Tar Spot of Corn in the U.S. and Canada

Tar spot is a foliar disease of corn that has recently emerged as an economic concern for corn production in the Midwestern U.S. It is not a new disease, having been first identified in 1904 in high valleys in Mexico. Historically, tar spot’s range was limited to high elevations in cool, humid areas in Latin America, but it has now spread to South American tropics and parts of the U.S. and Canada. It first appeared in the U.S. in 2015. During the first few years of its presence in the U.S., tar spot appeared to be a minor cosmetic disease that was not likely to affect corn yield. However, widespread outbreaks of severe tar spot in multiple states in 2018 and again in 2021 proved that it has the potential to cause a significant economic impact. With its very limited history in the U.S. and Canada, much remains to be learned about the long-term economic importance of this disease and best management practices.

Tar spot in corn is caused by the fungus Phyllachora maydis, which was first observed over a century ago in high valleys in Mexico. P. maydis was subsequently detected in several countries in the Caribbean and Central and South America (Table 1). Despite its decades-long presence in many of these countries, it was not detected in the U.S. until 2015.

Historically, P. maydis was not typically associated with yield loss unless a second pathogen, Monographella maydis, was also present, the combination of which is referred to as tar spot complex. In Mexico, the complex of P. maydis and M. maydis has been associated with yield losses of up to 30% (Hock et al., 1995). In some cases, a third pathogen, Coniothyrium phyllachorae, has been associated with the complex. Only P. maydis is known to be present in the United States but it has proven capable of causing significant yield losses, even without the presence of an additional pathogen.

The first confirmations of tar spot in North America outside of Mexico were in Illinois and Indiana in 2015 (Bissonnette, 2015; Ruhl et al., 2016). It has subsequently spread to Michigan (2016), Wisconsin (2016), Iowa (2016), Ohio (2018), Minnesota (2019), Missouri (2019), Pennsylvania (2020), Ontario (2020), Kentucky (2021), New York (2021), Nebraska (2021), Kansas (2022), South Dakota (2022), Maryland (2022), Delaware (2023), Virginia (2023), North Dakota (2024), and Quebec (2024). Its presence was also confirmed in Florida in 2016 (Miller, 2016) and in Georgia in 2021.

2018 Outbreak

During the first few years of its presence in the U.S., it appeared that tar spot might remain a relatively minor cosmetic disease of little economic impact. In 2018, however; tar spot established itself as an economic concern for corn production in the Midwest, with severe outbreaks reported in Illinois, Indiana, Wisconsin, Iowa, Ohio, and Michigan. Significant corn yield losses associated with tar spot were reported in some areas. University corn hybrid trials conducted in 2018 suggested potential yield losses of up to 39 bu/acre under the most severe infestations (Telenko et al., 2019). Growers in areas severely impacted by tar spot anecdotally reported yield reductions of 30-50% compared to 2016 and 2017 yield levels. Yield losses specifically attributable to tar spot were often difficult to determine however, because of the presence of other corn diseases due to conditions generally favorable for disease development. Instances of greatest tar spot severity in 2018 were largely concentrated in northern Illinois and southern Wisconsin, where other foliar diseases and stalk rots were also prevalent.

2019 and 2020 Observations

In 2019, tar spot severity was generally lower across much of the Corn Belt and appeared later and more slowly compared to 2018, although severe infestations were still observed in some areas. There is no clear explanation for why tar spot severity was lower in 2019 in areas where it was severe 2018. Less favorable conditions for disease development during the latter part of the growing season in 2019 may have played a role. Reduced winter survival may have been a factor as well. Winter temperatures in some tar spot-affected areas oscillated between warm periods and extreme cold, which may have affected fungal dormancy and survival (Kleczewski, 2019).

Despite the generally lower disease severity, tar spot continued to expand its geographic range in 2019. In Iowa, tar spot presence was limited to around a dozen eastern counties in 2018 but expanded to cover most of the state in 2019 (Figure 1). Tar spot was confirmed in Minnesota for the first time in September of 2019 (Malvick, 2019). Tar spot spread to the south and east as well, with new confirmations in parts of Missouri, Indiana, Ohio, and Michigan.

2020 brought another year of generally lower tar spot severity in the Corn Belt, with severe infestations mostly limited to irrigated corn and areas that received greater than average rainfall or developing late enough in the season that they had minimal impact on yield. Tar spot continued to spread, however; with the first confirmation of tar spot in Pennsylvania. Tar spot was also confirmed to be present in corn in Ontario, marking the first time the disease had been detected in Canada.

