(Maier et al. 2000 ). The size of infections and
the culturability of its host (Ectocarpus siliculo-
sus[Dillwyn] Lyngbye) mean that this phago-
myxid represents the best chance for ongoing
studies of these organisms.
No phytomyxid has been shown to com-
plete a life cycle in culture free of host cells.
There have been, however, successes in growing
P. brassicaeandS. subterraneain tissue culture
with their respective plant hosts. These studies
have used two approaches. First,P. brassicae
andS. subterraneahave been propagated suc-
cessfully in hairy root cultures established by
Agrobacterium rhizogenes(Asano et al. 2006 ;
Qu and Christ 2007 ). Secondly, bothP.brassicae
andS. subterraneahave been propagated for
significant periods in plant callus cultures
(Asano and Kageyama 2006 ; Bulman et al.
2011 ; Ingram 1969 ; Tommerup and Ingram
1971 ; Williams et al. 1969 ).
V. Conclusions and Future Prospects
Phytomyxea comprises a discrete taxonomic
group that contains several members of eco-
nomic importance. Despite the extensive
applied literature on the control of the plant
pathogensP. brassicaeandS. subterraneaand
the viral vectorsP. graminisandP. betae, sev-
eral unresolved questions about the life cycles
of members of the group remain. These include:
lWhere in the life cycle does karyogamy
occur?
lWhat determines when a resting spore
germinates?
lHow does a zoospore recognize a host cell?
lWhat determines whether a plasmodium will
follow sporogenic or sporangial develop-
ment?
It seems inevitable that Phytomyxea species
are more abundant and widespread than is cur-
rently known (Neuhauser et al. 2011 ). Searches
of potential hosts in other locations would be
rewarding, and studies of environmental DNA
samples may provide a new window into the
group by determining the presence of unde-
scribed species of Phytomyxea in terrestrial
and aquatic environments. Further studies
could include comprehensive Basic Local
Alignment Search Tool searches of anonymous
ribosomal RNA sequences in public databases
for the presence of sequences of likely phyto-
myxean origin (Lesaulnier et al. 2008 ), multiple
PCR-primer approaches (Stoeck et al. 2006 ),
and the use of PCR primers biased toward the
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