Ch. 8— The Application of Genetics to Plants • 157 
com. Epidemics like this one are, of course, not 
new. In the 19th centurv, the phvllo.xera disease 
of grapes almost desti'oved the wine industry of 
France, coffee I'ust disrupted the economy of 
Ceylon, and the potato famine triggered e.xten- 
si\ e local star\ ation in Ireland and mass emigra- 
tion to \orth .Amei'ica. In 1916, the red rust de- 
stroN'ed 2 million hu of wheat in the United 
States and an additional million in Canada. Fur- 
ther epidemics of wheat rust occurred in 1935 
and 1953. The corn hlight epidemic in the 
United States stimulated a stud\’ that led to the 
publication of a repoi't on the "Genetic \ ulner- 
ahility of .Major Crops”.-' It contained two cen- 
tral findings: that \ ulnerahility stems from ge- 
netic uniformity, and that some .American crops 
are, on this basis, highly \ iilnerahle. (See table 
29.) 
However, genetic variability, is only a hedge 
against \ ulnerahility. It does not guarantee that 
an epidemic will be avoided. In addition, path- 
ogens from abroad can become serious prob- 
lems when they are introduced into new envi- 
ronments. .As clearly stated in the study, a tri- 
angular relationship e.xists between host, path- 
ogen, and env ironment, and the coincidence of 
their interaction dictates the severity of disease. 
^'.National .Vcademv of Sciences. Genetic Vulnerabililv of Major 
Crops, Washington. D. C., 1972. 
The basis for genetic uniformity 
Crop unifoi'mity results most often from soci- 
etal decisions on how to produce food. The 
structure of agriculture is extremely sensitive to 
changes in the market. Some of the basic factors 
influencing uniformity are: 
• the consumer’s demand for high-quality 
produce; 
• the food processing industry’s demand for 
harvest uniformity; 
• the farmer’s demand for the “best” variety 
that offers high yields and meets the needs 
of a mechanized farm system; and 
• the increased world demand for food, 
which is I'elated to both economic and pop- 
ulation grow th. 
New' varieties of crops are bred all the time, 
but several can dominate agricultural produc- 
tion— e.g., Norman Borlaug and his colleagues in 
Mexico pioneered the "green revolution” by 
developing high-yielding varieties (HYV) of 
wheat that required less daylight to mature and 
possessed stiffer straw and shorter stems. Since 
the new varieties (see Tech. Note 13, p. 163) 
gave excellent yields in response to applications 
of fertilizer, pesticides, and irrigation, the in- 
novation was subsequently introduced into 
countries like India and Pakistan. When a single 
Table 29.— Acreage and Farm Value of Major U.S. Crops and Extent to Which 
Small Numbers of Varieties Dominate Crop Average (1969 figures) 
Crop 
Acreage 
(millions) 
Value 
(millions of 
dollars) 
Total 
varieties 
Major 
varieties 
Acreage 
(percent) 
Bean, dry 
1.4 
143 
25 
2 
60 
Bean, snap 
0.3 
99 
70 
3 
76 
Cotton 
11.2 
1,200 
50 
3 
53 
Corns 
66.3 
5,200 
197b 
6= 
71 
Millet 
2.0 
7 
— 
3 
100 
Peanut 
1.4 
312 
15 
9 
95 
Peas 
0.4 
80 
50 
2 
96 
Potato 
1.4 
616 
82 
4 
72 
Rice 
1.8 
449 
14 
4 
65 
Sorghum 
16.8 
795 
7 
7 
7 
Soybean 
42.4 
2,500 
62 
6 
56 
Sugar beet 
1.4 
367 
16 
2 
42 
Sweet potato 
0.13 
63 
48 
1 
69 
Wheat 
44.3 
1,800 
269 
9 
50 
3Com includes seeds, forage, and silage. 
^Released public inbreds only. 
•^here were six major public lines used in breeding the major varieties of corn, so the actual number of varieties is higher. 
SOURCE: National Academy of Sciences, Genetic Vulnerability of Major Crops, Washington, D.C., 1972. 