2021 Outbreak

The 2021 growing season proved that the 2018 outbreak was not a fluke, with a severe outbreak of tar spot once again impacting corn over a large portion of the Corn Belt. Wet conditions early in the summer appeared to be a key factor in allowing tar spot to get a foothold in the crop. Whereas in 2018, when tar spot appeared to be mainly driven by wet conditions in August and September, in 2021 many impacted areas were relatively dry during the latter portion of the summer. Wet conditions early in the summer were apparently enough to allow the disease to get established in the crop and enabled it to take off quickly when a window of favorable conditions opened up later in the summer. The 2021 season also provided numerous demonstrations of the speed with which tar spot can proliferate, enabled by its rapid reinfection cycle (Figure 2).

The availability of several fungicides labeled for tar spot allowed growers to get a better look at fungicide efficacy. Fungicide application timing proved to be critical for controlling tar spot in 2021. In some cases, two applications were necessary to provide adequate control.

2022: The Tar Spot Story Gets More Complex

2022 was another season with generally low to moderate tar spot severity in most affected areas, similar to the 2019 and 2020 growing seasons. Dry summer conditions experienced in many areas of the Corn Belt may have helped keep tar spot in check. Greater familiarity with the disease following several years of infestation and two major outbreaks may also be driving more a more proactive approach to management with foliar fungicides when symptoms begin to develop.

Tar spot made another substantial expansion westward in 2022, with its presence confirmed for the first time in numerous eastern Nebraska counties as well as a few counties in northeastern Kansas. Eastward spread was more limited, with only a handful of new confirmations in counties in Pennsylvania, New York, and Maryland. Infestation continued to spread in the southern U.S. with several new confirmations in Georgia.

A study published in 2022 (Broders et al., 2022) shed new light on the pathogen that causes tar spot, Phyllachora maydis. Previously, it was thought that P. maydis was not in the U.S. prior to 2015 and that it was not capable of infecting any species other than corn – results from the new study indicate that both of these hypotheses were wrong. Even more notably, the study revealed that P. maydis infecting corn in the U.S. is not one species but is actually multiple, related but genetically distinct, species. In light of these new findings, the authors proposed the term P. maydis species complex to refer to the causal pathogen for tar spot in corn pending further research.

The study assessed sequence diversity of numerous tar spot specimens from field samples as well as herbarium samples of corn and several other grass species. Results revealed five genetically distinct Phyllachora species, three of which are currently found in corn in the U.S.:

Species 1 (In U.S. Corn)

• Found only in corn.
• Found only in field samples from Indiana and Ohio.

Species 2 (In U.S. Corn)

• Found only in corn.
• Found in herbarium samples from Colombia and Puerto Rico and field samples from Puerto Rico, Mexico, Florida, Illinois, and Michigan.

Species 3 (In U.S. Corn)

• Widest geographic and host range.
• Found in several U.S. states and a dozen other countries around the world.
• Found in corn as well as 10 other host species, including monocots and dicots.
• Includes first isolate collected from U.S. corn in 2015 and the original specimen collected in Mexico in 1904.
• Herbarium samples indicate that Species 3 has been present in the Southwestern U.S. since at least the 1940s in native grass species, but not in corn.

Species 4

• Found in herbarium samples of corn from Guatemala and Venezuela.
• Found in field samples of other grass species in the U.S. but NOT in corn..

Species 5

• Not found in corn.
• Found in some of the same grass species as Species 4.

2023 and 2024 Observations

Drought and heat stress were major factors during the 2023 growing season across much of the Corn Belt, which tended to keep foliar diseases – including tar spot – in check in many areas. Tar spot pressure often stayed low or ramped up late enough in the season that it had little to no yield impact. A notable exception was parts of eastern Nebraska and northern Missouri along the Missouri River that experienced high levels of tar spot and areas with yield loss and standability challenges. Tar spot was detected in two new states in 2023 – Virginia and Delaware – and continued its westward expansion, with first detections in numerous counties in Nebraska, Kansas, and Missouri.

2024 was another year of generally low to moderate tar spot pressure. Tar spot continued its spread around the margins of previously infected areas, with a major expansion northwest in Minnesota, South Dakota, and into North Dakota. Tar spot was also found in Quebec for the first time.

Tar spot is the physical manifestation of fungal fruiting bodies, the ascomata, developing on the leaf. The ascomata look like spots of tar, developing black oval or circular lesions on the corn leaf (Figure 3). The texture of the leaf becomes bumpy and uneven when the fruiting bodies are present. These black structures can densely cover the leaf and may resemble the pustules of rust fungi (Figure 3). Tar spot spreads from the lowest leaves to the upper leaves, leaf sheathes, and eventually the husks of the developing ears (Bajet et al., 1994).

Under a microscope, P. maydis spores can be distinguished by the presence of eight ascospores inside an elongated ascus, resembling a pod containing eight seeds (Figure 4).

