Step 1: Identify the historic disease.
The mid-19th century Irish potato famine is associated with potato late blight.
Late blight rapidly destroys potato foliage and tubers in cool, wet environments.
Step 2: Name the causal organism.
*Phytophthora infestans*, an oomycete that propagates via sporangia and zoospores, causes late blight.
Step 3: Eliminate distractors.
*Alternaria solani* causes early blight, distinct from the famine-inducing late blight.
*Synchytrium endobioticum* causes potato wart disease, a quarantine issue but not the famine pathogen.
*Erwinia carotovora* (now *Pectobacterium*) causes soft rot, primarily affecting stored potatoes and wounds.
Step 4: Conclusion.
The Irish potato famine was caused by Phytophthora infestans.