Stalk Rot Threatening Corn This Fall
Stressful growing conditions anytime during the season is impacting late-season stalk quality, warns plant pathologist Tamra Jackson and agronomist Roger Elmore, both of the University of Nebraska. Extremely stressful growing conditions occurred during much of 2019 in that and other states. This included wet conditions early that delayed planting, record rainfall during July and August, and continued wet conditions into September.
Jackson and Elmore report that corn plants in many areas are showing poor stalk quality that may indicate a need to scout fields to determine which may need to be harvested first or earlier than planned to avoid losses due to lodged corn.
The risk of stalk rot diseases is increased in some fields, especially when:
• Leaf diseases, such as gray leaf spot, southern rust, Physoderma brown spot, and others, are severe. Loss of leaf area can lead to stalk cannibalization as the plant fills grain.
• Plants were in standing water.
• Stalks were wounded due to hail and or insect damage, allowing for infection by some pathogens.
• Susceptible hybrids were planted.
• Soil fertility challenges such as too much or (especially) too little nitrogen that may have leached away during wet conditions.
• Higher than recommended populations were planted.
• Fields have a history of stalk diseases, especially in continuous corn with carryover fungal inoculum from the previous season(s).
Diseases of concern this fall, Jackson and Elmore point out, include:
• Physoderma brown spot. This disease does not cause a true stalk rot disease, but spores can accumulate and infect at nodes, weakening them. Infected nodes may become brittle and snap when pushed, revealing darkly discolored stalk rinds often with healthy interior tissue.
The leaf phase of the disease (Figure 3) was common in many Nebraska fields this year. Infection usually occurs earlier in the season when wet conditions at V3 to V8 lead to water accumulation in the whorl of plants where infection usually occurs. Lesions on the leaf blades are yellow to brown and on the midrib and leaf sheath they are black. Lesions often occur in bands across the leaves. Corn hybrids vary in their resistance to the disease, so hybrid selection may help to reduce severity, although brittle stalks may develop in some hybrids with only low
• Anthracnose stalk rot. This pathogen creates dark, black splotchy lesions visible on the outside stalk rind. When the fungus becomes systemic, a top dieback may occur in nodes above the ear and was common in 2019 cornfields. Resistance is available in some hybrids for stalk rot, but is not always effective against the leaf blight phase of the disease. Generally, when 10% to 15% of plants are affected within 60 days of harvest, early harvest may be beneficial to minimize future losses.
• Fusarium stalk rot creates discolored, softened stalks that may have loose strands of vascular bundles inside. White, salmon, or light pink discoloration is common. Stalks may develop brown streaks visible on the outside or evidence of fungal growth or spores, especially on the nodes. Plants may die prematurely. Splitting stalks may reveal discoloration that is in the lower crown nodes (crown rot) of the plant, resulting in rapid plant death. Generally, when 10% to 15% of plants are affected within 60 days of harvest, early harvest may be beneficial to minimize future losses.
• Gibberella stalk rot may cause development of dark streaks on the lower internodes of stalks. Close examination may reveal raised, black specks (fungal fruiting bodies called perithecia) on the surface that cannot be scratched off. The pathogen also can cause scab disease in cereal grain and will be more common in crop rotations that include susceptible crops. Dark pink to red discoloration may develop inside stalks. Generally, when 10% to 15% of plants are affected within 60 days of harvest, early harvest may be beneficial to minimize future losses.