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Plants are strain to diseases, which slow down production rates.

"Diseases have a lasting affect on production of many types of food and plants. Diseases spread quickly in dense production conditions. This will cause the density of production to decrease and the demand for other growth regions to increase. Spreading out production is less efficient, therefore, the growth of diseases is harmful.

Detection and differentiation of plant-infecting phytoplasmas. Over the last few years, numerous emerging and economically limiting plant diseases worldwide have been reported to be associated with diverse phytoplasmas."

The limitation of supply based on a disease will increase prices, because supply is lower.

http://www.ars.usda.gov/SP2UserFiles/Program/303/NP303AnnualReportsPriorYears/NP303FY2008AnnualReport.pdf