7/21/2020

Physoderma Brown Spot

Pathogen Facts

  • Physoderma brown spot is caused by the fungal pathogen Physoderma maydis.
  • Infection is most common during the V5-V9 stages when water is in the whorls of plants due to wet weather or irrigation.
  • This disease is generally of minor economic importance.
  • Localized outbreaks may occur in years when weather favors disease development.
Physoderma Brown Spot - Corn Leaf, closeup

The diurnal cycle of infection often results in the banded pattern of lesions seen on leaves.

Disease Symptoms

  • Leaf symptoms of Physoderma brown spot are distinctive.
    • Infected leaves will have numerous small yellowish or brown spots.
    • Lesions often occur in bands across the leaf, a result of infection happening while leaves are in the whorl.
    • Lesions also occur on the leaf midrib – a key identifying characteristic. Midrib lesions are typically purplish or black.
  • As disease progresses, small lesions may coalesce to form larger affected areas.
  • Lesions occur on the mid-canopy mainly on leaves, but may also occur on leaf sheath, stalks, outer ear husks and tassels.
Physoderma Brown Spot - symptoms on corn leaf

Dark spots on the midribs are a key distinguishing characteristic.

Disease Cycle

  • Overwintering fungal structures, sporangia, survive in infected corn tissue or soil.
  • Sporangia germinate to produce infective zoospores under conditions of moisture and light.
  • With favorable water, light, and temperature conditions, infections often occur on a diurnal cycle when leaves are in the whorl, resulting in a banded pattern.
  • P. maydis is also the casual pathogen of Physoderma stalk rot.
  • Leaf symptoms are not necessarily predictive of stalk rot later in the season. It is not uncommon for Physoderma stalk rot to occur in fields with little to no foliar disease.
Physoderma Brown Spot - symptoms on corn stalk

Physoderma maydis can also produce lesions on the stalks.

Conditions Favoring Disease

  • Wet growing seasons are more favorable for disease development. Infection occurs when water has been in the whorl for extended periods of time.
  • Warm temperatures (75-85°F) and sunlight are also necessary for infection to take place.

Management Considerations

  • Inoculum levels can be reduced via crop rotation or tillage to promote the decomposition of old infected tissues.
  • Specific management for this disease is not typically required, as the occurrence is sporadic and effect on yield is minimal.
  • Some fungicides are labeled for control of P. maydis but there are limited data on efficacy and optimum application timing.

References



Author: Madeline Henrickson

The foregoing is provided for informational use only. Please contact your Pioneer sales professional for information and suggestions specific to your operation. Product performance is variable and depends on many factors such as moisture and heat stress, soil type, management practices and environmental stress as well as disease and pest pressures. Individual results may vary.