856 
Downy Mildew 
Pseudoperonospora cubensis 
(B. & C) Rost. 
Downy mildew, the most destructive 
of all cucurbit diseases, is especially in- 
jurious to cucumbers, but also attacks 
melons, squashes, pumpkins, gourds, and 
other related vines. 
Appearance 
The first indication of downy mildew 
in the field is a yellowing of the older 
leaves in the center of the plant. Faint- 
ly defined angular spots bordered by the 
veins will then be detected. These be- 
come more distinct, and if the weather 
is moist an obscure violet coating of the 
spores may be noticed on the under side 
of the spots. The disease progresses 
from the center of the hill outward, the 
young leaves at the tips of the branches 
living longest. It spreads slowly in bright 
weather, but under the more favorable 
conditions afforded by cloudy, humid 
weather it often develops with the great- 
est rapidity, so that the fields quickly be- 
come as if scorched by fire. 
Downy mildew has been known in this 
country since 1889, and in various years 
has caused serious loss, especially to the 
pickle industry on Long Island and in 
Ohio and other states. It is also de- 
structive to cucumbers in greenhouses. 
Cause 
Downy mildew is caused by a parasitic 
fungus closely related to the destructive 
downy mildews of grape, onion, etc, and 
to the late blight of potato. So far as 
known, it is spread entirely by its conidia, 
or Summer spores, produced on the low- 
er surface of diseased leaves. These are 
blown about by the wind, but are very 
thin-walled, delicate bodies, which perish 
quickly when dried. 
Conditions Favoring Development 
The disease lives through the winter 
in Florida and probably spreads north- 
ward each summer. There is also good 
evidence that it lives over in greenhouses, 
which may later become the centers of 
local epidemics. 
Spray frequently with half strength 
Bordeaux mixture, coating both sides of 
the leaves. 
ENCYCLOPEDIA OF PRACTICAL HORTICULTURE 
References 
Farmers’ Bulletin 231. 
Connecticut Station Bulletin 56. 
Ohio Station Bulletin 214. 
Eenworm. See Nematode, this section. 
Lear Burcar. See Cantaloup Diseases. 
Leaf Mould 
Not important. Yields to same treat- 
ment as downy mildew. 
Leaf Spot Diseases 
Aside from anthracnose, downy mil- 
dew, and leaf blight there are a number 
of other leaf spot diseases hardly distin- 
guishable from the above, all of which 
yield to the Bordeaux treatment. 
Mosaic Disease 
This disease of greenhouse cucumbers 
is analogous in character to the mosaic 
diseases of tobacco and tomatoes and to 
the yellows of the peach. It is due to an 
oxidizing ferment in the leaves and is 
transmitted like the tobacco mosaic dis- 
ease, by touching first diseased and then 
healthy plants. The fruitfulness of these 
variegated yellow plants is very low and 
it is best at all times upon the appearance 
of the disease to remove the diseased 
plants and destroy them. 
A. D. Sreupy 
Nematodes or Eelworms 
Heterodera radicicola (Greef.) Mull. 
These minute parasitic worms are of- 
ten very destructive upon cucumbers un- 
der glass. The greatest injury may oc- 
cur on the seedling plants, but plants of 
all ages are destroyed by the parasitic 
worms. Their presence may be known 
by the small, bead-like enlargements pro- 
duced upon the roots or rootlets. No 
remedy has been discovered that is ef- 
fective with plants once attacked by eel- 
worms. The time to prevent this trouble 
is in the selection or preparation or treat- 
ment of the soil for greenhouse benches. 
Indeed the nematodes seem to be present 
in old sod, and to some extent in decay- 
ing vegetable matter generally. An ef- 
fective remedy against eelworms consists 
in steaming and So treating the soil that 
the parasites will be destroyed. For this 
procedure see Ohio Bulletin 73. Also 
