- OOMYCETE DISEASES OF CUCURBITS 283
Some species of terrestrial oomycetes are plant pathogens (Kamoun
2003), which are among the most devastating pathogens of dicotyle-
donous plants (Alexopoulos et al. 1996; Erwin and Ribeiro 1996; Agrios
2005; Fry and Grunwald 2010). Examples includePhytophthora infes-
tans, the causal agent of late blight of potato and tomato; causal agent
of downy mildews of cucurbits (Pseudoperonospora cubensis), grapes
(Plasmopara viticola), onions (Peronospora destructor), and lettuce
(Bremia lactucae);Albugo candida, causal agent of white rust of bras-
sicas; andPythiumspecies, which cause seedling damping-off and root
rot of many plant species (Alexopoulos et al. 1996). Virtually every
dicot plant is affected by one or more species ofPhytophthora, and sev-
eral monocot species are infected as well. Major oomycete pathogens
of cucurbits arePhytophthora capsici,Pseudoperonospora cubensis,
and severalPythiumspecies.Phytophthora capsicicauses damping-
off, foliar blight, and fruit rot;P. cubensisincites downy mildew; and
Pythiumspp. cause damping-off, root rot, and fruit rot (Thomas 1996;
Agrios 2005; Schumann and D’Arcy 2006; Fry and Grunwald 2010).
Oomycetes produce filamentous hyphae that resemble those of the
true fungi (Alexopoulos et al. 1996; Erwin and Ribeiro 1996). Septa
are absent except at the bases of reproductive organs and occasion-
ally in older hyphae of some species. Hyphae of nonobligate para-
sitic oomycetes species (e.g.,PhytophthoraandPythiumspp.) grow
mainly intracellularly in plants, while those of the obligate parasites
(e.g., downy mildew pathogens) grow intercelluarly and produce spe-
cialized hyphal branches called haustoria (singular: haustorium) that
enter the host cells to absorb nutrients.
With the exception of the obligate parasites, most of the oomycete
species can be cultured in artificial media (Seymour and Fuller 1987;
Alexopoulos et al. 1996). Isolation of oomycetes from water, soil, or
plant tissues typically involves the baiting of soil, or water sample
with various seeds such as hemp or sesame, small pieces of shed snake
skin, or on selective culture media (Papavizas et al. 1981; Seymour and
Fuller 1987; Alexopoulos et al. 1996; Erwin and Ribeiro 1996). Gener-
ally, oomycete species grow quickly on artificial media, and it is possi-
ble to obtain pure cultures by transferring clean hyphal tips to a freshly
prepared culture medium.
The hypha of oomycetes is multinuclear because there are no cross
septa in the hyphae (Alexopoulos et al. 1996). Both mitotic and meiotic
divisions in the oomycetes are intranuclear and centric, which means
that the nuclear envelope remains intact until the end of division and
that centrioles are present at the poles of dividing nuclei (Heath 1977;
Beakes 1981; Alexopoulos et al. 1996).