- OOMYCETE DISEASES OF CUCURBITS 285
of some algae, a fact that links the oomycetes to the chrysophyte algae.
Reports suggest that K-bodies are involved in the adhesion of encysting
zoospores to substrates (Lehnen and Powell 1989).
Zoospores are released into water and they swim short distances to
find potential substrates or hosts. Then, zoospores encyst and produce
germ tubes. Various patterns of swimming and encystment of zoospores
from different species have been reported by Deacon and Donaldson
(1993). Each cyst zoospore germinates to produce a new thallus. Germi-
nated zoospores establish new infection sites when they contact new
hosts. Detailed information on the germination of zoospores and colo-
nization of plant tissues has been published (Robb and Lee 1986; Barron
1987; Heffer Link et al. 2002; Fry and Grunwald 2010).
C. Classification of Oomycetes
Oomycetes are classified into five orders (Alexopoulos et al. 1996; Noyd
2000).
A. Spores always formed within the sporangium, monomorphic or
dimorphic, rarely aplanatic B, BB.
Eucarpic; hyphae regularly constricted with “cellulin” gran-
ules at constrictions Leptomitales
BB. Holocarpic or eucarpic; hyphae when present without constric-
tions C, CC
Thallus holocarpic or eucarpic and mycelial; periplasm
minimal; usually several oospores in an oogonium Sapro-
legnials
CC. Thallus monocentric with rhizoids; periplasm, persistent;
usually a single oospore per oogonium; often in stagnant water
Rhipidiales
AA. Spores formed within the sporangium or, if not, then usually
within an evanescent vesicle arising from the sporangium;
monomorphic, reniform. D, DD
Holocarpic Lagenidiales
DD. Eucarpic PeronosporalesD. Plant Pathogenic Oomycetes
Plant pathogenic oomycetes are in the orders Saprolegniales and Per-
onosporales. Most members of Saprolegniales are aquatic and some
live in soil. Only the genus Aphanomyces in Saprolegniales is a
plant pathogen (Cerennius et al. 1987). Several soil-borne species of