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Tar Spot and Irrigated Corn: What we Saw in 2018 and Will it Happen Again?

By Bruce MacKellar, Martin Chilvers and Eric Anderson
 
Tar spot, a new fungal pathogen first seen in Michigan in Allegan County in 2016, spread like a wildfire across west Michigan in 2018. The disease, which is prevalent in production areas in Mexico, comes from higher altitude fields in that country. The fungal pathogen apparently is well adapted to our environment here and likely overwinters on infected corn residue. Based on the rapid spread last season, we anticipate there will be plenty of spores to go around in southwest Michigan and northern Indiana in 2019.
 
Levels of resistance to tar spot vary greatly among hybrids. The brown areas were severely injured by tar spot compared with the green strips planted to a different hybrid. 
 
One of the more troubling aspects of the disease is that it really caused yield reductions during 2018 in the irrigated areas of fields, with much less effect on the dry corners. In some cases, dry corners out-yielded irrigated areas with the same hybrid by 30-40 bushels per acre. That is not to say that there was no tar spot in the corners but that the pathogen showed more severity in those areas. In thinking back on 2018, we did not tend to irrigate fields a whole lot in areas where thundershowers were more prevalent. This has us worried that just a couple of irrigation events at the wrong time may have set the stage for rapid disease development.
 
Unfortunately, we were not able to conduct a lot of research and field observations on the situation because we had no idea that the disease would have this level of impact. We have been looking at a few fields that showed this level of reduction, tracing back irrigation events, cloudiness, rainfall, temperature, wind, relative humidity, leaf wetness values from the closest Michigan State University Enviroweather station and planting dates for the hybrids planted. This hopefully will give us a place to start in understanding what conditions in the field triggered severe infection last season.
 
Martin Chilvers, MSU Extension field crops pathologist, thinks that trying to manage irrigation to reduce tar spot is going to be a tough challenge, and success will likely depend on several weather factors, especially dew point. He believes limiting leaf wetness is going to be critical. Considering this, irrigating late in the day would most likely extend the time leaves remain wet and enhance tar spot development. If we have conditions where the leaves are already going to be wet, starting the pivot at, for example, 1 a.m., running it until the sun is up, then shutting the system off to let the canopy dry out may be the best option. How quickly leaf drying would occur depends on cloud cover, relative humidity and wind speed. We are going to be working on gathering leaf wetness data on both irrigated and dry corner corn plants this summer to help provide Chilvers’ crew with a better insight on how center pivot irrigation changes the duration of leaf wetness under field conditions.
 
Managing irrigation to minimize the duration of leaf wetness is not how we have thought about running center pivots in the past. Depending upon water demand and the capacity of a system to cover acres, you may not be able to do it in the future and maintain yield potential. But some aspects of irrigation triggered disease response in 2018.
 
Managing for tar spot
 
Unfortunately, the disease is so new to the U.S. that there has not been a lot of research compiled to determine the best ways of controlling the disease. Evaluation of university hybrid performance trials has shown there are substantial differences among hybrids in commercial corn. Initial recommendations would be to obtain as much information as possible from the companies about hybrid performance in areas with tar spot issues in 2018. Fungicide applications will likely increase on commercial corn grown in areas where the disease is prevalent. The thought is that products that provide both a protectant and curative function will likely be the most widely used in 2019. Early results showed that a DMI (triazole) fungicide application with or without a QoI (strobilurin) pre-mix fungicide resulted in the lowest disease severity.
 
The timing and duration of tar spot sporulation makes the best timing of fungicide application a bit harder to know. Chilvers suggests that if you were planning on making a single application of a fungicide, applying around tasseling would probably provide the best window for application. If two applications are being considered, the first application most likely would be applied during the later vegetative growth stages with the second applied closer to R2. As the seed industry completes more screening for hybrid resistance to tar spot and incorporates these genes into their breeding lines, we hopefully will be able to reduce our reliance on fungicides to control this disease.