Common potato diseases and physiological problems
Learn what to look for to identify common potato diseases.
Late blight
What to look for
- On leaves: brown lesions, usually with a light green halo. On underside of leaves a white fungal growth surrounds the lesions. Dry lesions do not have the white fungal growth.
- On stems: dark brown to black irregular lesions. If humidity is high, a white fungal growth may develop. Infected stems are brittle and break easily.
Distribution in field
- Anywhere in the field.
- Favored by high humidity.
- Spores need water to germinate.
- Low spots, border rows close to tree lines, pivot center point and pivot tracks, compacted and weedy areas should be monitored twice a week.
Early Blight
What to look for
- On leaves, brown, round to irregular spots with concentric rings. The spots are usually limited by major veins.
- First symptoms appear on older, lower leaves.
- Stressed plants are prone to early blight.
Distribution in field
- Early blight is uniformly distributed in the field.
- The incidence can be higher in spots where plants are under stress, for example, low spots with excessive soil moisture.
Blackleg and Aerial stem rot
What to look for
- Early in the season Infected plants are stunted, yellowish and look stiff.
- Leaflets tend to roll upward at the margins.
- Stems show an inky-black decay that starts from the seed. The rot is slimy and extends a variable distance up the stem.
- Secondary bacteria that invade the affected area produce a fishy odor.
- Aerial stem rot looks similar to blackleg but starts from aboveground parts of stems, not from the seed.
Distribution in field
- Anywhere in the field because it is a seed borne disease.
- Aerial stem rot can develop anywhere in the field. Low spots with high soil moisture can have higher disease incidence.
Rhizoctonia
What to look for
- Stunted, weak plants with brown cankers on underground stems.
- Aerial tubers produced either on leaf axils or at the base of the stems.
- At the base of the stems, white to grey mat of fungal growth (mycelium).
- Longitudinal brown cankers on underground stems.
Distribution in field
- Anywhere in the field but incidence of Rhizoctonia tends to be higher on wet spots.
Verticillium
What to look for
- Infected plants wilt during the day but recover at night.
- Only 1 or 2 stems per plant may be affected.
- Leaf yellowing is followed by browning and necrosis.
- The vascular area of stems turns brown.
- Fields die early
Distribution in field
- Usually spotty. Widespread in heavily infested soils if the season is dry. Fields of susceptible varieties may die early.
- Symptoms develop right after flowering
Mosaic and Leaf roll
What to look for
- Mosaic: Stunted plants, crinkled leaves with yellowish or light green colored mottling. Lower leaves turn yellow and die. Dead leaves cling to the stem.
- Leaf roll: Stunted, erect plants. Lower leaves are yellowish, leathery and rolled up.
Distribution in field
- Anywhere in the field.
Air pollution
What to look for
- Yellowing of leaves and pepper spotting or speckling. White or bleached spots.
- Brown to dark brown irregular lesions on leaves. The lesions are limited by major veins.
- One side of the leaves may be more severely affected than the other.
- Glazy bronzing on the underside of leaves.
Distribution in field
- Anywhere in the field.
- Higher incidence in fields close to busy highways.
White mold
What to look for
- Dense white mycelium on stems or leaves.
- Infected stem areas appear bleached.
- Severely affected stems become hollow.
- Black sclerotia are produced inside or on the stem.
Distribution in field
- Anywhere in the field. Dense canopies and high humidity favor white mold development.
Botrytis grey mold
What to look for
- Young lesions look like water soaked spots.
- Later the lesions turn brown with concentric rings.
- Lesions usually develop on tips and margins of leaves.
- In humid weather a dense grey mycelium forms on the lesions.
Distribution in field
- Anywhere in field. This disease is favored by humid weather and dense canopies.
Updated: October 17, 2024
Published: July 05, 2022