Be on the lookout for tar spot of corn again this season

LANSING, Mich. — 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 United States, 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 in 2016 late in the season near Lake Michigan in Allegan County. In 2017, the disease was again observed in Allegan County, but earlier in the season and with greater severity. In 2018, the Chilvers Lab, with assistance from farmers and private ag consultants, confirmed tar spot in 27 counties across western and central Michigan, with multiple reports of 50 bushel-per-acre 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-inch) black spots are a fungal fruiting structure that are capable of releasing spores to infect new corn plants.

According to Marin Chilvers, associate professor of field crop pathology with Michigan State University’s Department of Plant, Soil, and Microbial Sciences, the fungus Phyllachora maydis is the only pathogen associated with this disease that has been confirmed in the U.S.

In fields with severe disease, the corn will often appear frosted, will senesce early, and may lodge. In addition to the impact on grain yield and test weight, Chilvers has 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 quality by reducing the digestible component and energy value of the feed

 There are, however, no associated mycotoxins with this disease.

One challenging aspect of tar spot is the rapid progression of 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, Chilvers said none of our material has been screened and bred for this disease. His team assessed the MSU corn performance trial in Allegan (for details, see www.varietytrials.msu.edu/corn) and found no hybrids were immune; however, there were differences, with some hybrids being more resistant that others.

With every 10 percent increase in tar spot severity they noted a 5 bushel-per-acre 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 updates. With little information, it is best to spread risk by planting a few different hybrids.

Researchers have some early indications that fungicides will help in managing this disease, but 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 management.

At this point farmers will have to see what weather conditions and disease pressure are like in 2019. The pathogen is capable of overwintering on infested residue. 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 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. Chilvers’ team is 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. Leaf wetness is also a major driver of disease, and there have been incidences of much greater disease under pivots compared to areas not irrigated. Researchers have heard of at least two cases where yields were 50 bushels fewer under irrigation versus non-irrigated acres.

In order to track the disease this coming season, Chilvers would like to hear from farmers. If you observe tar spot in counties that have not been confirmed to date, email a picture of diseased leaves to chilvers@msu.edu or tweet it to Twitter user @MartinChilvers1

(This research was funded by the Corn Marketing Program of Michigan.)