30 



MISCELLANEOUS PUBLICATION NO. 1065, U.S. DEPARTMENT OF AGRICULTURE 



effectively that the greater part of the crop may 

 be lost. 



Handling of tobacco or tomato plants in trans- 

 planting, pruning, and staking may be followed 

 by 100-percent infestation of tobacco mosaic. 



Weed Seeds 



Farmers' fields can be infested by weed seeds 

 that are transmitted by wind, soil, birds, and 

 animals. In a sense, these seeds become infectious 

 agents, since weed pests can be just as harmful 

 to a crop as insects or diseases. Wind is the natural 

 disseminator of many weed seeds to millions of 

 acres of cropland. Use of irrigation water con- 

 taminated with weed seed reinfests thousands of 

 acres of irrigated land each year. Spreading ma- 

 nure, especially that with an abundance of bedding, 

 spreads weed seeds. 



Witchweed, a parasitic plant, produces thou- 

 sands of microscopic seeds that are easily dissemi- 

 nated by wind, water, or soil. The spread of these 

 seeds increases the cost of quarantine and eradica- 

 tion programs. 



Seeds of many weed species are poisonous to 

 livestock, birds, and wildlife. A number of 

 Crotalaria species have poisonous seeds (114). 



These natural mechanisms of weed seed dissemi- 

 nation provide for continual replenishments of 

 these weeds in soils. Some degree of weed control 

 is needed on 365 million acres of cropland each 

 year. Weed and brush control are urgently needed 

 on several hundred million acres of rangeland. 



Farmers spend about $2y 2 billion annually to 

 control weeds. Yet during the period 1951-60, the 

 estimated annual loss due to reduced crop yields 

 and quality caused by weeds varied from 3 to 25 

 percent, depending on the crop. 



The winds that transport infectious agents to 

 soil, water, crops, and trees can be just as devas- 

 tating in terms of financial loss and human suffer- 

 ing as the waters that transmit scarlet fever. 



Nematodes 



Nematodes move from one infested area to the 

 next on plants and soil and in water. Irrigation 

 water is suspected to be a principal means of dis- 

 tributing nematodes affecting alfalfa in the North- 

 west. Direct examination and bioassay of irriga- 

 tion water from a farm pond in south Georgia 

 indicated several species of plant-parasitic nema- 

 todes can be spread by irrigation water. 



Animal Disease Agents 



Agricultural losses caused by infectious agents 

 of livestock and poultry carried by air, water, and 

 soil have also been great. Some of the animal 

 diseases that may be so transmitted are leptospiro- 

 sis, salmonellosis, hog cholera, mastitis, foot and 

 mouth disease, tuberculosis, brucellosis, histoplas- 

 mosis, ornithosis, infectious bronchitis, Newcastle 

 disease, anthrax, blackleg, foot rot, coccidiosis, 

 blackhead of turkeys, erysipelas, and transmissible 

 gastroenteritis. 



Hog cholera was first noted in this country in 

 the 1880's, but it spread rapidly. The virus causing 

 the disease may be picked up from polluted water, 

 polluted soil, or polluted feed. Over the years, the 

 ravages of this disease have caused losses of hun- 

 dreds of millions of dollars to hog farmers. 



Brucellosis of cattle, swine, and goats can be 

 spread by water, feed, or soil polluted with the 

 causal bacteria. 



In the summer of 1966, approximately 130 clini- 

 cal cases of encephalitis in horses were reported in 

 southern Louisiana. Mosquitoes developing in the 

 waste waters of marshlands carried the infectious 

 agent. This example well illustrates how stagnant 

 water may provide the vectors for a serious infec- 

 tion afflicting agricultural endeavor. 

 Costs 



Infectious agents transported by the en- 

 vironment are causing heavy annual losses to agri- 

 culture and forestry. In fact, each crop and do- 

 mestic animal grown on a large scale in the United 

 States is susceptible to one or more highly infec- 

 tious diseases that may be transported by air, water, 

 or soil. 



During the 1950's, estimates of losses in potential 

 production of domestic food animals because of 

 diseases ranged from an average of 3 to 28 percent 

 annually. During 1966, there were over 100 mil- 

 lion cattle on U.S. farms valued at more than $14 

 billion. The value of all meat animals exceeded $17 

 billion. Thus, losses on the order of 10 percent are 

 staggering. The infectious agents of most all of 

 these diseases were carried in wind, water, or soil. 



Estimates of losses in production potential 

 owing to diseases of pasture and range plants 

 varied from 3 to 9 percent for the same period. 

 Comparable figures for fruit and nut crops and 

 for vegetables are 2 to 38 percent and 2 to 23 per- 

 cent, respectively. 



