Although tar spot has been observed to some degree in Illinois since 2015, 2018 was the year the disease made its mark, in terms of both incidence and severity. Some farmers in northern Illinois, especially, can attest to the damage the disease caused last season.
Tar spot is a fungal disease caused by Phyllachora maydis. Just as the name suggests, the disease appears as black speckles, called stromata, on leaves and sometimes husks. It’s important to note that these spots are raised, but they will not rub off when touched.
In some cases, necrotic tissue surrounds the stromata. This is referred to as a fisheye lesion. There are several explanations for this observation; however, more research is underway to better understand the phenomenon.
As with any disease, certain conditions favor disease development. For tar spot, moisture is critical for the stromata to release spores, and splashing from raindrops further spread the fungus. In fact, research from South America determined spores had the capability to travel 80 yards.
As if that isn’t enough, stromata may form within 12 to 15 days of infecting a healthy plant. Reflecting on weather conditions last year, the high humidity made an ideal environment for the disease.
In terms of yield impact, timing of infection, susceptibility of the hybrid and environmental conditions all come in to play. Generally, an earlier infection followed by good conditions for disease development will result in earlier plant death and the biggest yield hits.
Early plant death also coincides with poor stalk quality and low test weight. Last year, some claimed 30 bushels per acre lost on account of tar spot. Like gray leaf spot and northern corn leaf blight, yield losses are less dramatic if infection occurs later in the season.
Management strategies continue to be refined, but consider these options if you suspect tar spot is wreaking havoc on your fields.
1. Reduce residue in impacted fields. Because the inoculum overwinters in residue, encouraging the breakdown of residue via tillage may be an effective strategy for reducing disease incidence the following year.
2. Alternate crop choices. Consider rotating to soybeans in order to reduce the presence of the inoculum for the next season.
3. Explore fungicide options. Work with your agronomist to identify fungicides with a label to control tar spot.
4. Invest in high-quality hybrids. Select hybrids with good scores against tar spot, if ratings are available.
For more information, visit https://crop-protection-network.s3.amazonaws.com/publications/tar-spot-filename-2019-03-25-120313.pdf.