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account, sometimes known as “‘brown rot.” Compare with grape affected 
with anthracnose or black rot. (See illustration specimens.) 
Sometimes, particularly in the older berries, the fungus does not seem 
to be able to send forth the white downy fruiting structures. Such berries 
fail to ripen and have a whitish opaque color. This character 1s not well 
seen in preserved specimens. : 
SKETCH a bunch of grapes to show the contrast between diseased and 
healthy berries. 
ETIOLOGY 
The fungus which causes the downy mildew is a Phycomycete known 
as Plasmopara Viticola (Berkley and Curtis) Berlese and De Toni. It is 
a native of America and was introduced into Europe about 1878 where it 
has since wrought great destruction. 
Life-history. Thanks to the extensive investigations devoted to this 
pathogene, our knowledge of its life-history is now relatively complete. 
The Primary Cycles are initiated shortly after growth of the host 
starts in the spring. The sources of the primary inoculum are in the pre- 
viously diseased overwintered leaves on the ground. 
Pathogenesis. Examine bits of overwintered leaves or prepared 
mounts of the same. SEARCH carefully for:— 
12. Rather large, globose bodies,—the oospores, embedded 
in the tissues of the leaf. They are readily distinguished by the circular 
band-like appearance of the thick hyaline inner wall. These are resting- 
spores which serve to carry the fungus through the winter. 
These germinate during wet weather in the spring in the old leaves as 
they lie on the ground. The study of actual germination of these spores 
is attended with difficulties; therefore, study separates of Gregory’s article 
from Phytopath. 2:237, fig. 2. OBSERVE:— 
13. The large rough oospores embedded in the disorganized 
leaf-tissues. : 
14. The slender stalk,—conidiophore, sent forth from a crack 
in the wall of the oospore and bearing at its tip a large egg-shaped conidium 
(sporangium). 
copy, fig. 2aandb. Label fully. 
When mature, this conidium is readily broken off by the breeze or splash- 
ing rain-drops and carried to young leaves or growing canes. Here in a 
drop of water, the conidium, a potential sporangium, germinates by the 
division of its protoplasm into (usually) 5-8 swarmspores, which are 
emitted through the papillate apex as shown in Gregory’s article, fig. 3. 
If viable conidia are available, attempt to germinate them, proceeding 
as directed by the instructor. Watch the process carefully. 
Make prawincs, either from observed germination or from Gregory’s 
drawings, to show swarmspore-formation. 
These swarmspores swim about by means of two flagella (Gregory’s 
figures 4c and 7). copy drawings to show swarmspore-germination and 
invasion through a stoma. 
From the swarmspore-germtube, a mycelium is developed which rami- 
fies the tissue, passing between the cells. 
Make thin tangential sections through the cortex of a diseased cane or 
berry. Mount in chloral hydrate to clear. OBSERVE :— 
