Emily Unglesbee

The search for tar spot disease-management strategies continues.

Corn leaves that look as though they’ve traveled down a freshly oiled country road may be exhibiting a disease called tar spot.

Tar spot on corn may have arrived in the U.S. in 2015, but 2018 will go down as the disease’s first year in the major leagues.

The disease started catching eyes in August and September, when its telltale black spots began surfacing on corn leaves and husks in Illinois, Indiana, 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.

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

The past summer provided plentiful opportunities to rate the disease on different hybrids, Kleczewski says. However, since most plots weren’t specifically designed to test for tar spot, it was difficult to isolate it as the only contributor to yield loss and plant damage.

“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 says. “And, those infections were fairly independent of other diseases, so we were able to rate it in isolation.”

VARIETY MATTERS. 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 adds.

“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 explains. University scientists will make the results public on the Crop Protection Network at cropprotectionnetwork.org.

Growers should also talk to their seed company representatives to see what they have learned from this season. “If you suffered from severe losses and heavy infestations this summer, it might be a good idea go looking for these tolerant hybrids,” Kleczewski suggests.

MANAGE RESIDUE. Scientists think 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.

“It’s probably overwintering on residue, but there is potential for it to have some alternative hosts, like weeds,” Kleczewski says.

This makes it hard for scientists 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. It could be more of a regional disease that needs to be managed in a regional way,” he adds.

FUNGICIDE VARIABILITY. “Persistent wet weather that moves in as plants develop seems to favor the disease,” Kleczewski explains. “We saw that in 2015 and again this year.”

Fungicide efficacy against the disease was highly variable. Scientists found that the timing of the application mattered more than the product in many fields.

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

Source: Progressive Farmer, January 2019