Corn Diseases

Tar spot of corn (caused by the fungus Phyllachora maydis) was first confirmed in the United States in 2015 on dent corn in seven counties in northwest Indiana and 10 counties in north-central Illinois. The disease was detected very late in the growing season and no yield loss was reported in fields where the disease was first confirmed. In 2018, a yield-reducing epidemic of tar spot occurred in northern Indiana and in surrounding states. Following this epidemic, tar spot was detected in 172 counties across six states in the Midwest (Kleczewski et al., 2019). Tar spot has been confirmed in 86 Indiana counties. Fields in the most severely affected regions reached 100% disease incidence and over 50% severity on the ear leaf before the dent growth stage (R5/R6) with reports of 1,345‐4,035 kg/hectare (20‐60 bu/acre) yield loss.

Annual Distribution of Tar Spot

Our knowledge of how to economically and sustainably manage this new disease is limited, but it is important to understand how to identify tar spot if it appears on your farm.

This publication describes:

1. How to correctly identify tar spot.
2. Conditions that favor disease development
3. The disease’s potential impact

Identification

While a preliminary identification of tar spot can be made visually, a laboratory diagnosis is required to distinguish it correctly from other pathogens. Leaves with tar spot have small, raised black and circular spots, which are fungal structures called stromata (Figure 2).

Stromata can be present on healthy or dead tissue of leaf sheaths, stalks, and husks. Stromata can be surrounded by a narrow tan halo, which is known as a fish-eye lesion (Figure 3). The stromata are raised and bumpy, and vary in shape from small pinhead structures to more elongated structures.

Infection of tissues can vary from light to heavy depending on the amount of the pathogen in the field, the environment, and the hybrid (Figure 4). It is easy to confuse stromata with structures associated with other fungal diseases, such as the black pustules that the corn rust pathogen produces as it ages. A laboratory diagnosis is required to distinguish tar spot stromata from rust pustules or other pathogens. In Indiana, you can submit a sample to the Purdue Plant and Pest Diagnostic Laboratory ppdl.purdue.edu.

Tar spot can also be easily confused with the black saprophytic organisms that grow on dead leaf tissue. However, saprophytes usually have a dusty appearance and you can rub them off the leaf tissue. (Tar spot stromata cannot be rubbed off.)

Favorable Conditions

Most of the information we have about tar spot originated in Mexico and Central America, where favorable environmental conditions make the disease prevalent. In these areas, cool (59-70°F), humid conditions (85% relative humidity) with long periods of leaf wetness (greater than 7 hours) promote infection and disease development. Wind-driven rain and storms can spread spores of the pathogen to new plants and spread the disease. The fungus that causes tar spot is an obligate pathogen and requires a living host to grow and reproduce. Researchers believe that the fungus is surviving over winter in Indiana on infected corn debris on the soil surface within stromata. Other fungi related to the tar spot fungus overwinter in a similar fashion by infecting grasses and weeds. It is unknown how long the fungus will survive in this debris outside a living host or the range of hosts the pathogen can infect, although it is assumed to only infect corn. Researchers do not believe the tar spot fungus is seedborne, although it can infect husks, as mentioned above.

Potential Impact

It is still unknown what future impact tar spot could have in the United States, but we speculate that the disease will continue to occur and spread. Some corn fields in the most severely affected Midwestern states (Indiana, Illinois, Michigan, and Wisconsin) reached 100% disease incidence and 50% severity on the ear leaf before the dent growth stage (R5), with reports of 20-60 bu/A yield losses in 2018 (Telenko, et. al. 2019). Yield loss was associated with an early, rapid senescence that led to reduced ear size, poor kernel fill, and vivipary (a condition in which the seed germinated while still on the cob). Preliminary observations also suggest that stalk rot and lodging were increased with high tar spot severity. The rapid senescence could also reduce forage quality.

