"Monoculture is the agricultural practice of producing or growing a single crop or plant species over a wide area and for a large number of consecutive years. It is widely used in modern industrial agriculture and its implementation has allowed for large harvests from minimal labor. However, this ratio remains true only if the accounting for labor required is limited to the number of workers employed on the farm. If the indirect work of employees involved in producing chemicals and machinery are taken into account, the ratio of labor to output is higher.
Monocultures can lead to the quicker spread of diseases, where a uniform crop is susceptible to a pathogen. 'Crop monoculture' is the practice of growing the same crop year after year.
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The term is frequently borrowed for other uses, such as raising one species of livestock in a factory farm, or even in fields other than agriculture to describe any group dominated by a single variety, e.g. in the field of musicology to describe the dominance of the American and British music-industries in pop music, or in the field of computer science to describe a group of computers all running identical software.
​Monocultures used in agriculture are usually single strains that have been bred for high yield and resistant to certain common diseases. Since all plants in a monoculture are genetically similar, if a disease strikes to which they have no resistance, it can destroy entire populations of crops. Polyculture, which is the mixing of different crops, has natural variation and a likelihood that one or more of the crops will be resistant to any particular pathogen. Studies have shown planting a mixture of crop strains in the same field to be effective at combating disease.
Ending monocultures grown under disease conditions by introducing crop diversity has greatly increased yields. In one study in China, the planting of several varieties of rice in the same field increased yields by 89%, largely because of a dramatic (94%) decrease in the incidence of disease, making pesticides less necessary.
There is currently a great deal of international worry about the wheat leaf rust fungus, that has already decimated wheat crops in Uganda and Kenya, and is starting to make inroads into Asia as well. As much of the worlds wheat crops are very genetically similar following the Green Revolution, the impacts of such diseases threaten agricultural production worldwide." 

COURTESY WIKIPEDIA​