Corn Disease Pressure Builds in Nebraska

2025-08-22-corn-diseases-hero-1000x563119804-1

By Tamra Jackson-Ziems – Extension Plant Pathologist

Diseases are common and severe in some Nebraska corn fields this year. The sustained high relative humidity, abundant rainfall in some areas, and warm temperatures have been very favorable for numerous pathogens – especially fungi and some bacteria. Some parts of the state have or are experiencing record levels of southern rust (again). With cooler weather just around the corner, we should see southern rust slow, but tar spot could become more active, bringing more questions about what to expect in Nebraska corn fields.

Corn leaves showing yellow-orange pustules and speckling symptoms of southern rust infection in a field — **Figure 1.** Severe southern rust is in some eastern Nebraska fields this year. *Lily Ziehmer | FMC Technical Service Manager*

With the rapid increase in southern rust (Figure 1) in some fields lately, many are considering late fungicide applications, and sometimes second applications. However, based on the collective body of university data reported by the Crop Protection Network, fungicide applications are generally not recommended for corn in the dent (R5) stage because of the decreased probability of economic return — this is particularly true for second applications (Table 1). Historically, late-season fungicide applications have occasionally led to a yield increase in field experiments under more extreme conditions, such as in susceptible, late-planted corn under heavy disease pressure, as shown in Figure 2.

Table 1. Possible benefits (by growth stage) from applying fungicides to protect against southern rust, via the Crop Protection Network.
Crop Stage When Southern Rust is First dDetected	Possible Benefits from Spraying	Comment
Vegetative	Not likely to find southern rust at this stage unless corn is planted very late for the region	Scout fields for disease
VT (tasseling)	Yes	May need a second spray
R1 (silking)	Yes	May need a second spray
R2 (blister)	Yes	Less likely to need a second spray
R3 (milk)	Yes	No second spray needed
R4 (dough)	Maybe, with severe disease pressure	No second spray needed
R5 (dent)	Unlikely	No second spray needed
R6 (black layer)	No

Given that the window of protection provided by most contemporary foliar fungicides is about three weeks, if a second fungicide treatment is needed, it should be made three weeks following the first application to provide maximum benefits. Second applications are less likely to provide an economic return except in the case of heavy disease pressure.

ar chart of a 2008 fungicide timing trial in Nebraska showing corn yield by application date. Headline fungicide applied at tassel produced the highest yield at 257 bushels per acre, while non-treated plots yielded 231. Stratego applications showed yields ranging from 241 to 250 bushels per acre depending on timing. — **Figure 2.** In a 2008 field experiment at the UNL South Central Ag Lab near Clay Center, Nebraska, some late-season treatments tested in the later planted hybrids (April 30 vs. May 14) led to yield increases over the non-treated control under substantial gray leaf spot pressure (in the more sensitive hybrid) when applied as late as early dough, but not beyond that timing. Results and more details from the study can be viewed on CropWatch.

orn leaf with black fruiting structures and tissue death from tar spot infection. — **Figure 3a.** The classic symptom (sign) of tar spot is the presence of black, raised fungal structures on the leaf or husk surface, called ascomata or stromata. Young, smaller spots are very small and may only be on the upper leaf surface. *Nebraska Extension photo from September 2022*

lose-up of a corn leaf with a black tar spot lesion, highlighted by a red arrow. — **Figure 3b.** Over time, stromata spots lengthen, may appear pointed and grow through the plant so they are also visible on the underside of the leaf. These spots will not rub off the leaf surface (like insect frass). *Nebraska Extension photo*

Black Spots

Changing weather conditions over the next several days may bring some relief from southern rust in parts of Nebraska, but cooler temperatures will likely favor more tar spot development and spread. Late during the crop season, it is also common for the rust fungi (causing both southern rust and common rust) to begin producing black pigmented teliospores instead of the more common and recognizable orange/tan urediniospores (visible in Figure 1). Teliospores are produced on or near the old rust pustules and often in a ring or horseshoe shape around it (Figure 3). Their proximity to old rust pustules and shape are the best ways to differentiate them from the black stromata produced by the tar spot fungus, which are more pointed or diamond-shaped as they grow larger. Also, mature tar spot stromata should also have evidence of a black structure visible on both the upper and lower surfaces of the leaf.

Corn stalks and leaves with brown, irregular lesions from southern rust or common rust — **Figure 4a.** Southern rust and common rust fungi can produce black spores late in the season. Southern rust was common in some Nebraska fields in 2025 and the presence of black telia and teliospores around pustules might make differentiating them from tar spot stromata more difficult. Remember that black rust telia are usually produced adjacent to or around old rust pustules in rings (3a) or horseshoe shapes (3b). Lily Ziehmer | Technical Service Manager at FMC

Close-up of a corn leaf with irregular, water-soaked lesions and dark freckles characteristic from southern rust or common rust — **Figure 4b.** Black rust telia around old rust pustules in horseshoe shapes. *Montana Kunkel | Cropwise Consulting LLC*

Increased Risk for Stalk Rot Diseases and Stalk Lodging

There are several scenarios that could increase the risk for stalk rot diseases and lodging this year in some Nebraska corn fields. Development of substantial leaf diseases can inhibit plants’ ability to fill grain and lead to stalk cannibalization, which hollows out the stalk and weakens it.

Nutrient imbalances — such as nitrogen deficiencies that develop after heavy rainfall — leech nitrogen and can also lead to weakened stalks and several other crop stresses.

You can evaluate the risk for stalk lodging in each field by using a “push or pinch” test of >100 plants from different parts of the field and calculating the percent of those that lodge below the ear or are easily crushed by hand (in the lower internodes).

Prioritizing harvest of more vulnerable fields to be harvested first or earlier than normal if they have more than >10% of plants with weakened stalks can reduce harvest losses due to lodging later.

For More Information

If you see symptoms that look like tar spot on corn — especially in one of the counties where the disease hasn’t been confirmed in disease distribution maps — please reach out to someone in Nebraska Extension or send samples to the UNL Plant and Pest Diagnostic Clinic.

The tar spot fungus will continue to overwinter in Nebraska corn fields and disease will redevelop and continue to spread. Most fields have very minor disease with no impact on yield. Eventually, as the fungal inoculum builds in corn fields over time and disease becomes more severe, it may become a threat to yield in more areas and require active disease management to minimize and prevent economic losses to producers. Thus, it is important that we continue to monitor spread of the pathogen to new areas of the state so that local producers and those in the ag industry can be alerted.

Please see CropWatch and the Crop Protection Network for more information.

Submit samples to the UNL Plant and Pest Diagnostic Clinic and enclose a completed sample submission form.