LATE BLIGHT OF POTATOES 
This is the most destructive disease of the potato in the humid temperate 
regions of the world. It is especially severe in northeastern United States. 
SYMPTOMS 
During damp, muggy weather in August and September, circular 
areas in the fields are often to be observed in which all the tops of the pota- 
toes are dead or dying (photograph1). (See also N. Y. (Geneva) Bul. 241, 
pl. VII, and 264, pl. II, III, VIII, XIV, XV.) 
On the leaves. Examine the material provided and oBSERVE:— 
1. The large spots or lesions with indefinite margins. Their 
position in the leaflets (photograph 2). 
2. On holding them to the light, the dark water-soaked or opaque 
appearance of the diseased areas. : 
3. On the lower surface of the leaflet, a white frost-like mildew 
about the margins of thediseased spots. (See illustration specimen; photo- 
graph 2; or N. Y. (Geneva) Bul. 241, pl. XI.) In dry weather this mildew 
may be difficult to see. 
4. That the lower leaves are usually the first attacked. They 
die and shrivel on the stem. (See illustration specimen; or N. Y. (Geneva) 
Bul. 241, pl. X.) 
Compare these leaf-spot symptoms with those of the Alternaria blight 
as shown in the material and photographs provided. 
Show in DRAWINGS the difference in leaf-symptoms of these two diseases. 
Show both sides of the spots. 
On the tubers. The disease may also occur on tubers, causing a reddish 
brown rot. (Read Connecticut Ann. rept. 1904:364, second paragraph 
and see pl. XXXIII; also photographs 4a and 4 b.) 
Study the material provided and OBSERVE :— 
5. The sunken spots on the surface of the diseased tubers 
provided. To what due? 
6. The color and consistency of the diseased tissues shown on 
cutting the tuber open. How deep do the lesions extend? 
DRAW to show tubet-symptoms as observed in specimens and photo- 
graphs. ; 
This disease is also destructive to the tomato in some sections of the 
United States. If material is available study the symptoms on this host 
and show in pRawincs. (See California Bul. 175:9 and fig. 3, 4 and 5; 
and Virginia Bul. 192:7=10 and fig. 5, 6, 7 and 8.) 
ETIOLOGY 
One of the phycomycetous fungi, Phytophthora infestans (Montagne) de 
Bary, is the cause of this disease. The species of Phytophthora are all 
very virulent and destructive pathogenes. 
Life-history. So far as is certainly known this pathogene remains 
in continuous association with the living host throughout its life-cycles. 
The Primary Cycle originates from the activity of the mycelium in 
diseased tubers after they are planted in the spring. 
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