Introduction to Fungi, Third Edition

102 STRAMINIPILA: OOMYCOTA
haploid nuclei are present in each antheridium
just prior to plasmogamy (Sansome, 1963; Win-
Tin & Dick, 1975). The antheridium then attaches
itself to the oogonial wall and penetrates it by
means of a fertilization tube. Following penetration,
only three nuclei were counted in the
antheridium, suggesting that one had entered
the oogonium. Later still, empty antheridia were
found, and it is presumed that the three
remaining nuclei enter the oogonium and join
the oogonial nuclei degenerating in the periplasm.
Fusion between a single antheridial and
oosphere nucleus has been described. The fertilized
oosphere secretes a double wall, and the
ooplast appears in the protoplasm. Material
derived from the periplasm may also be deposited
on the outside of the developing oospore.
Such oospores may need a period of rest (afterripening)
of several weeks before they are
capable of germinating. Germination may be
by means of a germ tube, or by the formation of
a vesicle in which zoospores are differentiated
(Figs. 5.18d,e), or in some forms the germinating
oospore produces a short germ tube terminating
in a sporangium.
Ecological considerations
Pythium spp. can live saprotrophically and may
survive in air-dry soil for several years. They
are more common in cultivated than in natural
soils (Foley & Deacon, 1985), and appear to be
intolerant of highly acidic soils. As saprotrophs,
species of Pythium are important primary colonizers,
probably gaining initial advantage by
virtue of their rapid growth rate. They do not,
however, compete well with other fungi which
have already colonized a substrate, and they
appear to be rather intolerant of antibiotics.
The control of diseases caused by Pythium is
obviously rendered difficult by its ability to
survive saprotrophically and as oospores in soil.
Its wide host range means that it is not possible
to control diseases by means of crop rotation.
The effects of disease can be reduced by improving
drainage and avoiding overcrowding of
seedlings. Pythium infections are particularly
severe in greenhouses and nurseries, where
some measure of control can be achieved by
partial steam sterilization of soil. Recolonization
of the treated soil by Pythium is slow. The use of
certain types of compost instead of peat in
nurseries can provide good control (Craft &
Nelson, 1996; Zhang et al., 1996). The fungicide
metalaxyl (see Fig. 5.27) also gives good control
of seedling blight.
Pythium insidiosum
This species is associated with algae in stagnant
freshwater in tropical and subtropical regions.
When horses or cattle come into contact with
P. insidiosum-contaminated water, zoospores are
attracted to wounds and can infect them,
causing severe open lesions of skin and subcutaneous
tissues known as pythiosis insidiosi (Meireles
et al., 1993; Mendoza et al., 1993). If contaminated
water is consumed, gastrointestinal or systemic
infections may also arise. In addition to grazing
animals, infections in dogs and humans have
been reported. Pythium insidiosum is keratinophilic
and survives well at 37°C. Infections can be
treated successfully by immunotherapy in which
horses are injected with killed fungal material,
the immune response leading to healing of
infections (Mendoza et al., 1992). Pythium insidiosum
used to be known under different names,
but its taxonomy has been clarified by de Cock
et al. (1987).
5.3.3 Phytophthora
The name Phytophthora (Gr.: ‘plant destroyer’) is
apt, most species being highly destructive plant
pathogens. The best known is P. infestans, cause of
late blight of potatoes (Plate 2e). This fungus is
confined to solanaceous hosts (especially tomato
and potato), but others have a much wider host
range. For example, P. cactorum has been recorded
from over 200 species belonging to 60 families of
flowering plants, causing a variety of diseases
such as damping off or rots of roots, fruits
and shoots (Erwin & Ribeiro, 1996). Phytophthora
cinnamomi has the widest host range of
all species, being capable of infecting over 1000
plants and causing serious diseases especially on
woody hosts, including conifers and Eucalyptus
(Zentmyer, 1980). Several other Phytophthora spp.
and related Pythium spp. can also cause diebacks
and sudden-death symptoms of trees, with