Yield Impact of Tar Spot
- 2018 was the first time that corn yield reductions associated with tar spot were documented in the U.S.
- University corn hybrid trials conducted in 2018 suggested potential yield losses of up to 39 bu/acre under heavy infestations (Telenko et al., 2019).
- Severe tar spot infestations have been associated with reduced stalk quality. If foliar symptoms are present, monitor stalk quality carefully to determine harvest timing.
- There is no evidence that tar spot causes ear rot or produces harmful mycotoxins (Kleczewski, 2018).
Differences in Hybrid Response
- Observations in hybrid trials indicate that hybrids differ in susceptibility to tar spot (Kleczewski and Smith, 2018).
- Longer maturity hybrids for a given location have been shown to have a greater risk of yield loss from tar spot than shorter maturity hybrids (Telenko et al., 2019).
- Genetic resistance to tar spot should be the number one consideration when seeking to manage this disease, as it appears to have a greater impact on symptoms and yield loss than either cultural or chemical management practices.
- Several foliar fungicides are labeled for control of tar spot in corn (Table 1).
- Field research on tar spot has been limited so far, but has shown that fungicides can reduce tar spot symptoms and help protect yield.
- Specific management recommendations for fungicides in the Midwestern U.S. are still being developed.
Table 1. Efficacy of fungicides labeled for tar spot in corn (Wise, 2020).
Fungicide application timing is extremely important and needs to be made near the onset of the tar spot symptoms. Efficacy ratings based on limited site locations from 2018 and 2019. A 2(ee) label is available for several fungicides for control of tar spot, however, efficacy data are limited. Check 2(ee) labels carefully, as not all products have 2(ee) labels in all states.
- Research suggests that tar spot may be challenging to control with a single fungicide application due to its rapid reinfection cycle, particularly in irrigated corn.
Agronomic Practices to Manage Tar Spot
- The pathogen that causes tar spot overwinters in corn residue. How the amount of residue on a field’s soil surface affects disease severity the following year is unknown.
- Observations so far suggest that rotation and tillage probably have little effect on tar spot severity.
- Duration of leaf surface wetness appears to be a key factor in the development and spread of tar spot. Farmers with irrigated corn in areas affected by tar spot have experimented with irrigating at night to reduce the duration of leaf wetness.