By DOUG SCHMITZ
Iowa Correspondent

HARLAN, Iowa — Even though Iowa growers started off with some seedling blights and rots this summer, Iowa State University agronomists and farmers saw more crop diseases such as leaf diseases and crown rots, with the continued wet weather.

“Harvest will be fun for them, since they will either have to let dirt work and related equipment run down standing crops, or they will have to harvest and wait for dirt work to allow them to get to some of their fields,” said Clarke McGrath, ISU extension field agronomist and partner program manager at ISU’s Iowa Corn and Soybean Initiative.

The torrential rains that drenched Iowa’s farmland for the past few months not only delayed planting season several times this spring for growers but also introduced many crop diseases to the state’s already stressed cropland.

“We had both corn and soybean stand losses from seedling diseases earlier in the season, which resulted in some replanting where crops could be replanted,” said Jim Fawcett, ISU extension field agronomist in eastern Iowa.

While he said it would have been too early at that point this summer to tell the level of white mold, brown stem rot and Sudden Death Syndrome (SDS), there were reports of white mold mushrooms appearing in some soybean fields, which produce the spores that infect the plants.

“There has been at least one report of Sudden Death showing up already, which is earlier than normal,” he said. “Symptoms usually do not show up until August, but I would expect some problems with all of these.”

Virgil Schmitt, ISU extension field agronomist in southeastern Iowa, said the kinds of plant diseases that occur this time of year depend largely on current weather conditions.

“In corn, we had considerable seedling blight early in the season and are now finding eyespot, common rust and gray leaf spot,” he said. “But eyespot and common rust like cool, wet weather, so this heat should slow them down considerably.”

On the other hand, Schmitt said, gray leaf spot thrives on warm, wet weather such as Iowa had this summer.

“We usually don’t see a lot of gray leaf spot until late July or early August, but you can now find a few lesions on the lower leaves in many fields,” he said. “If we continue with hot, wet weather, hybrids susceptible to gray leaf spot may be at risk.”

In soybeans, Schmitt said his counties had some seedling rots, “a great deal of brown spot and bacterial blight and some phytopthora root rot early in the season.

“Bacterial blight and brown spot like cool, wet weather, so the temperatures now will slow down any future development,” he said. “It is too early to see white mold evidence, but the temperatures at infection time (early flowering) were generally too high.”

While it was too early for the heaviest infection from foliar diseases to surface this summer, McGrath said they would have eventually showed up if temperatures continued to stay warm and humid. He added that he would have expected Goss’ wilt, common rust and gray leaf spot in corn; and Septoria brown spot in soybeans, as well as perhaps Cercospora and frogeye.

In Iowa this year, there were also reports of bacterial, anthracnose, Fusarium and Diplodia stalk rots, according to Alison Robertson, ISU extension plant pathologist.

“Incidence (percent infected plants) ranges from field to field and is likely a function of genetics,” she said. “It’s a good idea to identify what stalk rot is predominant in the field to help with hybrid selection in subsequent years.”

She and fellow ISU plant pathologist Leonor Leandro said the combination of early planting and wet weather at key points during the growing season created the ideal circumstances for SDS to thrive.

“This year, we’ve had particularly severe SDS due to a combination of early planting into cool, wet soils and the continuation of a wet season with a wet July during reproductive stages, that is also thought to favor the disease,” Leandro said. “We also see the disease moving north and west year to year, probably as a result of an increase in the pathogen density in the soils that’s building up over the seasons.”

Robinson said survival of the fungus on corn debris may be another reason why SDS is so widespread this year.

“Recent soybean checkoff-funded research at ISU has shown the fungus can survive on corn kernels and cornstalks,” she explained. “The corn debris is likely allowing the fungus to carry over from year to year.”

Leandro said plants with SDS are likely to have some yield loss, though the extent would depend on how early SDS appeared during the reproductive stages of plant growth.

While SDS can’t be treated, Robertson said it can be managed with appropriate variety selection and improved field drainage.

In corn, Robertson told farmers at a June 30 field day at the ISU’s Nashua and Kanawha research farms that before applying a fungicide, they should weigh the price of corn – the price of product plus application and drying costs.

Growers should also factor in unseen variables such as hybrid susceptibility and their respective field’s disease history before using a fungicide, she added.