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.