genera Sclerospora and Peronosclerospora,
which Dick retained in their own family, the
Sclerosporaceae (Dick 2001). However, the
association of these downy mildews with Sapro-
legniomycete genera was not supported by
sequence data (Hudspeth et al. 2000 ), and this
order and family are rejected in this revision.
Molecular studies by other groups revealed that
bothPachymetra(Riethmu ̈ller et al. 1999 ) and
Verrucalvus (Telle and Thines, unpublished
results) are also part of the same clade asApha-
nomycesandPlectospira, and therefore theVer-
rucalvaceaeas the older available name should
take precedence over Leptolegniaceae for this
family.
Both morphological and molecular evi-
dence (Hudspeth et al. 2000 ;Le ́clerc et al.
2000 ) points to this “aphanomycoid clade”
(Fig.3.10) as being the most basal in the Sapro-
legniales (Figs.3.9and3.10).Aphanomycesspe-
cies have relatively slender delicate hyphae that
entirely convert to non-proliferating filamen-
tous sporangia (Fig. 3.9a, b) and discharge
naked spore initials that immediately encyst to
form clusters (balls) of primary aplanospores
(Fig.3.9a, b) (Johnson et al. 2002 ; Sparrow
1960 ), which is reminiscent of sporulation in
Aphanomycospsisand Pythiella. Members of
this family also all have single oospored oogo-
nia containing centric/subcentric oospores
(Fig. 3.9c, d). Some species, such as A.
euteiches, are economically important root-
infectingpathogens of legumes(Gaulin et al.
2007 ; Johnson et al. 2002 ; Levenfors and Fatehi
2009 ), whilst others, likeA. astaci, thecrayfish
plaguepathogen (Cerenius et al. 1988 ), andA.
invadans, the causal agent ofepizooitic ulcera-
tive syndrome(EUS; Lilley et al. 2003 ), are
significant pathogens of animals (Phillips et al.
2008 ). Phylogenetic trees based on internal
transcribed spacer sequences have shown that
the saprotrophs, animal parasites, and plant
pathogenicAphanomycesspecies fall into their
own separate clades (Die ́guez-Uribeondo et al.
2009 ; Lilley et al. 2003 ).
Incox2- and SSU rRNA-based phylogenetic
trees the relatively little known genusPlectos-
pirais sister toAphanomyces(Hudspeth et al.
2000 ;Le ́clerc et al. 2000 ).Plectospira(Fig.3.9e,
f) is a monotypic genus associated with bamboo
roots that has slender hyphae on which irregu-
larly inflated sporangia are produced with long
slender discharge tubes (Fig. 3.9e) (Johnson
et al. 2002 ; Watanabe 1987 ). The oogonium of
Plectospiracontains a single oospore and is
surrounded by a halo of fine antheridial hyphae
(Fig.3.9f).
Johnson et al. ( 2002 ) suggested thatSom-
merstorffia(Fig.3.9g–i) is also related toApha-
nomycesand therefore should be placed in this
family. Molecular sequences for a newly
described genus of freshwater rotifer parasites,
Aquastella, have also been found to cluster in
the Aphanomycesclade (Molloy et al. 2014 ),
supporting this placement. However, the place-
ment ofLeptolegniain this clade is problematicFig. 3.11(continued) ooplast vacuole (b). (c–e) Draw-
ings ofThraustotheca clavata.Mature sporangium
packed with aplanospores being released by terminal
dissolution of wall (c), secondary zoospore being
released from aplanospore (d), and multi-oospored
oogonium with eccentric oospores (e). (f, g) Drawings
ofBrevilegnia megasperma. Seriate rows of aplanos-
pores being released by general rupture of sporangium
wall (f) and single-oospored oogonium with eccentric
oospore (g). (h–j) Drawings ofDictyuchusspp. Sporan-
gium ofD. sterileshowing release of secondary zoos-
pores from net-like array of primary cysts (h) and
single-oospored oogoniumD. monosporuswith eccen-
tric oospore (i). Detail of secondary cyst case ofD.
sterileshowing thick tapered spines (j). (k–x)Drawings
of Saprolegniaceae genera.(k–n)Newbya(formerly
Achya)colorata. Discharged sporangium with attached
cyst ball (k); detached spore ball with discharged cysts
(l); secondary zoospore being released from cyst (m);
multi-oospored oogonium with centric oospores with
granular ooplast vacuoles (n). (o–s) Drawings ofSapro-
legniaspp. Immature sporangium ofS. ferax(o); chain
of internally proliferated sporangia ofS. parasitica(p);
four asexual spore types found inS. ferax—primary
zoospore, primary cyst, secondary zoospore, and sec-
ondary cyst (q). Detail of secondary cyst cases ofS.
ferax(r) andS. parasiticashowingbundle of boat-
hook hairs (s). (u, v) Drawings of Calyptralegnia
depicting a sporangium (u) showing terminal release
of primary cysts (u) and multi-oospored oogonium
with centric oospores (v). (w,x) Drawings ofPythiopsis
showing an empty zoosporangium (w) and multi-
oospored papillate oogonium with centric oospores
(x). (c,d,h,j,k,l,o,q,r,s) From Webster ( 1970 ). All
remaining figures adapted from Johnson et al. ( 2002 )64 G.W. Beakes et al.