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Keep Your Eyes Peeled For These Early-season Pests, Diseases

It's still really early in the growing season, but the weed, insect and disease pressures are already popping up in young and emerging corn and soybean fields around the Midwest, agronomists report.

They're all fairly familiar pests whose treatments will likely be just as familiar. But, chances are you'll still need to do some good old-fashioned crop scouting to get a sense for whether or not you'll have your hands full taking care of them both in the short and longer-term.

Heading toward mid-May, here are the pests and disease for which you'll need to be on the lookout.

Palmer amaranth

Agronomists have confirmed "waterhemp's big, bad brother" in fields in 5 counties in Iowa, and that means it's likely in a lot more spots than that, says Meaghan Anderson, Iowa State University Extension field agronomist in east-central Iowa. THis early, it's small, usually has 4 leaves and will likely be easiest to spot in sandier soils.

"Likely areas of infestation would be near cattle or swine operations that import feed or bedding from southern states, especially those that use cottonseed products or gin trash," Anderson says. "Two of the known infestations are near commercial grain handling operations; Palmer amaranth seed may have been brought on trucks transporting grain from areas with Palmer amaranth infestations."

A common issue with Palmer amaranth can be confusion with other amaranth species that can be less damanging to growing crops. At this point in the year, that's just about impossible, Anderson says, meaning if you think you have it now, it will be important to keep your eye on it until it develops some of the physical characteristics that distinguish it from other less-problematic subspecies.

"We would not suggest that identification of a population at this seedling stage is easy or even possible. The presence of Palmer amaranth in this area previously gives us confidence that the seedlings seen on May 4 are Palmer amaranth and not another of the many Amaranthus species present in the state," she says. "Many other Amaranthus species are present in Iowa, including redroot pigweed, smooth pigweed, waterhemp, and spiny amaranth, among others. These Amaranths have significant genetic variability, often making it nearly impossible to distinguish many individuals without the presence of inflorescences. Luckily, or unluckily, Palmer amaranth is expected to begin flowering in Iowa in late June to early July."

Worms and grubs

Bugs like cutworm larvae and white grubs can be sneaky robbers of yield potential as soon as young corn plants pop out of the ground. Cutworms, for example, can inflict damage even before emergence, and in some field situations, you may want to start scouting for them now, even if your seed was treated with an insecticide before planting.

"As corn begins to emerge, be alert to the potential damage that may be caused by early season insects. Most early season insect damage would be caused by cutworms, wireworms, or white grubs. Wireworms and white grubs are most often associated with fields that have been in pasture or CRP where the grasses were allowed to grow for more than one year. It is rare to see these problems in continuous corn, but exceptions happen. Since wireworms and white grubs feed underground and cutworms feed on or below the soil surface, scout for plant damage and then dig in soil around the plant to identify the insect causing the damage," according to University of Nebraska Extension entomologist Bob Wright. "Cutworms and other insects may hinder emerging corn plants this spring, even if seed was treated with insecticide or Bt corn hybrids were used. High populations of insects can overwhelm the protection method, regardless of whether it was an insecticide applied at planting or a Bt corn hybrid. Cutworms can cause serious damage to corn in the first couple weeks after emergence so it is important to scout fields early for damage. Several species of cutworms attack corn. The severity and the area affected will vary greatly, depending on species involved, previous crop history, and weather conditions."

Knowing specifically how these bugs get to your fields and when/how they inflict their crop damage is key to managing pests like cutworms and wireworms. Cutworms, for example, don't overwinter in the Midwest, but arrive via south-to-north moth flights about this time of year. And, they like big weeds.

"Fields with winter annual weeds or abundant crop residue are more attractive to the egg-laying black cutworm moths in the spring," Wright says. "Remember that early detection of a problem is essential because most of the cutting occurs within seven days of plant emergence. Generally, a postemergence 'rescue' treatment should be considered if cutting is observed on 3-5% or more of plants and the worms are one inch or less in length. Rescue treatments are effective in controlling soil cutworms."

If you're dealing with wireworms, though, management's quite a bit different. They do all their feeding underground; the larvae can continue to hatch from eggs laid as long as 6 years ago, and they can start feeding as soon as seed hits the dirt.

"There is no rescue treatment for wireworms, so the main decision at this time is whether there is sufficient stand reduction to warrant replanting," Wright says. "The use of seed treatments like Cruiser and Poncho has greatly reduced the incidence of wireworm damage. They are excellent early season stand protectors."

While you're out scouting, check for white grubs; they wait a little longer to start feeding than wireworms and unlike cutworms can overwinter in the soil and feed on roots up to corn's 6-leaf stage. " Like wireworms, there is no rescue treatment for white grubs. Again, high risk areas need to be treated at planting. Products for white grub control are similar to wireworm control," Wright adds.

Early-season diseases

Weeds, check. Bugs, check. How about disease? The longer things stay damp like they are right now in much of the nation's center, the better your chances of your crops developing disease problems, especially soybeans. Right now, your most likely specific disease pressure in soybeans depends on the mercury; University of Nebraska Extension plant pathologist Loren Giesler says diseases like Pythium will be more common in damp, cooler soils while Phytophthora root rot will hit you harder when things are warmer.

"Cooler soil temperatures will typically make [Pythium] worse and more common so earlier planted soybean fields will often see this if they were not properly protected with a seed treatment. Typical symptoms of Pythium include seed decay and pre-emergent seedling rot, and seedling damping off after emergence. If the plant has emerged, often the outer layer of its root system can be easily pulled off while the center of the root stays intact," Giesler says. "Warmer conditions are more conducive for Phytophthora, which will also be present now. Phytophthora is not as common as Pythium but is definitely a disease that we are seeing more of each year in Nebraska. Many times Phytophthora is indicated in fields planted with a standard rate of a seed treatment product but where there is still a significant stand reduction when wet conditions occur. This will be field-specific and doesn't occur as often as Pythium. Typical symptoms of Phytophthora are seed decay and pre-emergence seedling rot, and seedling damping off after emergence. Typical symptoms on seedlings are darkened stems at the base of the plant coming up from the soil line. When young plants are cut at the lower stem, the stem center will exhibit a dark color. Phytophthora can kill plants at any stage of development, but Pythium typically does not kill plants much past the V5 growth stage."

If you used a soybean seed treatment, you won't likely face as much pressure from these diseases. If you still see one or both of them pop up, though, your treatments may have been off the mark.

"There is a lot of overlap in symptoms for these two diseases, especially at the pre-emergence stage of development where many fields are now. I encourage you to get a diagnosis of any problems you're seeing in your fields so proper management actions can be taken in the future," according to Giesler. "Seed treatment and the use of resistant varieties (for Phytophthora) are the management actions which are modified based on the field history. In fields where a seed treatment fungicide was used and seedling disease is still developing, it can be the result of the wrong treatment or excessive moisture leading to product failure under extreme conditions. The most common example of a product rate issue is when mefenoxam or metalaxyl is put on at a rate too low for good Phytophthora control."

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