July 23, 2019
Healthy corn fields with the most promising yield potential at tasseling stage are not exempt from leaf, ear, and stalk diseases. Storms, overwintering inoculum, and temperatures influence what can infect corn between the seedling stage and grain fill. Crop management decisions are based on the identification and possible resolution of mid-season corn issues listed in the table below.
Gray Leaf Spot (GLS)
What You See
Small necrotic spots with halos turn into rectangular blocks running parallel to leaf veins.
When to Scout
Typically, two weeks prior to pollination and two weeks after. GLS has a 2-week incubation period after infection.
Cause
Fungus
How You Can Manage
In-season: fungicide application VT-R1 or earlier if lesions appear early in the season.1
Next season: Selection of corn products with good (or improved) GLS ratings is important in fields with a history of GLS pressure. Crop rotation.
Northern Corn Leaf Blight (NCLB)
What You See
Cigar-shaped tan-gray lesions. May turn darker during sporulation after periods of high moisture.
When to Scout
Typically, two weeks prior to pollination and two weeks after
Cause:
Fungus
How You Can Manage
In-season: fungicide application VT-R1.2
Next season:
- Selection of corn products with excellent NCLB ratings is important in fields with a history of the disease.
- Crop rotation
Fusarium
What You See
Corn with stunted growth and leaf yellowing or purpling in areas of stress, not fully recovered or killed from seedling disease, mild to moderate crown deterioration (tan to brown coloration).
When to Scout
Infection occurs at the seedling stage during stressful conditions. May not show symptoms until later in the season after a dry period. Any stress at grain fill will compromise stalk and crown health, allowing the pathogen to infect.
Cause:
Fungus (several species of Fusarium)
How You Can Manage
In-season: cultivation to mound soil around base of plants to support nodal development.3
Next season: seed treatment to prevent seedling diseases.
Southern Rust
What You See
Lesions develop after storms move up from the south with wet and humid weather. Circular to oval circles with light green/yellow halos. Orange to red pustules develop on leaf surfaces.
When to Scout
Typically, two weeks prior to pollination and throughout grain fill. Pustule formation can occur earlier under ideal conditions, especially in late-planted corn.
Infection before flowering causes the most risk of yield loss.
Cause:
Fungus
How You Can Manage
In-season: foliar fungicides soon after the first few pustules are observed. Multiple sprays may be needed in severely infected fields, but fungicide should not be applied on corn within two weeks of black layer. Fungicide should contain dual modes of action, including a strobilurin (Trifloxystrobin) and triazole (Prothioconazole).
Next season: Selection of corn products with excellenttolerance to Southern rust is important in areas at high risk for severe disease outbreaks. Early planting dates to minimize the window in July where storms can carry the fungus from the south and temperatures are high and humid.4
Bacterial Leaf Streak
What You See
Translucent, water-soaked streaks between veins, progress to longer yellowish/necrotic streaks and may coalesce.
When to Scout
Any stage of corn growth, observed as early as mid-June.5
Cause:
Bacteria
How You Can Manage
In-season: sanitize equipment between fields.
Next season: tillage to bury residue, possible differences between corn products. Control weedy field edges as the pathogen is known to infect other grass species.
Goss’s Wilt
What You See
Water-soaked lesions with wavy margins, freckles inside lesions, ‘ooze’ or shiny appearance.
When to Scout
Often spreads after silking but can be earlier with temperatures greater than 80°F and storms causing foliar wounds and splashing.6
Cause:
Bacteria
How You Can Manage
In-season: none
Next season: Selection of corn products with improved ratings for Goss’s wilt is crucial for fields with a known history of the disease, rotation away from corn, grassy weed control, tillage to bury corn residue.
Stewart’s Wilt
What You See
Flea beetle (vector) feeding appears as scratches and scrapes on leaves. Early phase of blight appears as bleached leaf streaks running the length of the leaf. Internal tissues can be discolored and ooze bacteria from the cut surface.
When to Scout
Symptoms can be found at all stages; however, the second phase of blight does not usually kill plants.7
Cause:
Bacteria vectored by flea beetles.
How You Can Manage
In-season: Scout soon after emergence and look for bleached leaf streaks and flea beetles. Foliar insecticides can be applied when there are greater than 6 beetles per 100 plants (susceptible corn products) or 25% of plants have severe feeding damage with 2 beetles/plant (tolerant corn products).8
Next season: Treated seed or in-furrow insecticides can protect young corn plants from flea beetles.
Sources:
1Jardine, D. 2016. Deciding whether to spray for gray leaf spot. No-till Farmer. https://www.no-tillfarmer.com/articles/5862-deciding-whether-to-spray-for-gray-leaf-spot
2Wise, K. 2016. Northern corn leaf blight. Purdue University Extension.
3Munkvold, G.P. 2002. Crown rot symptoms common in corn. Iowa State University. Integrated Crop Management News. 7-1-2002.
4Paul, P. 2017. Managing corn rust with fungicides. Agronomic Crops Network. Ohio State University Extension. https://agcrops.osu.edu/newsletter/corn-newsletter/2017-24/managing-corn-rust-fungicides.
5Jackson-Ziems, T., Korus, K., Adesemoye, T., and Van Meter, J. 2016. Bacterial leaf streak of corn confirmed in Nebraska, other Corn Belt states. University of Nebraska-Lincoln.
6Robertson, A. 2009. Goss’s wilt and northern corn leaf blight showing up in Iowa. Iowa State University. https://crops.extension.iastate.edu/cropnews/2009/07/gosss-wilt-and-northern-corn-leaf-blight-showing-iowa.
7Stack, J. and Jackson, T. Stewart’s wilt. University of Nebraska.
8Phillips, B. and Goldy, R. 2017. Stewart’s wilt in sweet corn 2017. Michigan State University. https://www.canr.msu.edu/news/stewarts_wilt_in_sweet_corn.
(Sources Verified 7/12/19) 5006_S1