Population biology is relevant to plant pathology because plant diseases are caused by populations of parasites. One pathogen lesion on one leaf does not have a significant economic or ecological impact. An epidemic that causes significant crop loss involves thousands or millions of infection events involving an entire population of parasites and their host plants. To control the disease, a plant pathologist must develop methods to control the entire pathogen population. Thus it is important to understand the population biology of plant pathogens in order to develop rational control strategies. Comparing to natural ecosystems, the evolution of plant pathogens in agricultural ecosystems is generally faster due to high-density monocultures, large-scale application of agrochemicals, and international trade in agricultural products.