Associate Prof Guy discussed the now little-known issue in a talk on "Botanical Heresies circa 1869" at the Otago Museum, as part of the 1869 Conference and Heritage Festival, which also began yesterday.
The conference and festival celebrate the 150th anniversary of the University of Otago, which began operating in 1869.
Prof Guy said that Sri Lanka, then called Ceylon, had been a major producer of the coffee previously consumed in Britain in the 19th century, but the arrival of fungal rust about 1869 unexpectedly wiped out coffee production there within about 20 years.
Sri Lanka was particularly vulnerable because of a "huge monoculture of coffee" - only one type of coffee tree was grown - and a lack of fungicides to fight the deadly rust.
Sri Lanka then successfully switched to tea production, and continued exporting to Britain.
Tea subsequently became much more popular in Britain than coffee had been, also becoming a preferred drink with all walks of life, and not just the relatively well-off.
Very few tea drinkers were now aware of the devastating fungal disease which had wiped out coffee production in Sri Lanka and heavily contributed to the modern popularity of tea in Britain, New Zealand and Australia.
Prof Guy, of the Otago botany department, often discussed this botanical disaster in his teaching.
"For teaching it's great-it shows people what a big impact plant diseases can have.
"We can't be complacent about them."