Tar Spot Look-Alikes

Common rust (Puccinia sorghi) and southern rust (Puccinia polysora) can both be mistaken for tar spot, particularly late in the growing season when pustules on the leaves produce black teliospores (Figure 5a). Rust pustules can be distinguished from tar spot ascomata by their jagged edges caused by the spores breaking through the epidermis of the leaf (Figure 5b). Rust spores can be scraped off the leaf surface with a fingernail, while tar spot cannot. Saprophytic fungi growing on senesced leaf tissue can also be mistaken for tar spot.

The mechanism by which tar spot arrived in the Midwestern and Southeastern U.S. and the reason for its recent establishment and proliferation in the U.S. and Canada, despite being present in Mexico and several Central American countries for many decades prior, both remain unclear.

Following its initial detection in the U.S. in 2015, numerous reports speculated that P. maydis spores may have been carried to the U.S. via air currents associated with a hurricane, the same mechanism believed to have brought Asian soybean rust (Phakopsora pachyrhizi) to the U.S. several years earlier. However, Mottaleb et al. (2018) suggested that this scenario was unlikely and that it is more plausible that spores were brought into the U.S. by movement of people and/or plant material. Ascospores of P. maydis are not especially aerodynamic and are not evolved to facilitate spread over extremely long distances by air. Tar spot was observed in corn in Mexico for over a century prior to its arrival in the U.S., during which time numerous hurricanes occurred that could have carried spores into the U.S. Chalkley (2010) notes that P. Maydis occurs in cooler areas at higher elevations in Mexico, which coupled with its lack of alternate hosts would limit its ability to spread across climatic zones dissimilar to its native range. Chalkley also notes the possibility of transporting spores via fresh or dry plant material and that the disease is not known to be seedborne.

As for the reason for tar spot’s establishment and spread as a disease capable of severely reducing corn yield, Broders et al. note two possible contributing factors. The first is that changes in climate have favored the disease. Shorter and warmer winters may be allowing P. maydis to overwinter further north than previously possible and greater temperature and precipitation could contribute to epidemics during the growing season. Second, is that the overall lack of resistance to P. maydis in North American corn genetics has made corn in the U.S. and Canada a particularly vulnerable host population. Corn hybrids have been shown to vary in their susceptibility to tar spot. Corn breeding programs in Central and South American – countries where tar spot has long been present – would have selected for more resistant genetics, whereas breeding programs in the U.S. and Canada, until very recently, would not.

Much is still being learned about the epidemiology of tar spot, even in its native regions, and especially in the U.S. and Canada. P. maydis is part of a large genus of fungal species that cause disease in numerous other species. P. maydis is the only Phyllachora species known to infect corn, and, until very recently, was believed to only infect corn (Chalkley, 2010). The recent confirmation of the existence of multiple, related P. maydis species infecting corn, some of which can infect others hosts as well, has added another layer of complexity to the situation.

P. maydis is an obligate pathogen, which means it needs a living host to grow and reproduce. It is capable of overwintering in the Midwestern U.S. in infected crop residue on the soil surface. Tar spot is favored by cool temperatures (60-70 ºF, 16-20 ºC), high relative humidity (>75%), frequent cloudy days, and 7+ hours of dew at night. Tar spot is polycyclic and can continue to produce spores and spread to new plants as long as environmental conditions are favorable. P. maydis produces windborne spores that have been shown to disperse up to 800 ft. Spores are released during periods of high humidity.

Yield Impact

2018 was the first time that corn yield reductions associated with tar spot were documented in the U.S. University corn hybrid trials conducted in 2018 suggested potential yield losses of up to 39 bu/acre under heavy infestations (Telenko et al., 2019). Pioneer on-farm research trials, along with grower reports, showed yield losses of up to 50% under the most extreme infestations during the 2018 season and again in the 2021 growing season.

Differences in Hybrid Response

Observations in hybrid trials have shown that hybrids differ in susceptibility to tar spot (Kleczewski and Smith, 2018; Ross et al., 2023). Tar spot affects yield by reducing the photosynthetic capacity of leaves and causing rapid premature leaf senescence. Longer maturity hybrids for a given location have been shown to have a greater risk of yield loss from tar spot than shorter maturity hybrids (Telenko et al., 2019).

Pioneer agronomists and sales professionals continue to collect data on disease symptoms and hybrid performance in locations where tar spot is present to assist growers with hybrid management. Pioneer hybrid trials have shown differences in canopy staygreen among Pioneer® brand corn products1 and competitor products under tar spot disease pressure (Figure 7). Genetic resistance to tar spot should be the number one consideration when seeking to manage this disease.

Stalk Quality

Severe tar spot infestations have been associated with reduced stalk quality (Figure 8). Stress factors that reduce the amount of photosynthetically functioning leaf area during grain fill can increase the plant’s reliance on resources remobilized from the stalk and roots to complete kernel fill. Remobilizing carbohydrates from the stalk reduces its ability to defend against soil-borne pathogens, which can lead to stalk rots and lodging.

