Tar spot: A new challenge for corn silage production in the Midwest – Dr. Damon Smith, University of Wisconsin-Madison

By Dr. Damon Smith, University of Wisconsin-Madison assistant professor and extension specialist 
The 2018 corn growing season has brought a new challenge for farmers in the upper Midwest.  Tar spot has hit heavy and hard.  Tar spot is a relatively new disease of corn.  It is caused by a fungus called Phyllachora maydis and was first identified in Indiana in 2015.  Since then, tar spot has been found in other Midwest states, including Wisconsin, since 2016.  Until 2018, tar spot had been considered a benign issue causing just cosmetic damage.  This season, tar spot is clearly more than a cosmetic issue.

Tar spot favors cool conditions (60 to 70 degrees F) and high relative humidity (averages above 75 percent).  When Phyllachora maydis occurs by itself, damage is cosmetic in nature and does not result in significant yield reductions.  In Latin America, where Phyllachora maydis can form complexes with Monographella maydis and/or Coniothyrium phyllachorae, “fisheye” symptoms, severe necrosis, and early dry down can be observed.  In Latin America, it has been reported that, when in complex, damage from tar spot can result in as much as a 30-percent grain yield loss.  The main method to manage tar spot in Latin America is to use resistant hybrids.  Little is known about effective fungicides, especially fungicides that might be available in the U.S.

We are not sure what the impact on yield, starch, and neutral detergent fiber digestibility (NDFD) will be.  This is a hard question to answer and will depend on when the epidemic started in a field and how severe symptoms will be.  It is having a major effect on corn dry down, especially when other diseases, like gray leaf spot or northern corn leaf blight, are present.  We have observed fields quickly passing ideal moisture levels, which will result in difficulty packing the bunker and facilitating the fermentation process.

Mycotoxins have not been implicated in the organisms reported to cause tar spot in Latin America.  However, that doesn’t mean other organisms that produce mycotoxins might not be present in silage.  While we don’t expect issues with mycotoxins, I would encourage you to continue to test feed to be sure mycotoxins are not present.

Scout fields this season to make timely chopping decisions.  One observation we have made is fields affected by early epidemics of tar spot, and other foliar diseases, have significant stalk issues because the stalks are being scavenged for nutrients to finish filling the ear.  This leaves stalk integrity highly questionable in some fields, with various stalk rots moving in.  Check moisture and stalk integrity and plan to chop fields as quickly as possible to try to get the best pack possible on the bunker.  Finally, be sure to scout hybrids this season.  We know tar spot severity has a significant correlation with early dry down of corn hybrids.  Choose hybrids that are still green this season for the 2019 season.