by Deborah Jeanne Sergeant

Though snow may cover your cornfields, now is a good time to learn about the latest in corn diseases. Gary C. Bergstrom, professor at Cornell’s School of Integrative Plant Science Plant Pathology and Plant-Microbe Biology, presented “Corn Disease Updates: Identification and Management” at the recent virtual Corn Congress.

Bergstrom looked back at the weather for the 2020 planting season. The May to July weather in the Northeast offered “a good slug of rain,” he said, during the months of planting and getting plants established. “Then things really dried out. We had reduced precipitation and slightly warmer than average temperatures.”

Northeastern farmers received heavy rainfall events in July, when it’s normally drier, followed by warmer, drier weather during the second half of the maturation and dry down period.

Although some farmers saw corn diseases like seedling blights, northern leaf blight, gray leaf spot and stalk rot, “diseases were not as big of a problem,” Bergstrom said. “Toward the end of the season, we had some local hotspots with northern leaf blight and gray leaf blight, but overall, it was less severe than normal.

“Bacterial leaf spot – we had a few outbreaks of that. Diseases were fairly low, from what I’m hearing back from grain producers and buyers,” he added.

Corn fungal leaf blights like northern leaf blight, gray leaf rot, northern corn leaf spot and anthracnose “continue to be the main fungal blights we see, more prominent in river valley areas,” Bergstrom said.

Corn fungal ear rot and corn stalk rot varieties were a little less common in 2020 too, depending upon the weather. “We have seen a bit in very stressed situations,” Bergstrom said. “We’ve also seen a bit of anthracnose stalk rot. In areas with massive rains, we saw a little bacterial stalk rot.”

Tar spot is what Bergstrom calls “the new corn enemy at our gates.” Latin name Phyllachora maydis, it’s the only known Phyllachora species to infect corn.

“It is a very wide genus of fungus,” Bergstrom said. “We find species on many of our forage and wild grasses. We believe it’s very specific to corn in its host range but we have a lot to learn about it.”

It produces raised stromata on foliage and husks. The sooty spots vary from small, oval-shaped pinheads to more enlongated shapes. There may or may not be necrotic areas around the stromata.

“As we talk about spread, there have been questions as to whether it’s seedborne, but it could be transmitted with leaf fragments associated with seed,” Bergstrom said. “That mechanism is a real possibility.”

When corn season ends, infected debris allow the fungus to overwinter and start the life cycle all over once moisture and warm temperatures return.

“It can be confused with – especially on stubble – the black rust stage of common rust,” Bergstrom said.

Historically, tar spot plagued the upper Midwest; however, “it quickly established in Indiana and has not gone away.” The disease has spread to 10 states and Ontario, Canada. “I fully expect to see this in the next season or two,” Bergstrom said. “I had honestly expected to see it [in New York] this year.” He foresees that the Western New York/Finger Lakes region will be compatible for the fungus.

Environment appears to be a strong driver for tar spot. Photos from the University of Michigan of a fungicide-treated cornfield indicated that areas affected by tar spot were irrigated areas of the field – not the drier corner areas.

Currently, Cornell is undertaking projects to help farmers better understand and prepare for tar spot. Researchers are developing a risk model, studying the impacts of cultural practices on the disease, identifying resistance in U.S. inbred lines and learning about pathogen biology, including overwintering, diversity and host range.

“There’s a great consortium with plant breeders and researchers,” Bergstrom said. “We still have an awful lot to learn.”

Bergstrom also shared results from a tar spot trial by Purdue University, Michigan State University, Univeristy of Illinois and University of Wisconsin-Madison which looked at uniform fungicide efficacy. Only five of the 12 fungicides evaluated are availble for use in New York: Aproach Prima 2.345C, Delaro 325SC, Headline AMP 1.68SC, Quilt Xcel 2.2SE and Trivapro 2.21SE. All the fungicides tested reduced tar spot compared with the control areas when applied at VT/R1.

Bergstrom said that future work will look at trial data from other areas to see if any consistent trends emerge. “We’re starting to see some differences between these products,” Bergstrom said.

He also noted using a moderately resistant hybrid “can significantly reduce tar spot over a succeptible variety.”

“Once here, it is here to stay and it’s going to spread,” Bergstrom said. “It’s likely to be an episodic disease similar to white mold or fusarium head blight. Be aware of the disease and manage accordingly. Scout your fields and pay attention to weather and reports to determine if within-season management is required.”

He encouraged farmers who see tar spot to photograph it, take samples and contact their local Extension office or the Cornell Field Crops Pathology Lab. “We really want to track this thing down,” Bergstrom said.