Phytopthora infestans, the suspect behind irish famine

Late blight caused by Phytopthora infestans is the disease that triggered the Irish potato famine in the 1840s. It also was the first plant disease for which a microorganism was proved to be the causal agent, leading to the birth of plant pathology as a science. Potato crops failed for a number of years during the cool and rainy "hungry '40s." Although poor people who were dependent on potatoes for food suffered in many areas, the disaster was greatest in Ireland. One and one-half million people starved and a similar number emigrated during the famine, resulting in a large Irish diaspora in many parts of North America. As with many famines, politics enhanced the suffering. Many Irish peasants grew cereal crops to pay their rent. Although the grain was harvested, it could not be eaten, and was exported to the English landlords throughout the famine.

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Hemileia vastatrix, The Culture Changer

Hemileia vastatrix are pathogen that cause coffe rust disease. Coffee rust is the most economically important coffee disease in the world, and in monetary value, coffee is the most important agricultural product in international trade. Before European people have a culture to drink a cup of tea they were drinking coffee. By the early seventeenth century, coffeehouses had sprung up in all the major cities of Europe, with the Dutch being the major coffee supplier. A taste that began with the nobility and the wealthy soon drew in the common folk as well. Coffeehouses became the places where the intelligentsia gathered to discuss philosophy, religion, and politics

Rust disease was first reported by coffee farmer in Ceylon (now Sri Lanka) in 1867. Later  the diseases were spread uncontrolled and destroyed most of coffee plantation in Ceylon and some place in the world. The vigor and productivity of the coffee plantations declined to the point where they were no longer economically viable. Ward had arrived too late to save the coffee, and his warnings about the dangers of monocultures went unheeded. Following a period of severe economic and social upheaval, British planters shifted to planting tea as extensively as they had coffee. And from this moment, the British coffee drinkers began drinking tea.

source :here