Tar spot of corn does not result in a severe outbreak every year. However, during severe outbreaks, yield loss estimates have ranged from 20 to 60 bushels/acre across north central US corn fields.

We have not experienced such an outbreak in Missouri to date. However, we should not get complacent. Three years after tar spot was confirmed in Indiana and Illinois, the first severe outbreaks were observed. Tar spot has been observed in Missouri since at least 2019, so it seems reasonable to speculate that we have enough pathogen present in northern Missouri for a severe outbreak to occur under favorable environmental conditions.

Here are 3 thoughts on tar spot in Missouri as we enter 2023.

1. Know where tar spot is being reported during the season.

Figure 1 As of 2022, tar spot has been confirmed in gray-shaded counties of Missouri. Image from corn.ipmpipe.org/tarspot/

Fungicide applications near the onset of disease are recommended during severe outbreaks. Keep yourself updated on where the disease is being observed in-season and scout fields routinely. The Corn IPM Pipe website has a county-level map that can be used to track tar spot during the season.

Tar spot showed up late in Missouri during 2022 but gained ground. The state had 9 counties with confirmed incidences as of 2021, mostly restricted to the northeast. The number grew to 20 counties scattered across the north and central portions of the state in 2022 (Figure 1).

The fungal pathogen that causes tar spot, Phyllacora maydis, can survive Missouri winters. Once the disease is confirmed in a region we can presume the pathogen is there to stay and plan management accordingly.

If you observe tar spot in-season, please report it. Reported incidences are confirmed by myself or MU Plant Diagnostic Clinic Director, Dr. Peng Tian, before appearing on the Corn IPM Pipe map.

If you are unsure whether you have tar spot, please submit a physical or digital sample to Dr. Tian at the MU Plant Diagnostic Clinic for confirmation.

2. Be familiar with symptoms that can be mistaken for tar spot.

The black lesions (stromata) indicative of tar spot (Figure 2) cannot be scraped off leaf surfaces. These lesions can resemble insect frass (or poop). This is especially true when tar spot symptoms first appear and there are few lesions. Look for indicators of insect damage and try to remove the black spot. Insect frass can be removed, although sometimes it requires wetting the leaf. Figure 3 was taken of a corn leaf in Marion County that had insect frass and early onset of tar spot.

Figure 2 Tar spot disease symptoms on a corn leaf. The black bumps (stromata) cannot be scraped off.

Figure 3 Corn leaf with insect damage and frass, highlighted by the gold circle and tar spot lesions beginning to appear. Two of the latter are marked by the 2 white arrows.

Physoderma brown spot is another corn disease in which dark purple to blackish colored lesions form in the midvein. These lesions cannot be scraped off; however, lighter brown lesions will also form on the leaf.

Other symptoms mistaken for tar spot last season included the black residue from foliar applications of zinc and the smokey black lesions on dead corn tissue at harvest. The latter is caused by saprophytic fungi that degrade the dead corn tissue.

If you question whether you have tar spot, please submit a physical or digital sample to Dr. Tian at the MU Plant Diagnostic Clinic for confirmation.

3. Keep up-to-date on the latest with tar spot management

This disease is new enough that there are still many questions. Colleagues to our north and east, where tar spot outbreaks occurred initially, are addressing many of these questions.

What we know:

• In most years a single pass fungicide application at VT/R1 can reduce disease severity and protected yield.
• There is evidence that when tar spot shows up early (vegetative stage) and severe outbreaks occur a two-pass program may offer added yield protection.
• When disease shows up later than the R3 growth stage, such as in 2022, fungicide applications have not influenced yield.
• Hybrids that are more tolerant to tar spot exist.

Additional resources:

The Crop Protection Network publishes a fungicide efficacy chart annually that includes ratings for tar spot control.

The Tarspotter App is a free mobile app that can assist with site-specific fungicide application timing. However, models used in the app assume the pathogen is present. Therefore, this app is likely most useful in northeast and northwest Missouri, for now.

For more information on tar spot of corn, visit the Crop Protection Network.