This is just a quick note to say that I am on the home stretch of my PhD, and won’t be posting for a bit. Direct Transmission will return to its regular programming in mid-February.
Aliases: potato blight, late blight
The Irish Potato Famine infamously upended a nation. In the wake of a series of blight-plagued potato crop failures from 1845 to 1850, Ireland lost over a quarter of its population. During the Famine, 1.5 million Irish died of starvation or related disease, and nearly the same number emigrated to other countries.4 The Famine arguably changed the course of history, and it stands as a stark reminder of the consequences of social oppression, and fragility of the agricultural systems we depend on.
The British colonization of Ireland left the Irish peasantry disenfranchised and landless. In order to eke out a living, they were forced to live in close quarters and farm whatever land they could reclaim. The potato became critical to survival, as it produced high calories and fairly good nutrition in a small plot of land (only a ¼ of what wheat or other grains require). By the mid-1840s up to ½ of the Irish population depended on potatoes; they were an essential food source for peasant families.2 Prior to the Famine, the average Irish man ate 12 pounds of potatoes a day.4
The concentration and poverty of the peasant population resulted not only in human suffering, it also created the ideal conditions for a potato pathogen to spread. Because potatoes were so important, they were everywhere, providing ample biomass for disease to attack. Their ubiquity also forced potato fields close together, allowing for easy spread across the landscape. Finally, there was little genetic diversity in the potatoes grown by the peasantry, who did not have access to global markets, making their crops particularly vulnerable to disease.3
The blight swept through, devastating Ireland2, and it may not be done yet. Potato blight has recently reemerged, reaching epidemic levels in North America and Europe, despite greater awareness and precautions. And again, humans are not blameless. This renewed vigor is in part the result of growing fungicide resistance in the pathogen, following over a century of heavy use.4 It turns out, even knowing history’s mistakes, there are still plenty of new ones we can make.
Cause: Potato blight is caused by the nefariously named Phytophthora infestans, an oomycete. (Oomyocetes appear similar to fungi, but unlike true fungi, they contain cellulose rather than chitin in their cell walls and produce motile zoospores. They are actually more closely related to brown algae. And now you know). The exact geographic source of the pathogen is still unknown4; regardless, it travels fast. Its spores are easily transmitted by wind and water, and germinate almost as soon as they land on a host plant.2 The pathogen can also be transferred large distances in infected plant tissue.4
Consequence: Blight causes an array of symptoms in infected potato and tomato plants (Surprise! The pathogen is also devastating in tomatoes), including wilting, chlorosis (the insufficient production of chlorophyll), and the rotting of roots and other organs.1 The results of this can be catastrophic, leading to partial or total crop failure.
Cure: The preventative use of fungicides (anti-fungal chemicals) has been the only consistently successful containment measure for potato blight. In many major production regions, potatoes cannot be grown without fungicide. But this security comes at a high price: in 1995, blight control was estimated to cost more than US$30 million in the Columbia potato-growing region of Washington and Oregon alone. Heavy fungicide use has also spurred the evolution of fungicide-resistant strains of blight, which can wreak havoc on crops.4 Sometimes you really can’t win for losing.
- Akino, S, D Takemoto, & K Hosaka. 2014. Phytophthora infestans: a review of past and current studies on potato late blight. Journal of General Plant Pathology, 80:24-37.
- Braa, DM. 1997. The great potato famine and the transformation of Irish peasant society. Science & Society, 61:193-215.
- Fraser, EDG. 2003. Social vulnerability and ecological fragility: building bridges between social and natural sciences using the Irish Potato Famine as a case study. Conservation Ecology, 7:9.
- Ristaino, JB. 2002. Tracking historic migrations of the Irish potato famine pathogen, Phytophthora infestans. Microbes and Infection, 4:1369-1377.
Image source: Creative Commons, https://en.wikipedia.org/wiki/Great_Famine_(Ireland)