Tar spot is making in-roads in the Northeast. Gary Bergstrom, plant pathologist with Cornell University, presented “Let’s Talk about Field Crop Diseases” as part of the Southwest New York Crop Webinar recently.

A newcomer to the Northeast, tar spot is a fungal disease. “It’s not new in the world, but new in the U.S.,” Bergstrom said.

There have been confirmed sightings in Chautauqua, Livingston and Yates counties in New York and in northern Pennsylvania. Tar spot has been well-known in South America and parts of Mexico for 100 years.

Six or seven years ago, it made a jump from its host range into some isolated corn fields in Indiana in Illinois, Bergstrom said. It’s also been seen in Florida, Georgia and southern Ontario.

“We expect this expansion to continue, and especially in growing seasons where we have particularly wet conditions during the grain filling period,” Bergstrom said.

Tar spot is known to only affect maize. It produces raised stromata on leaves and husks, appearing as black oval “pinheads.” It may or may not have necrotic areas around the stromata. The stromata exude a mucous-like substance – “orange goo.” The disease is thought to not be seedborne but transmitted through leaf fragments associated with seed. The initial black markings of tar spot can resemble feces spots left behind by insects; however, Bergstrom said, the latter may be easily wiped away with a rag. Tar spot will not.

Tar spot persists in residue. The effects of tar spot “can be rather dramatic,” Bergstrom said, with infection spreading quickly under the right weather conditions. Environment is a strong driver of tar spot disease.

A new generation of fungal spores can develop every two weeks. Bergstrom noted that a field that received fungicide at silking (R1) showed tar spot only in the areas irrigated.

“I want to really hit home how important leaf wetness is to this disease,” Bergstrom said.

Tar spot spotted in NY and PA

Farmers should look for necrotic areas around the spots, or stromata. Photo courtesy of Gary Bergstrom

Wind and water splashes spread the spores. Cool temperatures and high relative humidity, along with leaf wetness for at least seven hours, create the idea environment for spore germination. Spores may blow in from a neighboring field and the cycle can repeat throughout the growing season to spread infection.

Since tar spot overwinters, any affected residue left in the field will manifest the following year under the right conditions.

For those producing silage, “you’re going to remove and ferment a large part of that potential inoculum,” Bergstrom said, “but there’s still some left.” Farmers who plant corn on corn and reduce tillage can expect tar spot to proliferate.

So far, no seed treatment is highly effective against tar spot, but multi-state tar spot fungicide trials in the Midwest and Ontario have found reduction of tar spot severity based on visual assessment.

“All of the fungicides are effective to a varying extent and we’re starting to gather more data now,” Bergstrom said. “Yet we are seeing some materials that are both protective against symptoms and yields.”

Revytek and Veltyma are among the top performers. Bergstrom advised two or more modes of action and that jug mixes tend to do better. Applying between tassel emergence and brown silk stage appears to be the best timing. After the R3 stage, “there’s really no solid evidence that it’s going to get a significant return on your investment,” Bergstrom said.

He foresees tar spot as endemic; however, its severity depends upon the hybrid, its treatment and the weather. He believes that rotation and tillage can help reduce or delay the onset but won’t resolve the problem.

He encouraged farmers to scout their fields and consider reporting via the TarSpotter app, free from the University of Wisconsin.

The mix of weather contributed to the development of “pretty much every disease that I’m used to seeing in New York State by the end of the growing season,” Bergstrom said, “but generally these occurrences were late with relatively minor impact on our yields.”

He added that northern leaf blight leads as the most widespread issue across the region. Northern leaf blight and northern corn leaf spot were widespread in September. The former is typified by light streaks parallel to the leaf’s length; the latter exhibits markings that Bergstrom calls a “string of pearls.”

“I’ve talked with some folks in the hybrid seed industry and they think there’s probably been some slippage in the resistance level,” Bergstrom said.

Although he attributes the spread of these pathogens to lower resistance levels, he doesn’t think that it is responsible for lowering yields.

He also noted that 2022 brought some gray leaf spot and common rust, especially in areas with persistent humidity like river valleys. In stressed corn, some stalk rot was observed, but it “wasn’t very uniform, and obviously this is something that varies with hybrid as well.”

Bergstrom received a few reports of anthracnose top dieback in southwestern New York and a few other places around the state. He also said that mycotoxins on corn, such as Cladosporium, Stenocarpella and Trichoderma, are making appearances but so far appear to only slightly impact corn quality and not quantity. Although unattractive, these mycotoxins produce very low levels of toxins that at this point.

Penicillium ear rot has become problematic in the Northeast. It can contaminate silage and should be of particular concern to those feeding horses and swine – as little as 5 ppm can cause severe illness. Penicillium appears toward the tip of the ear as green-blue powdery mold between the kernels.

Another big problem in New York is Gibberella ear rot. It appears as withered ear tips and pinkish-hued kernels.

“Overall, we had pretty low levels of toxins in silage but we had higher levels in some areas that had higher rain,” Bergstrom said. “Late-planted corn that experienced drought stress followed by heavier rains towards the end of grain filling and then delayed harvest developed stalk rot and mycotoxins in stover. Some whole silage had higher DON than ears.”

Research on applying fungicide in furrows appears to offer season-long control of fungal leaf blights, including Xyway 3D and Xyway LFR, both with active ingredient Flutriafol. “You may need additional spray, but in some cases, this is enough for effective control,” Bergstrom said.

In 2022, he heard only isolated reports of Sclerotinia stem rot (white mold) among soybeans and relatively low severities of most foliar fungal diseases, but increased prevalence of Cercospora leaf blight, which correlates with purple seed stain. Downy mildew also affected soybean quality.

Phytophthora root and stem rot, stem canker, sudden death syndrome and brown stem rot were also present.

Soybean cyst nematode has been identified in 38 New York counties (in low numbers) and in all of the soybean/dry bean production regions of the state.

Bergstrom noted that for more than 20 years, more than 95% of all soybean cyst nematode resistant soybean varieties have included resistance from the PI 88788 breeding line, meaning that “nematodes are becoming resistant to the resistance.”

He recommended for low infestations to choose a high-yielding, SCN-resistant varieties and rotating with non-host crops. For moderate to high infestations and use of resistant varieties, farmers should do an HG-type test and choose a suitable resistant variety or rotate with a non-host crop.

by Deborah Jeanne Sergeant