286 M. BABADOOST
Aphanomycesare destructive pathogens of important crops such as
peas, radishes, and sugar beets. Sporangiophores ofAphanomycesare
filamentous and indistinguishable from somatic hyphae (Cerennius
et al. 1987).Aphanomyces euteichescauses root rot of legumes (Hughes
and Grau 2007);A. cochlioidescauses root rot of sugar beet (Harveson
2013); andA. raphaninis the causal agent of black root rot of radish
(Kendrick 1927; Wenham 2011).
Peronosporales includes aquatic, amphibious, and terrestrial species.
Many species in this order are destructive pathogens of economic
plants. They survive under different climate conditions and have wide
host ranges (Griffith et al. 1992). Their oospores, chlamydospores, spo-
rangia, zoospores, and, in some cases, mycelia are capable of surviving
for various lengths of time outside their hosts and also may be dispersed
by wind or water. Many also are resistant to chemicals used to control
them or true fungi that are plant pathogens.
Mycelium of Peronosporales is well developed, consisting of coeno-
cytic slender hyphae that branch freely. The hyphae of parasitic species
are intercellular and produce haustoria of various shapes. Haustoria of
A. candidaare small spherical structures (Coffey 1975), while those of
Peronospora pisi(Hickey and Coffey 1977) andPhytophthora infestans
(Hohl and Sossel 1976; Hohl and Suter 1976) are more elongated.
As with other oomycetes, asexual reproduction in Peronosporales is
with sporangia and zoospores (Alexopoulos et al. 1996). Sexual repro-
duction in Peronosporales is by means of antheridia and oogonia, which
are borne on the same or on different hyphae. After fertilization, the
oosphere develops a thick wall and becomes an oospore (a thick-walled
spore). After overwintering or oversummering, the oospores germinate,
either by giving rise to zoospores, thus behaving as zoosporangia, or
by putting out germ tubes that soon afterward produce sporangia.
The type of germination varies with the species (Alexopoulos et al.
1996).
There are three families in the order Peronosporales, including Albug-
inaceae, Peronosporaceae, and Pythiacease (Alexopoulos et al. 1996;
Noyd 2000). Albuginaceae includes the plant pathogenic genusAlbugo,
which causes white rust in plants (Alexopoulos et al. 1996; Noyd
2000). Peronosporaceae contains highly specialized plant pathogens,
all obligate parasites of vascular plants, causing diseases known as the
downy mildews. Well-known genera of this family areBremia,Per-
onospora,Plasmopara,Pseudoperonospora,andSclerospora. Members
of the Pythiaceae generally bear their sporangia directly on the somatic
hyphae and they are inhabitants of water or soil. Two major genera of
this family arePhytophthoraandPythium.