As corn acres across the region have increased, so has the disease pressure. In North Dakota alone, corn acres have increased from 1 million to around 4 million acres since 2000.

The climate continues to change at the same time, and as it has become warmer and wetter, disease pressure has risen. Here is a list of commonly found diseases and management strategies for limiting the economic impact of these diseases.

TAR SPOT

How to spot: Small, raised black lesions that don’t rub off. Often surrounded by yellowing tissue as the disease progresses. It literally looks like someone splattered the plant with tar.

Prefers: Moderate daytime temps (60-70F) with cool nights and relatively high humidity (>70%RH). Extended leaf wetness (>7 hours). It does overwinter in fields with tar spot history.

Of concern: Reduces photosynthesis, leading to early plant shutdown and lost yields.

Look-a-like: Insect droppings or dirt, which will rub off.

How to manage:
• Use hybrids with better tar spot tolerance where risk is higher
• Apply preventive fungicides, preferably at VT-R1
• Use multiple modes of action
• Scout early and often due to long latent period
• Rotate to nonhost crop in any field with previous history of tar spot

NORTHERN CORN LEAF BLIGHT

How to spot: Long (usually 1”-6”), gray-green to tan, cigar-shaped lesions usually appearing on the lower leaves first and moving up the plant canopy as disease develops.

Preferences: Moderate temps (65-80F) and frequent rain or heavy dew (>6 hours daily).

Of concern: Reduces effective photosynthetic leaf area, which can limit yield.

Look-a-like: Gray leaf spot, which has more rectangular lesions bounded by leaf veins.

How to manage:
• Use hybrids with good tolerance
• Manage residue to reduce inoculum in the soil
• Apply fungicides before the upper canopy becomes infected

SOUTHERN RUST

How to spot: Bright orange to rust-colored pustules that rub off easily; primarily found on the top of leaves.

Preferences: Heat (+80F) and humidity – needs weather systems to push the spores north annually as it doesn’t overwinter in our region.

Of concern: Can spread rapidly, causing loss of green leaf area during grain fill.

Look-a-like: Common rust, which is darker, more scattered and found on both sides of leaves.

How to manage:
• Increase scouting during periods of heat and humidity
• Apply fungicides when disease is detected
• Plant hybrids with a good natural tolerance

COMMON RUST

How to spot: Small, brick-red to cinnamon-brown pustules on both sides of the leaf that release powdery spots when ruptured.

Preferences: Cool (65-70F) and moist environments; often arrives earlier in the season than southern rust. Overwinters in tropical regions and moves in on the wind similar to southern rust.

Of concern: Usually low impact to yield unless unusually heavy and early in the season.

Look-a-like: Southern rust, whose pustules are more orange and confined to the top of the leaf.

How to manage:
• Most fields don’t require treatment
• Consider fungicides only if severe pressure develops early or increases rapidly
• Seed corn field/inbreds are most susceptible to common rust

GOSS’ WILT

How to spot: Caused by bacterial infection. Water-soaked lesions with dark freckles and a shiny, varnished appearance. Infected lesions will have wavy edges.

Preferences: Bacteria can infect plant through the stomata or injury from hail, wind/etc., followed by warm temps.

Of concern: Infection can reduce grain fill and cause premature plant death.

Look-a-like: Northern corn leaf blight, which lacks dark freckles and water-soaked margins.

How to manage:
• Select tolerate hybrids
• Reduce residue to minimize plant injury in high-wind events
• No effective in-season chemical control available