Tar spot is considered the most important foliar disease in Latin America, particularly Mexico. In Mexico and Central America, P. maydis is not widely considered to cause economic damage when present alone, although there were isolated reports of damage in old literature. However, significant yield losses were reported from the tar spot complex, consisting of P. maydis and another fungus (Monographella maydis) associated with tar spot.

Monographella maydis was not detected in any U.S. tar spot samples from 2015 to 2019. However, 2018 observations indicate that the secondary fungus is not required to cause damage. Here in Indiana, P. maydis alone can cause yield loss under favorable environmental conditions. Farmers need to monitor fields in order to help track this disease and determine if management tactics are warranted.

Management Options for Indiana

Fungicides

Several fungicides are efficacious on tar spot. However, optimum application timing will be critical for a fungicide application to be effective and economical. Current fungicide efficacy ratings are available here.

Hybrid Resistance

The most popular and effective tool for managing tar spot in Latin America is to plant hybrids with resistance to the disease. In 2018, tar spot symptoms were observed across a range of hybrids in Indiana. All of these hybrids were susceptible to tar spot infection, but disease severity, the formation of halos or fisheyes, and rapid senescence varied greatly (Figure 5), and some hybrids appeared more resistant to the disease. Data indicated that no particular seed company brand outperformed others.

Crop Rotation and Sanitation

Any practice that reduces infested corn residue and encourages decomposition of fungal survival structures has the potential to reduce the negative impacts of tar spot compared to fields not implementing these practices. Two of the most common practices for reducing local disease inoculum include crop rotation and tillage. Rotating away from corn allows infested corn residue to decompose. However, the amount of time a previously affected field must be rotated away from corn previously infected with tar spot is unknown. Similarly, tillage should bury inoculum and reduce local spore dispersal within a field.

Currently no studies have examined the impacts of these practices on tar spot in the United States.

Efforts are underway to try to gain a better understanding of the biology and epidemiology of tar spot. This information will help farmers formulate fungicide application decisions in the future. If you suspect tar spot is present in an area, submit corn samples to a National Plant Diagnostic Network university diagnostic lab for diagnosis. In Indiana, contact the Purdue Plant and Pest Diagnostic Laboratory (PPDL) for information about collecting and processing samples: ppdl.purdue.edu.

Citations

Telenko, D. E. P., Chilvers, M. I., Kleczewski, N., Mueller, D., Plewa, D., Robertson, A., Smith, D., Tenuta, A., and Wise, K. 2020. Tar spot. Crop Protection Network. CPN-2012-W. doi.org/10.31274/cpn-20190620-008

Acknowledgements

The authors would like to thank Dr. Kiersten Wise and Gail Ruhl for their contribution to the previous version of this publication (BP-90-W), Nathan Kleczewski and Damon Smith for providing peer-review, and the Indiana Corn Marketing Council for their support.

Reference in this publication to any specific commercial product, process, or service, or the use of any trade, firm, or corporation name is for general informational purposes only and does not constitute an endorsement, recommendation, or certification of any kind by Purdue Extension. Individuals using such products assume responsibility for their use in accordance with current directions of the manufacturer.

Gray leaf spot on corn, caused by the fungus Cercospora zeae-maydis, is a perennial and economically damaging disease in the United States. Since the mid-1990s, the disease has increased in importance in Indiana, and now is the one of the most important foliar diseases of corn in the state. Hybrid susceptibility and weather strongly influence disease development, which is why gray leaf spot can be locally severe, but not cause widespread damage throughout the state.

Identifying the disease

Early gray leaf spot symptoms are observed on leaves as small, pinpoint lesions surrounded by yellow halos. At this stage, it can be hard to correctly identify the disease, but as lesions mature, they elongate into narrow, rectangular, brown to gray spots. Lesions expand parallel to leaf veins and may become 1.5 to 2 inches long. On susceptible hybrids, lesions may also appear on leaf sheaths and husks. The major leaf veins restrict lateral expansion of leaf lesions, giving the lesions a blocky shape (Figure 1). Under favorable conditions, lesions can coalesce to form large, irregular areas of dead tissue on the leaves.

