Agronomy Talk

Agronomy Talk: Foliar Corn Diseases

Published on Thursday, July 18, 2019

CLICK HERE FOR A DOWNLOADABLE VERSION OF THIS AGRONOMY TALK UPDATE

Corn foliar diseases can have similar symptoms. See below for a refresher on 6 of the most common foliar diseases in Beck's marketing area. For help with a specific situation, or to learn more about management options, reach out to your local Beck's representative.

 

Disease: Northern Corn Leaf Blight (Exserohilum turcicum fungus)

  • Signs and Symptoms: 
    • Begins on lower leaves and works its way up the canopy
    • Large (1 to 6 in.) elliptical lesions that form parallel to the leaf
    • Begin as greyish-green lesions which then turn tan; not restricted by leaf veins
  • Inoculum Source:
    • Infested corn residue from the prior year(s)
    • Spread primarily by wind
  • When to Scout: As early as the V6 growth stage
  • Conditions Conducive for Infection: 
    • Cool (64 to 81° F), wet, and cloudy
    • Wet, humid weather that keeps the leaves wet for long periods
  • Management Strategies: 
    • Can quickly cover the canopy (and impact yield) due to large lesion size
    • If inoculum is present, rotate to a non-host crop
    • Reduce corn residue on the soil surface
    • Select for a hybrid with a good NCLB rating

Disease: Gray Leaf Spot (GLS) (Cercospora zeae-maydis fungus)

  • Signs and Symptoms: 
    • Medium (0.5 to 2 in.) rectangular lesions
    • Red or brown and later turn grey
    • Lesions restricted by leaf veins
    • Begins on lower leaves and works its way up the canopy
  • Inoculum Source:
    • Infested corn residue from prior year(s)
    • Spread by wind or splashing water
  • When to Scout: Normally seen starting two weeks prior to tassel
  • Conditions Conducive for Infection: 
    • Hot and humid (80° F and above) and cloudy
    • Wet, humid weather that keeps the leaves wet for long periods
  • Management Strategies: 
    • If inoculum is present, rotate to a non-host crop
    • Reduce corn residue on the soil surface
    • Select for a hybrid with a good GLS rating

Disease: Southern Rust (Puccinia polysora fungus)

  • Signs and Symptoms: 
    • Small (1/8 in.) round pustule
    • Begin as light green or yellow and turn to orange raised pustules
    • Typically found on the upper leaf surface and seldom on lower
    • Lesions can be on the husks and stalks in addition to the leaves
    • Lesions can form concentric circles around the darker original pustule
  • Inoculum Source:
    • Does not overwinter in the Corn Belt
    • Inoculum survives the winter on corn in more southern locations and is carried north by wind
  • When to Scout: Reaches the Corn Belt in July or later
  • Conditions Conducive for Infection: Humid and warm (around 80° F)
  • Management Strategies: 
    • Develops faster than common rust
    • Can be found throughout the Corn Belt, but severe cases have been limited to the southern states
    • Severity depends on when southern rust arrives
    • Crop rotation and residue management are irrelevant because the inoculum does not overwinter
    • Damage is often more severe in later planting dates or late-maturing hybrid
    • Hybrids vary in resistance to southern rust

Disease: Common Rust (Puccinia sorghi fungus)

  • Signs and Symptoms: 
    • Small (1/4 to 3/8 in.) round
    • Begin as light green or yellow and turn to brick-red raised pustules
    • Lesions can be on husks and stalks in addition to leaves
    • Can sometimes form a band along the leaf
  • Inoculum Source:
    • Does not overwinter in Corn Belt
    • Inoculum survives the winter on corn in more southern locations and is carried north by wind
  • When to Scout: Normally seen between the V6 and R2 growth stages
  • Conditions Conducive for Infection: 
    • Humid with moderate temperatures (60 to 77°F)
    • Younger leaf tissue is more susceptible to rust, so later planted corn may show higher levels of rust than early planted corn
  • Management Strategies: 
    • Severity depends on when common rust arrives
    • Crop rotation and residue management are irrelevant because the inoculum does not overwinter
    • Most modern hybrids have adequate resistance to common rust

Disease: Physoderma Brown Spot (Physoderma maydis fungus)

  • Signs and Symptoms: 
    • Very small (1/8 to 1/4 in.) purple or black circles
    • Lesions found on the leaves in the middle of the canopy
    • Usually form a band along the leaf midrib, but it can also form on the stalk and leaf sheath
  • Inoculum Source:
    • Overwinters in infested corn residue from prior year(s)
    • This water-loving pathogen that can also survive in the soil
  • When to Scout: Typically symptoms first appear in mid to late vegetative stages. The lesions can be mistaken for Common Rust
  • Conditions Conducive for Infection: 
    • Prolonged wet periods early in the season with warm temperatures (75 to 85°F)
    • Infection takes place when the whorl is full of water for an extended period of time
  • Management Strategies: 
    • If inoculum is present, rotate to a non-host crop
    • Reduce corn residue on the soil surface
    • Corn plants become more resistant in the late vegetative growth stages
    • Disease has historically been of minor importance

Disease: Goss’s Wilt (Clavibacter michiganensis bacterium)

  • Signs and Symptoms: 
    • Large (1 to 6 in.) elliptical lesions with small black freckles\
    • Lesions appear wilted and water-soaked and start off as dark green to black and eventually turn tan
    • Most often seen in the top of the canopy
  • Inoculum Source:
    • Infested corn residue from prior year(s)
    • Spread by splashing water, but bacteria are also carried by the wind
    • Can move throughout infected plants
  • When to Scout: Leaves can be infected at any time, but the disease is usually seen around the VT growth stage
  • Conditions Conducive for Infection: Warm, wet conditions (65 to 82°F)
  • Management Strategies: 
    • Can quickly cover the canopy (and impact yield) due to large lesion size
    • If inoculum is present, rotate to a non-host crop
    • Reduce corn residue on the soil surface
    • Select a hybrid with a good Goss’s Wilt rating
    • Fungicides are not an effective means of control

 

CLICK HERE FOR A DOWNLOADABLE VERSION OF THIS AGRONOMY TALK UPDATE

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