Blight on Potatoes: Early Blight, Late Blight, and How to Tell Them Apart
Two diseases named "blight" affect potatoes, and they are not the same organism, do not cause the same damage pattern, and do not require the same management. Early blight is a fungal disease that is common and manageable. Late blight is a different organism -- an oomycete (water mold) -- that is.
Two diseases named "blight" affect potatoes, and they are not the same organism, do not cause the same damage pattern, and do not require the same management. Early blight is a fungal disease that is common and manageable. Late blight is a different organism — an oomycete (water mold) — that is the pathogen responsible for the Irish Potato Famine of 1845–52 and can destroy an entire potato planting in 5–10 days under favorable conditions.
Treating late blight as if it were early blight is a serious mistake. The fungicides effective against one are not necessarily effective against the other.
Early Blight
Pathogen
Early blight is caused by Alternaria solani, a fungus that is widespread in soils where potatoes and tomatoes have been grown. Per Penn State Extension, it overwinters in infected plant debris and soil; spores are dispersed by wind and rain splash.
Symptoms
Per Cornell Cooperative Extension:
- Dark brown spots on lower, older leaves first
- Spots are 1/4 to 1/2 inch, angular to circular
- Target pattern (concentric rings): Spots develop characteristic rings-within-rings that look like a target or bulls-eye — this is the most reliable visual identifier for early blight
- Yellow halo around the dark spot
- In severe cases: defoliation progressing upward through the canopy
- Tubers sometimes develop slightly sunken, dark lesions on the skin, but tuber infection is generally minor
Conditions
Per NC State Extension, A. solani is favored by:
- Warm temperatures (75–86°F optimal)
- Alternating wet and dry conditions
- Older plant tissue, especially leaves below the flowering zone
- Plant stress (drought, low nitrogen)
Early blight is a "mature plant" disease — it rarely causes significant damage on young, vigorously growing plants. It becomes most problematic after flowering when plants begin to senesce.