Search Site   
News Stories at a Glance
Deere 4440 cab tractor racked up $18,000 at farm retirement auction
Indiana legislature passes bills for ag land purchases, broadband grants
Make spring planting safety plans early to avoid injuries
Michigan soybean grower visits Dubai to showcase U.S. products
Scientists are interested in eclipse effects on crops and livestock
U.S. retail meat demand for pork and beef both decreased in 2023
Iowa one of the few states to see farms increase in 2022 Ag Census
Trade, E15, GREET, tax credits the talk at Commodity Classic
Ohioan travels to Malta as part of US Grains Council trade mission
FFA members learn about Australian culture, agriculture during trip
Timing of Dicamba ruling may cause issues for 2024 planting
   
Archive
Search Archive  
   
Heavy rains don’t always see an increase in crop diseases

By DOUG SCHMITZ

HARLAN, Iowa — With the heavy rains and unseasonable temperatures, the increase in crop diseases may be a concern for farmers, most of whom are still waiting to start planting; however, a number of variables need to be in play for disease to even be a likely scenario, according to Iowa State University crop experts.

“When it comes to diseases, wet weather and saturated soils are bad news, as we know, but they don’t always automatically mean we will have disease issues now or later in the year,” said Clarke McGrath, on-farm research and extension coordinator for ISU’s Iowa Soybean Research Center.

For diseases to get a grip on the crop, he said multiple factors work in what pathologists call the “disease triangle.” “There has to be a susceptible host, pathogens present, and environmental conditions that favor pathogens. Pathogens are sometimes here already, and sometimes they come in with the weather.”

Alison Robertson, ISU professor of plant pathology and microbiology, explained weather conditions certainly have much to do with disease development.

“Delayed planting due to the excessive rain and cold temperatures this spring means that grain fill will likely occur later into the summer when temperatures are warmer and more favorable for leaf diseases,” she said.

The earlier during grain fill a disease occurs, the more impact it can have on yield. “This is because the amount of leaves with disease increases, thus photosynthesis and grain fill is impacted more,” she said. “If the precipitation continues, we could definitely see disease: normal to above-normal temperatures – gray leaf spot and frogeye; cooler-than-normal temperatures – northern corn leaf blight and tar spot.”

McGrath said corn is susceptible to seedling diseases, “but nearly all of our seed corn is treated to help reduce the risk.

“I’d still recommend scouting and ensuring the stands are okay in those cornfields, though,” he added,” for both beans yet to be planted – and if your beans are in, scouting and taking stand counts to double-check that no replanting is needed.”

He said wet and possibly compacted soils from last fall and early-season planting may expose soybean seed to seedling disease, such as Phytophthora root and stem rot, and Pythium seedling blight and root rot.

“With moist soils, even high-quality seed is at risk for infection by these pathogens,” he said. “Phytophthora can infect soybeans at any growth stage. Root rot is more severe in no-till fields, poorly drained soils, or low areas prone to flooding. This pathogen is an oomycete (water mold), which means it can move through and survive in water.”

When soil temperatures reach approximately 60 degrees Fahrenheit and are saturated, he said oospores of the pathogen germinate, and infection of plants occurs. “Warm, saturated soils, especially right after planting, are conducive to disease. Seed treatments that include metalaxyl, mefenoxam, or ethaboxam fungicides target oomycetes and can provide some protection.”

He said Pythium – also an oomycete pathogen – can cause both seed and root rotting.

“This pathogen is prevalent in the northern U.S. and is capable of infecting plants at lower temperatures, between 50 to 60 degrees,” he said. “Planting in cold, wet soil should be avoided to reduce infection. Seed treatments targeting oomycetes can be effective.

“Reduced sensitivity to metalaxyl, mefenoxam, and ethaboxam has been documented in some species of Pythium. So while we don’t always recommend seed treatments on later-planted beans, with the wet soils it still could pay off because of the pathogens that like warmer soils.”

He said another soybean disease that comes to mind this spring is sudden death syndrome (SDS).

“Cool, wet field conditions shortly after planting favor early root infections and disease establishment, and wet soils midseason can favor early symptom expression,” he said. “Along with planting soybean varieties that have some genetic resistance to SDS, there are seed treatments like fluopyram that can help as well.

“It’s hard to tell how bad SDS will be in a year like this. As we get later with planting and encounter warmer soils, it does reduce the risk of SDS infection some. But on the other hand, if we are still having rains mid-season, saturated soils at that time typically increase our SDS problems.”

Virgil Schmitt, ISU field agronomist in southeastern Iowa, said the development of these diseases depends on the weather later in the season.

“If mid- to late summer is hot and humid, then gray leaf spot will likely be an issue on corn and frogeye leaf spot may be an issue on soybeans,” he explained. “If mid- to late summer is cool and wet, Northern corn leaf blight may flourish.

“If the above-normal rainfall does come to pass, then we will most likely see issues with either gray leaf spot or Northern corn leaf blight. If temperatures are normal or above, along with the rainfall, then frogeye leaf spot may be an issue in soybeans.”

He said in any growing season, good scouting is important to observe the beginning of disease development. “If the weather is appropriate for the disease to flourish and if the genetics of the crop are susceptible to the disease, then making that foliar fungicide application may well be warranted.”

McGrath said farmers need to stay up to date on disease progression in the Midwest by checking alerts from the different universities and agribusinesses, and be vigilant with weekly scouting trips in their fields.

“Those are good plans in any crop year, but even more important in a year like this,” he added. “While there are a lot of variables, the risk of diseases is higher with the wet conditions we are facing now, and probably generally higher with the later-planted crops, combined with what looks like saturated soils for at least the next few weeks, if not longer.”

Robertson said growers are encouraged to pay attention to disease reports from their local extension offices. “CropProtectionNetwork.org is also a good resource. Extension news releases, tweets, and videos updating growers and agricultural business on what is going on in field crop production are available here.

“This year, pathologists are testing a beta version of an app developed by Damon Smith at the University of Wisconsin for predicting tar spot and applying a fungicide to manage the disease,” she said. “Dr. Smith also has an app for white mold on soybeans, Sporecaster, which farmers can use to determine if they need to spray a fungicide to manage white mold.”

5/23/2019