Early Blight

Image credit: OMAFRA

Fungal disease in potatoes caused by Alternaria solani. It is commonly found in early maturing varieties or plants undergoing other stresses related to fertility, other pathogens, or water stress.

Symptomology:

Early Blight appears on older leaves under the leaf canopy in late July. These are dark spots ¼” to ½” in size, that are circular to angular and expand outwards around their target spot. Large veins in the leaves limit their growth though these spots are surrounded by a chlorotic halo. When infections are severe the entire leaf can be affected, and it can spread upwards through the plant. When tubers are affected, the lesions are dark brown/black, somewhat sunken and round/irregular in shape. A raised violet-blue border is often present with internal tissue decaying, becoming dark brown and dry with a corky texture.

Disease Cycle

• Spores (conidia) and fungal threads (mycelium) overwinter in soil on affected plant debris
• Conidia are released by wind or water splashing in spring, under adequate temperature and moisture
• Conidia infect foliar plant parts including leaf & stem surfaces forming lesions
• Lesions develop conidiophores, which release additional spores in the air & water, causing secondary infections
• Wounds on tubers during harvest can also be an infection point
• Infected plant debris along with conidia return to soils

Risk Factors:

• Low fertility, especially N fertility
• Presence of Verticillium or Fusarium wilt
• Early maturing cultivars, along with immature or damaged tubers
• Closed crop canopies provide optimal environmental conditions for disease development & ease of spread

Management:

• Crop rotation of min. 3-4 years
• Adequate fertilizer and water
• Cultivating plant debris after harvest
• Harvest 14 days after vine death for adequate tuber skin firmness
• Minimize tuber damage at harvest
• Adequate air flow between plants

References

https://extension.umn.edu/disease-management/early-blight-tomato-and-potato#:~:text=Identification,is%20its%20optimum%20temperature%20range. Date accessed January 7, 2026

https://cropipm.omafra.gov.on.ca/en-ca/crops/potatoes/diseases/7ac96e09-4163-4635-89e6-0ebdfb7eee0d Date accessed January 7, 2026

https://www.gov.mb.ca/agriculture/crops/plant-diseases/early-blight-potatoes.html#:~:text=Province%20of%20Manitoba%20%7C%20agriculture%20%2D%20Early,to%20occur%20from%20this%20infection. Date accessed January 7, 2026

https://vegpath.plantpath.wisc.edu/diseases/potato-early-blight/ Date accessed January 7, 2026

Hielke De Jong, et al. The Complete Book of Potatoes : What Every Grower and Gardener Needs to Know. Portland, Timber Press, 2011. p. 92-93, 122