- OOMYCETE DISEASES OF CUCURBITS 301
infection usually starts on the side of the fruit contacting the soil. In
watermelon, the decay frequently starts at the blossom end. In cucum-
ber, a brown to dark green, liquid-filled blister often precedes the dark
green, water-soaked areas that develop into the watery soft rot.
- Pathogen Biology. Pythium ultimumproduces spherical sporangia,
12–28μm in diameter, which are usually terminal but also may be
intercalary (Gubler and Davis 1996b). Sporangia germinate directly
and initiate infection. Sporangia formed in culture are barrel shaped
and measure 14–22.9×17–27.8μm (Gubler and Davis 1996b). Oogonia
are smooth, usually terminal, rarely intercalary, and spherical, 19.6–
22.9μm in diameter. Oospores are spherical, 14.7–18.3μmindiameter,
and do not fill the oogonium. Oospores germinate directly by one or
more germ tubes. Antheridia arise from the oogonial stack directly
beneath the oogonium. Usually one antheridium is produced per
oogonium.
Pythium irregulareproduces hyphae 2–6μm in diameter, with many
side branches (Gubler and Davis 1996b). Sporangia are spherical to pyri-
form, 10–30μm in diameter, terminal or intercalary. Sporangia produce
spores in a vesicle or germinate directly. Zoospores, a few to 15 or more
per vesicle, measure 4–6×10–12μm. Oogonia are terminal or inter-
calary, sessile or stalked, and spherical to cylindrical, and vary in shape.
Oogonia, 14–26μm in diameter, have either smooth wall or several pro-
jections. Oospores are smooth, 10–20μm in diameter, and do not fill the
oogonium. Antheridia are stalked, androgynous or diclinous, and very
rarely hypogynous. Usually one but as many as three antheridia are pro-
duced per oogonium.
Pythium aphanidermatumproduces branched hyphae, 2–8μmin
diameter (Gubler and Davis 1996b). Sporangia are filamentous, com-
posed of a lobulate, inflated mass of branches, cut off by cross walls
from the remainder of the mycelium, and have a long or short discharge
tube. Sporangia produce zoospores in vesicles, 15–40 or more per vesi-
cle. Zoospores are reniform and biciliate, measuring 6×12–14μm.
Oogonia are terminal, smooth, and spherical, 16–34μm in diameter.
Oospores are smooth and spherical, 12–18μm in diameter, and do not
fill the oogonium. Antheridia are stalked or intercalary and hypogynous
or diclinous; usually one antheridium per oogonium.
Pythium myriotylumproduces main hyphae up to 8.5μmwide.
This pathogen produces appressoria, which are clavate, knoblike or
sickleshaped, often up to 60× 11 μm (Gubler and Davis 1996b). Spo-
rangia are terminal or intercalary, and mostly 7–17μm wide. Sporangia
produce zoospores, 10–12μm in diameter. Oogonia are subglobose to