Tar spot is a disease of corn previously reported in Central and Latin America. In 2015, tar spot was found for the first time in the U.S. in Indiana and Illinois. Since then, the disease has been confirmed in Michigan, Wisconsin, Ohio, Iowa and Florida.
In Michigan, the disease was first observed late-season in 2016 near Lake Michigan in Allegan County. In 2017, the disease was again observed in Allegan County, but earlier during the growing season and with greater severity.
In 2018, the Chilvers lab, with the assistance of farmers and private ag consultants, confirmed tar spot in 27 counties across west and central Michigan, with multiple reports of 50 bu/A losses where the disease was severe.
As the name suggests, the disease appears and feels like flecks of black tar on the leaves, which cannot be rubbed off the leaf. These small (1/16”) black spots are the fungal fruiting structure which is capable of releasing spores to infect new corn plants.
The fungus Phyllachora maydis is the only pathogen associated with this disease that has been confirmed in the U.S. In Mexico, an additional fungal species is suspected of causing fish-eye symptoms, which is seen as dead leaf material around the black spots. We see the fish-eye symptoms in the U.S. However, to date, we have not found any secondary species associated with these symptoms.
In fields with severe tar spot prevalence, the corn will often appear frosted, will senesce (wilt and age) early and may lodge. Aside from the impact on grain yield and test weight, we have also observed the impact of this disease on silage quality.
When severe, the disease results in corn that is too dry for silage production, and it reduces silage quality by reducing the digestible component and energy value of the feed. Thankfully, there are no associated mycotoxins with this disease.
A challenging aspect of the disease is the rapid progression of the disease. In some fields, the first signs of disease were observed in early July, with widespread symptoms at the start of August that led to complete senescence at the field level by early September.
As with the management of any disease, the selection of hybrids with good disease resistance packages is essential. However, as tar spot is so new to North America, none of our material has been screened and bred for this disease.
We assessed the MSU corn performance trial in Allegan (for more info see (www.varietytrials.msu.edu/corn) and found that no hybrids were immune. However, there were differences with some hybrids being more resistant than others.
With every 10 percent increase in tar spot severity, we noted a 5 bu/A yield loss. Additional screening of hybrids and inbreds will be necessary to identify and incorporate sources of resistance into available hybrid varieties. It is recommended that farmers talk to their seed salespeople for any updates. With little information, it would be best to spread risk by planting a few different hybrids.
Although the disease is new, we have some early indications that fungicides will help in managing this disease. However, do not expect 100 percent control. Fungicide products with multiple modes of action appear to provide the best control. Fungicide timing will also be critical for maximizing disease control.
At this point, we will have to see what weather conditions and disease pressure are like in 2019. The pathogen is capable of overwintering on infested residue, so in areas where the disease is becoming established, there will be greater availability of disease inoculum to initiate disease.
Scouting fields will be essential to stay ahead of this disease. It’s likely that a single application of fungicide at the VT/R1 timing may prove to be the best strategy, but this will depend on hybrid susceptibility, disease pressure and weather; in some situations, it may make economic sense to make two fungicide applications. We are also currently working with collaborators to develop fungicide spray forecasting models.
Consider harvesting tar spot infected fields first. Try to avoid planting corn-on-corn, which will increase the local pathogen inoculum pressure and the speed at which disease will develop. However, even fields under a soybean-corn rotation have been significantly impacted, most likely as the spores are readily dispersed on the wind and capable of moving some significant distance.
Leaf wetness is also a major driver of disease and there have been incidences of greater disease under pivots compared to areas not irrigated. We have heard of at least two cases where yields were 50 bu/A less under irrigation versus non-irrigated.
In order to track the disease this coming season, we would like to hear from you. If you observe tar spot in counties that have not been confirmed to date, please send a picture of diseased leaves to us directly at the email [email protected] or the Twitter handle @MartinChilvers1.
For more information on tar spot and other diseases see www.cropprotectionetwork.org