The potato blight is caused by the Phytophthora infestans mold, a type of oomycete (a.k.a. water mold). Sporangia are carried by wind or water, and spread best in wet, cool conditions.
But rewind to 1845. The potato blight had been around for years, in America, in Europe, but most critically, in Ireland. The average Irish subsistence farmer lived almost entirely on potatoes and milk, and when the blight came, people died. 1845 was the biggest blight year ever seen, and it wasn't just the Irish that were impacted, although the Irish potato famine would be the talk of the English-speaking world for years to come. So in 1845, people wanted to know, what caused the blight?
Well, it tended to hit hardest after a period of rain, so perhaps it lived in the rain, or fell from the clouds separately from the rain (cool, overcast days also seemed to be correlated). On the other hand, only the underground portions of the plant were affected, leaving the leaves green and healthy, so perhaps it was something under the soil?
It was probably caused by 'mortiferous vapors' rising from 'blind volcanoes' (by which were meant underground magma pools); nothing to be done about that.
Or maybe... as it had been getting worse over the years (well, it's worse this year, isn't it?), that shows a correlation with the spread of those infernal steam engines, spraying sulphurous steam and smoke into the atmosphere. When will we learn not to play around with technology that runs counter to God's plan?
Well, no, it was clearly a disease of the earth, carried by guano imported for foreign lands and used as fertilizer, a comparatively new fad that was clearly to blame... never mind that the Irish peasantry were not big importers of guano.
Science was not absent from the proceedings, with one local experimentalist in County Clare noting that when a field was covered with cloths laid out to dry, those covered patches -- and only those covered patches -- were spared the blight. Clearly, this indicated that the blight was carried through the air.
The most authoritative voice was perhaps the well-known (for a botanist, anyway) Dr. John Lindley, who believed that it was a type of potato dropsy caused by too much rain causing the potatoes to absorb more water than they could handle, and then rotting.
Despite lively debate and many sharp minds -- and many more creative minds -- being turned to the problem, they were all wrong, completely wrong... except that one person drying their sheets. It was something in the air, and filtering it out apparently worked.