Tar spot seems to be particularly adept at causing stalk quality issues due to the speed with which it can infest the corn canopy, causing the crop to senesce prematurely. If foliar symptoms are present, stalk quality should be monitored carefully to determine harvest timing.

Fungicide Treatments

Research has shown that fungicide treatments can be effective against tar spot (Bajet et al., 1994; Da Silva et al., 2019; Ross et al., 2023). A multistate university study conducted in 2020 and 2021 showed that fungicide treatments with multiple modes of action were better at reducing tar spot severity and protecting corn yield than those with only a single mode of action (Telenko et al., 2022).

Severe infestations of tar spot may be challenging to control with a single fungicide application due to its rapid reinfection cycle, particularly in irrigated corn. A 2019 Purdue University study compared single-pass and two-pass treatments for tar spot control using Aproach^® (picoxystrobin) and Aproach^® Prima (picoxystrobin + cyproconazole) fungicides under moderate to high tar spot severity (Da Silva et al., 2019).

Fungicide treatments were applied at the VT (August 8) and R2 stage (August 22), and disease symptoms were assessed on September 30. Results showed that all treatments significantly reduced tar spot symptoms relative to the nontreated check, with Aproach Prima fungicide applied at VT and two-pass treatments at VT and R2 providing the greatest reduction in tar spot stroma and associated chlorosis and necrosis on the ear leaf (Figure 9).

Aproach^® Prima fungicide applied at VT and the two-pass treatments all significantly increased yield relative to the nontreated check. Aproach Prima fungicide applied at VT followed by Aproach^® fungicide at R2 had the greatest yield, although it was not significantly greater than Aproach followed by Aproach Prima (Figure 10).

On-farm fungicide trials conducted in 2021 appeared to confirm concerns that the rapid reinfection rate of tar spot would make it difficult to control with a single pass fungicide treatment. Precise application timing was often critical, and two applications were necessary in some cases to provide adequate tar spot control. Disease forecasting models such as Tarspotter, developed at the University of Wisconsin, may be helpful in optimizing timing of fungicide applications. Tarspotter uses several variables including weather to forecast the risk of tar spot fungus being present in a corn field.

Several foliar fungicide products are available for management of tar spot in corn (Table 2).

Agronomic Practices

The pathogen that causes tar spot overwinters in corn residue but to what extent the amount of residue on the soil surface in a field affects disease severity the following year is unknown. Spores are known to disperse up to 800 ft, so any benefit from rotation or tillage practices that reduce corn residue in a field may be negated by spores moving in from neighboring fields. Evidence so far suggests that rotation and tillage probably have little effect on tar spot severity. A 3-year Purdue University study that compared strip-till and conventional tillage found no effect on tar spot severity in the subsequent growing season (Ross et al., 2023).

Agronomists have noted that infestation may occur earlier in corn following corn fields, where infection proceeds in a “bottomup” manner from inoculum present in the soil, in contrast to rotated fields that more commonly exhibit “top-down” pattern of infection from spores blowing in from other fields.

Duration of leaf surface wetness appears to be a key factor in the development and spread of tar spot. Farmers with irrigated corn in areas affected by tar spot have experimented with irrigating at night to reduce the duration of leaf wetness, although the potential effectiveness of this practice to reduce tar spot has not yet been determined.

Yield potential of a field appears to be positively correlated with tar spot risk, with high productivity, high nitrogen fertility fields seeming to experience the greatest disease severity in affected areas. Research on P. maydis in Latin America has also suggested a correlation between high nitrogen application rates and tar spot severity (Kleczewski et al., 2019).

Mycotoxins

There is no evidence at this point that tar spot causes ear rot or produces harmful mycotoxins (Kleczewski, 2018).

Mottaleb et al. (2018) used climate modeling based on long-term temperature and rainfall data to predict areas at risk of tar spot infection based on the similarity of climate to the current area of infestation. Model forecasts indicated the areas beyond the then-current range of infestation at highest risk for spread of tar spot were central Iowa and northwest Ohio. Observations in subsequent growing seasons have been consistent with model predictions, with further spread of tar spot to the east in Ohio, Ontario, and Pennsylvania and a dramatic expansion of tar spot across Iowa and into parts of Minnesota and Missouri.

As of 2023, spread of tar spot in the Corn Belt has proceeded largely as predicted back in 2018, with some expansion to the north and south but primarily to the east and west. The two primary remaining areas to which tar spot expansion was predicted but has not yet occurred are eastward across New York and westward across South Dakota, so corn growers in these areas should be on the lookout for tar spot in the coming years.

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