New Corn Disease Complications

Tar Spot: What You Need to Know to Manage the Disease in 2019

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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The spots are tiny, but clustered together in severe infestations, tar spot in corn can be visible from the air -- note the dark bronze leaves in this cornfield. (Photo courtesy Martin Chilvers, Michigan State University)

ROCKVILLE, Md. (DTN) -- Tar spot on corn may have arrived in the U.S. in 2015, but 2018 will definitely go down as the disease's first year in the major leagues -- and it definitely earned Rookie Pathogen of the Year.

The disease started catching eyes in August and September, when its telltale black spots began surfacing on corn leaves and husks in Indiana, Illinois, Iowa, Michigan and Wisconsin. As farmers, agronomists and plant pathologists raced to scout corn, the number of infected fields began to rise, as did growers' concerns.

While plant standability and yield loss remain the biggest potential problems from tar spot right now, some scientists are already thinking ahead to next year.

Hybrid selection, residue management and fungicides are all possible control options in 2019, although data on all three is still pretty thin, said Nathan Kleczewski, a plant pathologist at the University of Illinois.

HYBRID SELECTION

The past summer provided plentiful opportunities to rate the disease on different hybrids, Kleczewski said. However, no one designed plots specifically to test for tar spot, so in many fields, it was difficult to isolate it as the only contributor to yield loss and plant damage, he said.

"Fortunately, in university trials in Wisconsin and Illinois at least, each of us had at least one site that contained severe enough tar spot to rate," he said. "And those infections were fairly independent of other diseases, so we were able to rate it in isolation."

There was a "pretty dramatic difference in the amount of disease" among different corn hybrids, so natural tolerance to the disease is definitely out there, Kleczewski said.

"What we are seeing across the board, is that, regardless of company or maturity or stay-green characteristics, all the companies have a lot of susceptible hybrids, but also within each are a handful of hybrids that are more tolerant," he said. University scientists are crunching their hybrid data now and will make the results public on the Crop Protection Network later this fall, he said.

Here is the link to the Crop Protection Network:

https://cropprotectionnetwork.org/…

Growers should also talk to their seed company representatives to see what they have learned from this season, he added.

"If you suffered from severe losses and heavy infestations this summer, it might be a good idea go looking for these tolerant hybrids," Kleczewski concluded.

MYCOTOXINS AND YIELD LOSSES

There are no known mycotoxins associated with tar spot yet, but infected fields are likely to have lodging and other ear rots, which heightens the risk for them, Kleczewski said. He urged growers with severe infestations to get their grain tested, just as they would for other diseases.

Yield losses also remain a question. So far, only one tar spot pathogen, Phyllachora maydis, has been confirmed in the U.S.

In Latin America, that pathogen can join forces with another one, Monographella maydis, to create the tar spot complex, which can cause yield losses of up to 50%. So far, M. maydis has not yet been found in the U.S., and scientists and growers aren't sure how the P. maydis has affected yields here yet.

RESIDUE MANAGEMENT

Scientists are pretty certain that tar spot is capable of overwintering, given its appearance in the Midwest for multiple years, far from the disease's origins in Mexico and South and Central America.

However, they haven't nailed down exactly where the pathogen beds down for the winter, Kleczewski said.

"It's probably overwintering on residue, but there is potential for it to have some alternative hosts, like weeds," he said.

This makes it hard for scientists like Kleczewski to advise growers on how to manage infested fields for next year.

"At this point, we're not too sure of the overall effects of residue management and rotation," he said. "It could be more of a regional disease that needs to be managed in a regional way."

FUNGICIDE USE

Some hints on the disease's biology did emerge this year, however.

"Persistent wet weather that moves in as plants develop seems to favor the disease," Kleczewski said. "We saw that in 2015 and again this year."

Moreover, fungicide efficacy against the disease was highly variable, and scientists found that the timing of the application mattered more than the product in many fields, he said.

"We might have some information next year on the efficacy of fungicides, but timing of applications will be the big question," he said. "And until we understand what causes tar spot spores to release and land on plants, that will remain a question."

For more information, see this Purdue publication on tar spot:

https://www.extension.purdue.edu/…

Or watch this video from Michigan State: https://www.youtube.com/…

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

You can also find a recent article by Kleczewski on some common myths about tar spot in corn here: http://cropdisease.cropsciences.illinois.edu/….

Follow her on Twitter @Emily_Unglesbee

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Emily Unglesbee