Biology Reference
In-Depth Information
Fig. 1
Downy Mildews Fruiting from Stomata on Under-
side of Leaves.
Basidiophora
, sporangiophoore with swol-
len apex;
Bremia
, sporangiophore tip enlarged to a disc,
dichotomous branching;
Peronospora
, sporangia on
sharply pointed terminal branches; Plasmopara, on obtuse
tips; a and a
1
, sporangium; b
1
, zoospore
Peronospora arthuri
Downy Mildew
of gode-
tia, clarkia, gaura, and evening primrose.
Peronospora destructor
Onion Downy Mil-
dew
,
Blight
, general on onion, shallot, Vidalia
sweet onion, and garlic. One of the more serious
diseases of onion, reported in the United States in
1884. All varieties are susceptible, but red onions
have some resistance. Reduction in yield may be
as high as 75 %. The first sign of onion mildew is
the production of conidiophores with a purplish
tinge a short distance back from tips of older
leaves. Leaves turn yellow, wither, and break
over; seedstalks may be infected. Onion mildew
is sporadic, abundant in years of heavy rainfall.
Spores, produced in great numbers in rain or
when plants are wet with dew, lose vitality
quickly when exposed to sun. Low temperature,
optimum 50
F, favors infection. The fungus
winters as mycelium in bulbs, in overwintering
plants in mild areas, or as oospores in soil. Peren-
nial onions in home gardens are considered an
important source of primary inoculum, but
oospores have been known to survive 25 years
in soil.
Control
Calred is a resistant variety adapted to
California. More onion seed is being produced in
Idaho, where dry summers preclude mildew.
Peronospora
Oomycetes, Peronosporales
Mycelium intercellular; haustoria in a few spe-
cies short and knoblike, but in most filamentous
and more or less branched. Sporangiophore with
erect trunk two to ten times dichotomously
branched, with branches somewhat reflexed and
terminal branches sharp-pointed; sporangia col-
ored, lacking an apical papilla, germinating from
an indeterminate point on the side. Oospores
smooth or variously marked, germinating by
germ tubes (see
Fig. 1
).
Peronosclerospora sorghi (formerly
Peronospora sorghi
). Downy mildew, on sweet
corn.
Peronospora antirrhini
Snapdragon Downy
Mildew
, reported from California, Oregon, Okla-
homa, Pennsylvania, and Maryland.
Peronospora arborescens
Downy Mildew
of
prickly-poppy on leaves, buds, and capsules. Yel-
low or light brown blotches on upper leaf surface
turn dark, with light gray mold on the underside.
The fungus winters in old plant debris in soil.
Remove and burn infected plants. Use clean seed.
