Introduction to Fungi, Third Edition

212 ZYGOMYCOTA
Fig 7.4 0 Carcass of a nettle aphid (Microlophium) incubated
on tap water agar for12 h. Note the halo of discharged conidia
of Erynia neoaphidis.
surrounding the host so that they come under
the influence of wind and gravity, falling at a
velocity of about 1 cm s 1 (Hemmati et al., 2002).
About 200 000 conidia are produced per cadaver
of adult pea aphid over a period of 2 3 days.
Immediately after discharge, primary conidia
may germinate to produce secondary conidia
which are wider and more ovate than the
primary conidia (see Figs. 7.41e,f). Direct germination
by the production of a germ tube from
one or both ends of primary and secondary
conidia also occurs (Fig. 7.41c). As in many
Entomophthorales, cytoplasm is concentrated
into a few terminal cells, leaving empty intercalary
segments cut off by retraction septa
(Fig. 7.41c).
Brobyn and Wilding (1977) and Butt et al.
(1990) have described the process of infection of
the pea aphid Acyrthosiphon pisum. Conidia adhere
to any point on the aphid cuticle, often in
clumps, and may germinate by forming secondary
conidia or germ tubes. The tip of a germ tube
can penetrate any part of the cuticle. Clavate or
globose appressoria develop at the tips of the
germ tubes. After penetrating the cuticle and
epidermis, the hyphal tips branch and fragment
to form multinucleate protoplasts which become
rapidly dispersed throughout the haemocoel
within about 12 24 h. The protoplasts may
increase in number by budding (Butt et al.,
1981). It is believed that the switch to the
protoplast form is in response to contact with
the nutrient-rich haemolymph. Later, as the
nutrients in the haemolymph are exhausted,
the protoplasts develop cell walls and are
transformed into hyphal bodies. Protoplasts
and hyphal bodies colonize fat bodies, nerve
ganglia and muscle tissue. Infected aphids die
some 72 h after inoculation, and shortly before
death rhizoids develop from enlarged hyphal
bodies and emerge from the ventral side of the
abdomen, making contact with the leaf on which
the aphid has been feeding, then branching by
bifurcation to form digitate holdfasts. About
15 30 rhizoids may develop from a single aphid
before it dies. Soon after death, pseudocystidia
and conidiophores emerge. Under natural conditions,
infected aphids die in the late afternoon
and sporulation begins at night. The moist
conditions and dew formation after sunset play
a role in enhancing spore production and
discharge. Hemmati et al. (2001) found that
concentrations of air-borne conidia among
wheat crops were usually highest at night and
in the early morning and relatively low during
the day, peak concentrations being correlated
with high relative humidity.
It is rare for an infected aphid to produce
both conidia and resting bodies. Germinating
resting bodies form branched or unbranched
germ tubes bearing retraction septa. They are
terminated by an apical conidium, followed by
one or two lateral conidia. These conidia closely
resemble those which develop on infected
aphids, and they are discharged by septal eversion
(Tyrell & MacLeod, 1975). The germination
of resting bodies is markedly stimulated by longday
conditions of more than 14 h of light per
day (Wallace et al., 1976). In the pea aphid,
E. neoaphidis does not form resting bodies, surviving
instead as hyphal bodies in aphid cadavers.
Artificially infected cadavers can be stored and