Common Potato Diseases and Physiological Problems

Table of Contents

  1. Late Blight
  2. Early Blight
  3. Blackleg and Aerial stem rot
  4. Rhizoctonia
  5. Verticillium
  6. Mosaic and Leaf roll
  7. Air pollution
  8. White mold
  9. Botrytis grey mold

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 e.g. 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 tirregular 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

For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca
Author: Eugenia Banks - Potato Specialist/OMAFRA
Creation Date: 21 June 2005
Last Reviewed: 4 August 2010