Growers should be on alert for presence of disease in corn, soybean fields

Badger CropDoc
During July, crop experts observed and received reports of some fields with Phytophthora root and stem rot of soybeans.

While its been a quiet growing season in regard to the presence of disease in corn and soybean fields, University of Wisconsin and Division of Extension officials say that could change as the month progresses.

The Soybean Situation

According to the Badger CropDoc Wisconsin Soybean and Corn Disease update for August 2, 2021, officials observed and received reports of some fields with Phytophthora root and stem rot of soybean.

Damon Smith, Associate Professor and Extension Specialist and Field Crops Pathologist says he has fielded many questions on why is happening and whether growers are seeing changes in the races of the Phytophthora organism in Wisconsin.

"My short answer is probably not. However, we have found a new species of Phytophthora in some fields that can affect soybean," Smith said in his Badger CropDoc blog.

The new species, Phytophthora sansomeana, can be found in mixed infections with P. sojae.

"Thus, even if we deployed the proper Rps resistances genes in our varieties, this “new” organisms might be causing some of the damage we observed this year," he said.

Smith says they are also are seeing fewer available soybean varieties with Rps 1K form of resistance. This form of resistance should be effective on about 99% of the fields in Wisconsin.

Instead, Smith says they are seeing varieties deployed that have the Rps 1c, Rps 1a, or no Rps gene indicated. Rps 1C is effective on about 75% of the acres in Wisconsin.

"I don’t think this is necessarily an issue where we have seen race shifts in P. sojae, but a combination of issues where perhaps we aren’t deploying the correct resistance genes and we might have a new species of Phytophthora adding to the mix," he said.

Smith noted that seed treatments can also be used to manage Phytophthora root and stem rot. 

What’s up with White Mold?

For most of the state of Wisconsin, Smith observes that growers are through the critical bloom time for infection by the white mold fungus.

"It is now really too late to make a fungicide application that is economically viable," Smith said.

He recommends scouting fields through August to help determine what worked, what didn’t, and to figure out an operation's harvest order.

"Remember, a great way to move the white mold fungus around is by contaminated combines at harvest," he pointed out. "Start harvesting fields with no or low white mold incidence and work your way to those fields that look worse."

Smith says that farmers should also consider cleaning combines between fields to limit movement of the fungal survival structures (a.k.a apothecia) from one field to the next.

Look out for SDS

Smith says that now is a good time to be scouting for sudden death syndrome (SDS) in soybeans.

"I’m not sure we will have a bunch of SDS this year in Wisconsin, but we will see pockets for sure," he said. "Knowing where you see it and what you did in 2021 can help with making variety and seed treatment decisions for 2022 and beyond."

He notes that growers do have reasonable partial resistance to SDS in many commercial varieties.

"Start here and choose the most resistant variety that fits your environment. Then consider layering a seed treatment (either Saltro or ILeVO) for improved management of SDS.

Crop disease scouting excursions have confirmed that tar spot is present in at least 5 counties so far in Wisconsin.

The Corn Situation

Corn in Wisconsin has been reasonably free of disease up to this point this season. However, Smith says he and his colleagues have noted a few foliar diseases beginning to pop up.

To date they have observed gray leaf spot (GLS) becoming easy to find in most fields, while northern corn leaf blight (NCLB) is starting to show up in a handful of fields they visited.

"We are also paying close attention to the tar spot and southern rust situations," he said.

How bad is tar spot?

According to Smith, the Tarspotter app has been running at moderate to high risk of tar spot increase over the last couple of weeks in Wisconsin.

"Our scouting has confirmed that tar spot is present in at least 5 counties so far in Wisconsin. All but Grant County show tar spot to be easy to find, but it is present in the lower canopy at low severity," Smith said. "In Grant County, we had to hunt a long time to find 2 spots in a research field on a known susceptible."

Smith says that these observations align with Tarspotter as it indicated just moderate risk in the southwest quadrant of Wisconsin, with high risk from south central to the north.

Those planning on spraying a fungicide to manage tar spot should do it soon, prior to the R3/R4 growth stage.

"The goal here is to protect the leaves from the ear leaf up from continued increase by the fungus," he said. 

For more information on the latest crop diseases visit https://bit.ly/3rRsJcJ