by Deborah Jeanne Sergeant
While tar spot on corn is still rare in New York, the yield-sapping disease has begun to infiltrate the Empire State. Dr. Gary Bergstrom, plant pathologist and plant-microbe biologist with Cornell University, presented “Tar Spot is Here!” at the recent virtual Corn Congress.
“We run into ear mold problems nearly every year in some form, and we saw more with the unique weather patterns this past year,” Bergstrom said. “In the Finger Lakes and Southern Tier, we saw rather intense development with Gibberella ear rot … We’re seeing DON (Deoxynivalenol) levels we haven’t seen since 2018.”
Bergstrom said the 2022 Cornell Field Crops Guide added a few new brands to its fungicide table: Xyway 3D and Xyway LFR, both soil-applied fungicides added at planting, and Veltyma and Revytek, foliar fungicides. “We’ve seen some pretty amazing season-long control of gray leaf spot and northern leaf blight,” Bergstrom said.
He said the presence of tar spot was confirmed in two well-separated fields in Erie County in October 2021. “It has exploded across the Midwest,” Bergstrom said. “Toward the end of the 2021 growing season, it exploded in Western New York.” (It’s not a new problem, however; tar spot was first described in Central America in 1904.)
Bergstrom shared tar spot information gathered by his colleagues in the Crop Protection Network, which includes Cornell, University of Wisconsin-Madison, Purdue University, Michigan State University and the Ontario Ministry of Agriculture, Food and Rural Affairs.
Tar spot presents with black, shiny fungal marks and stromata. “One of the things remarkable about this disease is that is occurs fairly late during the season, forming during grain development,” Bergstrom said.
Within two weeks, a healthy-appearing field of corn can be completely devastated with all the foliage destroyed by tar spot. This is why vigilant scouting is necessary. The fungal bodies can vary in size. When scouting for tar spot fungal bodies, Bergstrom said to look for “fish-eye” lesions and tiny black spots. Tar spot can also resemble insect frass and common rust. To tell the difference, remember that frass may be scraped or washed off and that common rust pustules appear burst, not as defined circles.
With a lifecycle of two to three weeks, tar spot quickly spreads. Infected debris can start the cycle all over again.
“At this point, we don’t know of any other hosts beyond corn,” Bergstrom said. “This seems to be specific to corn. Under warm, moist conditions, it takes four weeks to produce another generation of spores … Once it’s into the debris in the region, it can be transmitted to some degree in the air.”
Risk factors include moderate temperatures, average relative humidity greater than 75%, an average of seven hours of leaf wetness per night, 10 to 20 foggy days per month and monthly rainfall totaling at least 5.9 inches – “hence the epidemic we saw this year,” he commented.
Many companies, universities and government entities are working on developing hybrids that are resistant to tar spot. “There’s not truly resistant varieties identified, but there are moderate response hybrids available,” he added.
Midwestern states that are more affected are looking at using foliar-applied fungicides. They’ve found the best time to apply them is when the corn is at VT, V6+VT and R2. “A number of these single applications have at least delayed epidemic development,” Bergstrom said. “Even the moderately effective materials, like Tilt, do pretty decent job compared with untreated.”
He encouraged producers to use more than one mode of action, as the research indicated that more than one mode of action works better than just one (for example, using a resistant hybrid with fungicide).
Twelve fungicides are currently labeled in New York State for tar spot control in corn. Of these, most are rated “very good” for their efficacy. “Our arsenals are full and we have good tools to deal with until we identify the best hybrids to fight this epidemic,” Bergstrom said.
He encouraged farmers to download TarSpotter, a free app, to monitor daily risk of tar spot infection before symptoms, which can give them time to control it. In addition, the Crop Protection Network is trapping spores to see if it travels by air.
Bergstrom wants producers to scout their fields; talk about hybrid resistance with their seed providers; apply fungicide, leaving check strips; help monitor areas where tar spot has yet to be confirmed; and to not forget about other diseases, both new and established. “I suspect it’s much more widespread than what’s being reported right now,” he said of tar spot.
Another new-for-New York disease is bacterial leaf streak, discovered in 2021. “We confirmed this in five counties in central New York,” Bergstrom said (Delaware, Herkimer, Jefferson, Lewis and Oneida). “In the early stages, it could be confused with gray leaf spot.” Its symptoms include long, narrow lesions with bright yellow haloes when help up against light.
“The characteristic thing is to cut through a lesion over a bucket of water and you see the bacterial stream coming out,” Bergstrom said.
Bacterial leaf streak is a pathogen that survives season to season in corn debris. “I would expect to find it in excessively high rainfall seasons,” Bergstrom said. “The jury is still out as far as this having any major impact on corn yield … It’s something we want to keep an eye on.”