DOWNY MILDEW OF CUCURBITS 
This is a well known and much dreaded disease of cucumbers and musk- 
melons, ranging from the tropics to the north temperate regions. It 
sometimes affects squashes and watermelons. 
SYMPTOMS 
The lesions of this disease are confined to the leaves. The oldest leaves 
are first affected. The fruit is dwarfed by the loss of food-supply. | _ 
The general effect of the disease on the plant as a whole is quite striking. 
The affected leaves at the center of the hill, yellow and soon die and shrivel 
like leaves killed by the frost. (Study Connecticut Ann. rept. 1904, 
pl. XXX and read N. Y. (Geneva) Bul. 119:158.) 
On the leaf. Study the specimens provided. OBSERVE:— 
1. The location or distribution of the spots in the leaf-blade. 
2. The form, size, nature and color of the spots, both above and 
below. In the fresh condition the lower surface of the spot shows a pur- 
plish tinge due to the color of the conidia of the fungus. This color is lost 
in drying. 5 ; 
3. Relation of the margin of the lesions to the leaf-veins. Hold 
to the light to see this. 
Make a pRAwING to show the character of the spots on upper and 
lower surfaces. 
4. That after the spots have coalesced the entire leaf becomes 
brown, shrivels and drops. j 
Make a prawING of a leaf showing this advanced stage of the disease. 
ETIOLOGY 
This disease is caused by the phycomycetous fungus Peronoplasmopara 
cubensis (Berkeley and Curtis) Clinton. It is one of the ‘“downy mildews.” 
Life-history. Unlike most of the members of the order Peronosporales, 
P. cubensis appears to develop no sexual or resting-spores (oospores). 
It is believed that it depends entirely on conidia for reproduction, passing 
the winter on living hosts in the tropical islands of the West Indies or the 
extreme southern United States, spreading northward with the advancing 
summer weather. It may in some cases winter on greenhouse cucumbers. 
There are then strictly speaking no primary cycles. 
Secondary Cycles initiated in the south repeat themselves, spreading 
northward so that they reach the region of northern United States in late 
July and August. No saprogenic phase is certainly known for this patho- 
gene. 
Pathogenesis. The conidia produced in abundance on the lower 
surface of the lesions constitutes the inoculurh. Scrape some from the 
diseased leaves provided, mount and OBSERVE :— 
; 5. The large lemon-shaped conidia each with a papilla at the 
tip. The thin wall and the brown granular appearance of the contents. 
Make an enlarged prawinc of a single conidium. 
6. That the conidia germinate by the formation of swarmspores. 
copy the figures showing the different stages of germination from Connec- 
ticut Ann. rept. 1904, pl. XX XI. 
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