Why think about crop disease pressure in March? Last season’s disease pressure could impact this season, says Angie Peltier, a University of Illinois Extension educator, adding that many pathogens survive in the soil and residue from previously infected crops. Add the right conditions — like cool, wet soils in the spring — and you have a perfect environment for disease.

Identifying diseases in-season is ideal compared to a postmortem diagnosis, says Stephanie Porter, a sales agronomist for Burrus Hybrids. “It’s so much easier to tell when it’s green,” she notes. “It’s so hard to figure out what’s going on after harvest.”

Potential disease pressure may also impact management tools like planting different hybrid maturities, selecting resistant varieties and adjusting planting dates.

Peltier and Porter share five corn and soybean offenders in 2016 and what to look for this season:

1. Diplodia ear rot. The fungus that causes diplodia ear rot lingers on or in the soil on corn residue until the right conditions occur. For diplodia, conditions for spore release and infection would be a pattern of dry weather followed by wet conditions at or right after silking, Peltier explains. Scout for corn ears with bleached leaves, and then pull back the leaves and look for dull, grayish-brown kernels. Finally, Peltier recommends cracking the ear open to look for pycnidia, or black fruiting structures.

While diplodia ear rot doesn’t cause mycotoxin issues, Peltier says lower test weights and kernel breakage can be a challenge. Broken kernel reports in 2016 ranged from 0 to 50% across the state.

Fields with diplodia ear rot issues in 2016 may have challenges in 2017. “If infected, everything going out of the combine will hang out in the field,” Peltier notes. Management options include rotating to soybeans, as corn is the only host, and selecting genetically resistant hybrids.

“No hybrid is immune, but plan now by checking diplodia disease scores on your hybrids,” Porter says. Planting different corn maturities and spacing out planting dates will help reduce the risk of diplodia ear rot by spreading silking dates, she notes.

TELLTALE SIGN:  Angie Peltier says corn ears with bleached leaves are the first indication of diplodia ear rot. (Source: Angie Peltier, University of Illinois)

2. Tar spot. Tar spot is a new corn disease to the Corn Belt, Peltier notes. It was confirmed in Illinois and Indiana in 2015 and verified in Wisconsin, Michigan, Florida and Iowa in 2016. Tar spot's black bumps may be confused with common or southern rust pustules, making accurate diagnosis difficult. “If you run your finger across the pustule and it goes away, it’s rust,” Peltier explains. “If it stays, it could be tar spot.”

The good news is that confirmed Midwest tar spot cases lack the second fungal pathogen and fish-eye lesions found in Central and South America, which have been known to reduce yields up to 70%.

3. Bacterial leaf streak. Peltier says bacterial leaf streak is another new corn disease confirmed in nine states in 2016, including Illinois. Symptoms may appear on lower leaves in the early V stages or in the upper canopy after silking. The narrow yellow, orange, brown or tan lesions appear between and/or along the blade’s veins or midrib.

Bacterial leaf streak’s slightly wavy lesions may be easily confused with gray leaf spot’s straight lesions. When in doubt, Peltier recommends holding the infected blade up to the sun and looking for “bright yellow wavy halos that extend from the lesions” when backlit, indicating bacterial leaf streak. University researchers are still studying the pathogen that causes bacterial leaf streak and researching potential management strategies.

THE INSECT FACTOR:  Did you know aboveground insects like earworm can spread ear rot pathogens? Stephanie Porter, a sales agronomist for Burrus Hybrids, recommends selecting hybrids with a good insect trait package to protect corn plants from aboveground insects and help reduce additional spread of ear rot pathogens. 

4. Pod and stem blight. “I think this should be on our radar,” Peltier notes. Black, bumpy dots appear in linear rows on the outside of soybean stems, she explains, which helps distinguish pod and stem blight from charcoal rot. Pod and stem blight may lead to cracked, withered and moldy seeds, and ultimately yield loss.

“I’ve been warning farmers about the Diaporthe complex,” Porter notes. Phomopsis pod and stem blight can be associated with stem canker, she explains. “I had a few field calls last year where I found stem canker, and it can be a nasty yield robber of soybeans. The key to this disease is to have resistant-soybean varieties.”

5. Sudden Death Syndrome. “We’re seeing more and more of this particular disease,” Peltier says. The pathogen that causes SDS infects germinating soybean seedlings, but symptoms may not appear until well into the season. Rain events during podding move pathogen-produced toxins up into the leaves, resulting in yellowing and brown discoloration between the veins. Yield loss occurs when leaflets shrivel and prematurely fall, leaving only petioles behind.

Selecting SDS-resistant varieties is the most effective management tool, Peltier notes. Farmers can find university-tested variety options on the University of Illinois Extension website.

“ILeVO is insurance against this disease on your soybeans,” Porter says. She adds that soybeans planted earlier in the season may be exposed to the cool, wet conditions ideal for SDS infection. 

For more crop disease information and identification tips, visit cropprotectionnetwork.org.