genus is related to Phyllactinia. Its members
are common in warmer countries and occasionally
in glasshouses in cooler areas. Leveillula
taurica attacks a wide range of mainly herbaceous
plants including tomato and pepper. It is