Tar spot. Southern rust. Japanese beetles. Soybean gall midge. Oh no.

Crop scouting for these and other diseases and insects is part of every good crop management plan. Knowing when and where these common diseases and pests are prevalent this season can help direct scouting efforts.

Ritika Lamichhane is the Nebraska Extension crops and water educator in the southeast part of the state and says that tar spot and southern rust are becoming a problem in the surrounding areas. When scouting soybean fields, soybean gall midge and herbicide damage are relevant problems. In both corn and soybean fields, Japanese beetles have also been found.

“If you look at the 2024 Guide for Weed, Disease, and Insect Management published by the University of Nebraska-Lincoln, look for a fungicide rating for different diseases,” Lamichhane says. “That would be a really good resource for looking at the kind of fungicide and when to spray those fungicides.”

When going out to the fields to scout for diseases and pests, keep a close eye on these problems that have been plaguing the Midwest:

Tar spot. This disease has been a hot topic of conversation across the Midwest the past couple of years. Unfortunately, because it overwinters, it can infect corn year after year. Insect feces can sometimes be mistaken for tar spot because of the raised black spots that show up on the corn leaves. But to test to see if there is tar spot in the field, rub the leaves. If the spots do not leave the plant, it is likely tar spot.

This year, the extra precipitation and lower temperatures early in the growing season set up the perfect conditions for the disease.

“If the leaf wetness is seven hours or more, then it creates an ideal condition for the disease,” Lamichhane says. “With all the rain that we have been getting this year, we need to be more cautious of the disease.”

To best combat tar spot, Lamichhane says to use a foliar fungicide that is rated good to very good in the 2024 weed guide.

There are many different institutions doing research on tar spot, including the University of Nebraska-Lincoln and Purdue University, to find better prevention and management strategies.

Southern rust. This disease can sometimes be mistaken for common rust, which is not an economically damaging disease that producers should be worried about. However, southern rust has been shown to affect yield.

The best way to tell the difference between these types of rust is the color and location. Common rust will have a brownish-red appearance scattered across the leaf. Southern rust will be more orange in color and densely packed on the surface.

“They start at the southern states, and the wind will start blowing the spores to fields in the Midwest,” Lamichhane says. “And these fungi, like all other fungi, need moisture to germinate and infect. So high relative humidity, rainfall and irrigation will favor the disease.”

A foliar fungicide can be effective to control the spread of southern rust in fields.

Japanese beetles. This pest is found in both corn and soybean fields across the Midwest. Their favorite foliage to eat includes the leaves of linden or basswood trees. Fortunately, they are not usually an economically damaging insect in corn and soybeans unless they become uncontrollable. Japanese beetles will start to defoliate the leaves and clip the silk in corn, which may cause reduced pollination.

“If the damage in soybeans is about 30% in the vegetative state or 20% in the reproductive state, then it should be treated with an insecticide,” Lamichhane says.

If the beetles are kept to a low number, producers do not need to be concerned about these insects.

JAPANESE BEETLES: While Japanese beetles might not always cause a yield loss in corn and soybean fields, it is important to keep these insects at low numbers to limit excessive defoliation. (Photo by Ritika Lamichhane, UNL)

Soybean gall midge. Soybean gall midge has been a major problem in eastern Nebraska for some time. Entomologists at UNL are conducting several research studies to test different management strategies to control these pests.

“Adult gall midge overwinters in the previous soybean field and flies to the adjacent field next year and lays eggs on young soybean plants,” Lamichhane says.

Because these insects will travel to another field, Lamichhane says it is crucial to scout the field borders where farmers have seen a history of colonies.

When controlling soybean gall midge, a foliar insecticide has not provided a consistent solution. Crop rotation can help reduce gall midge populations, but it is important to watch for areas where corn and soybeans are near each other because the adults can move between fields.

Herbicide damage. In the past few weeks, Lamichhane has received calls about herbicide damage on soybean fields. While there is nothing that producers can do to prevent drifting onto their own fields, this problem is not necessarily always economically damaging to the crop.

The most noticeable symptom of herbicide damage is cupping of soybean leaves.

“If there is damage once the soybean has already flowered, then it could cause some yield reduction,” Lamichhane says.

If producers notice any disease or pest damage, it is recommended to work with a local Extension educator to find the cause and create a management plan.

The 2024 Guide for Weed, Disease, and Insect Management can be found here to help with management efforts.