produce only a single(secondary or principal)
type of zoospore (Dick2001a) (Figs.3.13fand
3.14d, q, w), and all have single-oosporedoogo-
nia(Figs.3.12b, c,e,3.13g, and3.14j, t–v)in
which the cytoplasm differentiates into an egg
(oosphere) surrounded by an outerperiplasmic
layerof cytoplasm (Figs.3.12band3.15q). This
periplasm may contribute significantly to the
outer oospore wall in genera such asAlbugo
(Beakes 1981 ). The centric to subcentric ooplast
vacuole is always homogeneous in appearance
(Dick 1969 ).
Molecular studies (Hudspeth et al. 2003 ;
Thines et al.2009c) (Fig. 3.6) revealed two
early diverging orders in the peronosporomy-
cete clade, theRhipidiales(Fig.3.12) andAlbu-
ginales(Fig.3.13). However, further order-level
splitting and genus assignment in this class
remain problematic at present. Waterhouse
( 1970 , 1973 ) adopted the historical position of
Rabenhorst by placing thePythiaceaeandPer-
onosporaceaetogether in a single order,Pero-
nosporales(Table3.3). Dick (2001a), however,
split this order into thePythiales(Fig.3.14) and
Peronosporales(Fig.3.15), assigning genera as
summarized in Table3.4. However, molecular
studies almost immediately revealed that some
genera had been misplaced. For instance,Phy-
tophthoraspecies (Blair et al. 2008 ; Cooke et al.
2000 ) clustered with the downy mildews
(Fig.3.5) rather than amongstPythiumspecies.
To avoid the proliferation of new orders that
are hard to distinguish with no or few synapo-
morphic features, we decided to retain a
broadly definedPeronosporales, as argued by
Hulvey et al. ( 2010 ) and Runge et al. (2011a).
Until more of the less-studied holocarpic repre-
sentatives of the clade are included in multi-
gene analyses, it is thought premature to split
the Peronosporales s. lat. further at present.
Whilst members of the familyPeronosporaceae
seem to form a well-supported monophyletic
clade (Fig.3.5), those in the familyPythiaceae
s. lat.do not (Fig.3.5b). However, the enor-
mous size and diversity of Peronosporales
necessitates the recognition of subgroups
(Table3.5) using informal clade names that
have been previously published.- Rhipidiales
Rhipidiales(Fig.3.12) contains a single family,
theRhipidiaceae,containing around ten genera
(Table3.3), all of which are saprotrophs, typi-
cally isolated from submerged twigs and fruit,
often under fermentative conditions (Dick
1973a,2001a; Gleason 1976 ). They generally
have small, relatively slow-growing, determi-
nant thalli that produce fine, rhizoid-like, basal
hyphae (Fig.3.12a) (Dick1973a,2001a; Sparrow
1960 ). Genera in this order have segmented
thalli, often occluded by plugs of wall material
(Fig.3.12c–e). InAraiosporaand Rhipidium,
mature zoospores are released into a transient
vesicle (Fig.3.12d), which is typical of Peronos-
poromycete genera. They form single-oospored
oogonia, and some genera, such asAraiospora
(Fig.3.12b), have a broadperiplasmic layerand
oospores with complex multilayered walls
(Fig. 3.12c, e) (Sparrow 1960 ). However, it
should be noted that the current phylogenetic
placement of the Rhipidiales (Figs.3.5and3.6)
is based solely on sequence data from just one
taxon,Sapromyces elongatus. The phylogenetic
position of this taxon has proved problematic in
that it may form the basal-most clade to both
main classes, depending on which gene is
sequenced and what other taxa are included in
the analyses. In both SSU rRNA-based (Lara andFig. 3.15(continued) Adapted from Weston and Uppal
( 1932 ) and (m) from Migula (1897). (n) Aerial sporan-
giophores ofPlasmopara pusilla(DMPH clade)on
Geranium pratenseshowing short terminal branches
bearing relatively large conidia-like sporangiospores
(n), together with detail of small pyriform haustoria
typical of this clade in anotherPlasmoparasp. (o). (n)
From Webster ( 1970 ), (o) from Voglmayr and Constan-
tinescu ( 2008 ). Aerial sporangiophore ofBremia lactu-
cae (DMPH clade) on Senecio vulgaris showing
terminal cluster of branches bearing conidiospores
(p). (q,r) Drawing ofaerial conidiophore of brassico-
lous downy mildewHyaloperonospora parasiticaon
Capsella bursa-pastoris (q) together with detail of
hyphae within host tissue illustrating their elongate
lobed haustoria (r). Developing oospore ofPeronos-
pora viciae(in DMCC clade)showing periplasm (s)
and mature oospore with reticulate wall (t). (p,q)
Adapted from Webster ( 1970 ), (s–t) from originals of
Migula ( 1897 )Systematics of the Straminipila: Labyrinthulomycota, Hyphochytriomycota, and Oomycota 69