Symptoms vary by hybrid susceptibility. Hybrids with partial gray leaf spot resistance may not experience the characteristic lesion expansion. These hybrids restrict lesion growth, so they may have lesions that remain small and have a round or jagged shape — instead of the long, rectangular shape characteristic of lesions on more susceptible hybrids. Gray leaf spot symptoms may be confused with symptoms of other foliar fungal diseases such as anthracnose leaf blight, eyespot, or common rust.

Conditions Favoring Disease Development

The fungus survives the winter on infected corn residue at the soil surface. As temperatures rise in the spring, the fungus on corn residue produces spores that splash onto young corn leaves, which is why symptoms are commonly observed first on the lower leaves of the plant. Wind may transport spores from a field with corn residue to neighboring fields. Infection occurs during prolonged warm (75°F to 85°F), humid (more than 90 percent relative humidity) periods.

Symptoms are commonly observed following long periods of heavy dew and overcast days and in bottomlands or fields adjacent to woods where humidity will be higher and dew will persist longer into the morning. C. zeae-maydis spores can cease development during low humidity periods, and then resume the infection process once humidity rises. Each lesion can produce many spores, which are splashed or blown to the upper leaves or to other plants where they can survive until conditions are favorable for infection. This cycle makes it appear that the disease is moving up the plant. Due to the length of the infection process, symptoms may not be noticeable for up to two weeks after infection, depending on weather conditions and hybrid susceptibility.

Hot, dry weather will restrict disease development and spread. Corn planted late will experience initial infection at earlier growth stages, which can result in higher levels of infection and increased yield loss.

Disease impact

Gray leaf spot can have a substantial impact on yield under favorable conditions. The relationship between the amount of leaf tissue affected by gray leaf spot and the amount of yield loss is unclear. However, the lesions reduce the amount of photosynthetic areas on leaves available to contribute carbohydrates to the developing grain. Yield loss may depend on the number of lesions and how far up in the canopy they occur as the plant enters tasseling and pollination. If lesions have reached the ear leaf or higher during the two weeks before and after tasseling, yield loss could occur. If lesions develop on upper leaves later in the season, the economic impact will be less.

Reduced photosynthetic area from gray leaf spot lesions can also contribute to stalk rots and lodging. conditions favoring disease development The fungus survives the winter on infected corn residue at the soil surface. As temperatures rise in the spring, the fungus on corn residue produces spores that splash onto young corn leaves.

Managing the disease

Preventative management strategies can reduce economic losses due to gray leaf spot. In-season disease management options, such as fungicides, are also available. Susceptible hybrids planted in no-till or reduced-till fields are at high risk for gray leaf spot development, but weather is the primary influence on disease development. Still, preventative management is especially important for high-risk fields.

Select Resistant Hybrids: Choosing a hybrid moderately resistant to gray leaf spot is important in areas of Indiana that have perennial and severe problems with the disease — such as areas of southern Indiana. While no hybrid is immune to gray leaf spot, there are hybrids that have good levels of resistance. These hybrids may be described as tolerant to gray leaf spot, or marketed as “defensive” hybrids. Careful hybrid selection is especially important in continuous corn and reduced-tillage systems. These systems have higher corn residue levels that host the fungus and are a source of disease for the following crop. Most seed companies publish gray leaf spot resistance ratings for their hybrids. When choosing a hybrid, pay particular attention to the rating scale — individual companies use different numerical values to indicate levels of resistance. Some gray leaf spot-resistant hybrids may have lower yield potentials in the absence of disease, so it is important to consult with your local seed dealer, and place moderately resistant varieties in areas at high risk for gray leaf spot development.

