The first accurate maps of outbreaks of potato blight — a disease caused by the fungus-like pathogen Phytophthora infestans that was responsible for the Irish potato famine between 1845 and 1852 — in the USA between 1843 and 1845 are presented in a study published in Scientific Reports. The findings improve our understanding of the spread of potato blight before the disease reached Europe.

potato blight1

Source: Jean Ristaino

An infected potato tuber

Jean Ristaino and colleagues at North Carolina State University mapped outbreaks of potato blight in North America between 1843 and 1845 by analysing historic agricultural reports published in the USA during this period.

Marching across states

The authors found that the disease was first reported in 1843 in five locations in the states of New York, Pennsylvania, New Jersey, Delaware, and Connecticut. By the end of 1844 the disease had spread to 107 additional locations, a further six US states (Ohio, Massachusetts, Rhode Island, Vermont, New Hampshire, and Maine) and the Canadian province of Nova Scotia.

Potato blight2

Source: Jean Ristaino

Late blight on potato in field in a field in North Carolina in 2012. The disease still wreaks havoc on crops worldwide.

In 1845 the disease spread to 53 new locations, including in four additional US states (Michigan, Illinois, Indiana, Maryland) and the Canadian province of New Brunswick. Contemporary reports suggested that the disease led to crop losses of between 33 and 50%. 

Theories and remedies

The authors also used their approach to characterise historical theories on the source of potato blight and remedies for treating the disease between 1843 and 1845.

Proposed causes of the disease during this period included insects, weather conditions, poor quality potato varieties, and a fungus. In addition, the authors identified a widely described debate in reports about whether the fungus was the cause or a consequence of the disease.

Suggested treatments for the disease included calcium oxide (known as lime), sulfur, copper sulfate (known as bluestone copper), and salt. Infected imported potato seed tubers from locations including Nova Scotia, France, and Bogota, Colombia were suspected as sources of the disease.

Together, the findings provide insight into the spread of potato blight in the USA and into public understanding of disease in the mid-19th century.