Manage Residue Production: Practices that encourage residue decomposition will reduce the amount of fungus present to infect the next corn crop. Continuous corn and no-till or reduced-tillage systems are at high risk for disease development because of the amount of residue they leave on the soil surface. A one-year rotation away from corn, followed by tillage is recommended to prevent disease development in the subsequent corn crop. In no-till or reduced-till fields with a history of gray leaf spot, a two-year rotation out of corn may be needed to reduce the amount of disease in the following corn crop. The fungus that causes gray leaf spot is able to survive on residue for more than one year, and economically damaging disease levels have been observed in Indiana fields with two-year-old corn residue.

Use Fungicides Effectively: Fungicides are available for in-season gray leaf spot management. It is important to remember that a fungicide application is an additional cost to corn production, so growers must consider economic factors (corn market price and fungicide application cost) and other disease factors before deciding whether to apply a fungicide to manage gray leaf spot.

Research in Indiana indicates that strobilurin and strobilurin/triazole premix fungicides are most effective at preventing yield loss when applied in response to disease presence, and at the tasseling to early silking (VT-R1) growth stage. Scouting fields around V14, or just prior to tassel emergence, can help determine the level of disease pressure in a field. Fungicide efficacy chart for foliar disease of corn tables are available to aid in fungicide decisions.

Information adapted from Purdue Extension Publication BP-56-W

Southern Rust:  Southern rust is caused Puccinia polysoa. It is important to distinguish southern rust from common rust (caused by P. sorghi). Rust diseases can cause economic damage in Indiana and are particularly damaging in seed corn, sweet corn, and popcorn production. These diseases may increase in intensity very quickly when weather conditions are favorable, which can result in substantial yield loss if they are not properly controlled.

 

Common rust was particularly severe in Indiana in 2000, resulting in heavy infections in some hybrid corn fields. Southern rust is less common, but is increasingly damaging in southern states and can cause damage if it moved into the state as seen in 2012. Although rust diseases rarely cause economic yield loss in Indiana, it is important to accurately diagnose these diseases and be aware of practical management options.

Figure 1. Distribution of southern rust in Indiana (https://corn.ipmpipe.org/southerncornrust/).

Southern rust pustules generally tend to occur on the upper surface of the leaf, and produce chlorotic symptoms on the underside of the leaf (Figure 3). These pustules rupture the leaf surface and are orange to tan in color. They are circular to oval in shape. We are seeing a lot of common rust as well and both diseases could be present on a leaf.

There are a few characteristics to use to try to distinguish southern rust from common rust. Common rust will form pustules on both sides of the leaf. In addition, common rust pustules tend to be spread out across the leaf, and less densely clustered. Common rust pustules have a brick red to brown coloration and may be more elongated than southern rust pustules.

Check out the southern rust publication for more images of southern rust and other diseases that might mimic it. This publication also has good information on determining when a fungicide application will be beneficial. The publication is at following link: https://crop-protection-network.s3.amazonaws.com/publications/cpn-2009-southern-rust.pdf

Each year the rust spores (urediniospores) travel on air currents from tropical regions to fields in Indiana. Short periods of leaf wetness are required for infection by both rust fungi. Morning dews in Indiana can provide the six hours of moisture required for infection and disease development. Generally, southern rust prefers warmer temperatures — with infection occurring between 77-82°F. Southern rust is usually detected in Indiana late August and September and generally not something to worry about. Now that we have found it late-July it will be very important to keep eye out for southern rust in your field.

Favorable weather can cause the infection to repeat in a disease cycle as short as seven days, resulting in secondary infections and new pustules. Each pustule can produce thousands of spores that can infect corn leaves and produce additional pustules. Disease intensity can reach epidemic levels very quickly as these cycles continue. The speed at which corn rust can reach damaging levels is why it is necessary to pay careful attention to the level and timing of initial disease infection in susceptible hybrids. Young leaves are more susceptible to rust infection than mature leaves. Our late-planted corn may be at greater risk for infection since the rust spores are now here in Indiana. Recent weather conditions continue to favor disease development therefore I cannot stress enough how important it will be to scout your corn fields and be on the lookout.