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Now’s the time to be scouting for diseases

By Staff | Jul 6, 2018

Gray Leaf Spot, shown here is beginning to show up in corn fields across Iowa. Now is the time to begin scouting for the disease.



Although conditions have not been all that favorable for producers this year, they have been favorable, however, for diseases.

Iowa State University Extension field agronomist, Angie Rieck-Hinz advises it is now the time for producers to get out into their fields and begin looking for potential disease pressure, especially before making the decision to apply a fungicide.

“We want to be out scouting because we don’t want to use fungicides prophylactically,” she said. “We don’t want to develop resistance to our fungicides. We want to be able to use them because we need them, not use them because we can.”

Corn diseases

At this point and time, Gray Leaf Spot, Physoderma Brown Spot and Bacterial Leaf Streak are the primary diseases producers should be on the look out for.

Gray Leaf Spot, Rieck-Hinz said is showing up in low levels in North Central Iowa. This particular disease likes it warm and wet and tends to thrive in temperatures of 80 degrees and above and humid conditions of 90 percent-plus humidity for 12 hours.

“We obviously are seeing that,” she said. “Not only are we getting a lot of rain, it is also humid in that canopy.”

If the weather would happen to turn warm and dry, the progression of Gray Leaf Spot may slow down, but right now, Rieck-Hinz said although she hasn’t seen a whole lot of pressure, the disease is definitely out there.

“People are going to want to scout and monitor that right now because it always starts in the lower canopy and we want to make sure it doesn’t move up the canopy above the ear,” she said. “We need green tissue above that ear for photosynthesis to fill the ear.”

While scouting, it is also recommended to look through the hybrid list and see what might be rated to have a higher susceptibility to Gray Leaf Spot.

“You might want to make plans to treat those fields first,” she said.

Rieck-Hinz said Gray Leaf Spot shows up as brown/grayish rectangular lesions with square edges that are confined between the veins on the corn leaf.

Physoderma brown spot

Rieck-Hinz said Physoderma brown spot has had some reports of showing up in the corn fields.

This disease is a leaf disease, however it can also be a node-rot disease located on the lower nodes of the stalk, rotting it and causing plants to snap off.

Physoderma brown spot, according to ISU Extension, are very small round-to-oval lesions that are yellowish-brown in color and occur in high numbers and in broad bands across the leaves.

In addition, dark-purple to black spots occur on the midrib. These midrib lesions help to distinguish this particular disease from other diseases such as eyespot and southern rust, Because infection requires a combination of light, free water and warm temperatures, alternating bands of infected and non-infected tissues commonly develop on the plant. Symptoms may also appear on the stalk, leaf sheath and husk.

Rieck-Hinz said scouting for Pysoderma brown spot should be going on now through the R1 stage.

Management for this disease, she added is mostly done as hybrid susceptibility.

“Although there is a fungicide labeled for Pysoderma brown spot, there is not a lot of good evidence out there on what they do and don’t do for the disease,” she said. “We don’t see this disease very commonly, but it is out there this year and people have seen it and reported it.”

Bacterial leaf streak

Bacterial leaf streak is a new corn disease to the state of Iowa being first diagnosed here in 2016.

Like its name implies, it is a bacteria, so fungicide applications, Rieck-Hinz said will not combat it.

“Bacterial leaf streak can very easily be confused with Gray Leaf Spot, so if you are treating what you think is Gray Leaf Spot and it’s not, a fungicide does not work on a bacterial infection,” she said. “It is important if you are not sure what you are looking at, that you get the proper diagnosis.”

Rieck-Hinz said the best piece of advice she can give when it comes to diagnosing bacterial leaf streak is to send a sample to the Plant and Insect Diagnostic Lab at ISU or speak with an agronomist.

“It is very difficult to diagnosis in the field,” she said.

According to Iowa State University, symptoms of bacterial leaf streak are long, narrow lesions that are less than one-inch to several inches long. The lesions are tan, brown or orange and occur between the veins of the corn leaves but can also occur close to the midrib or across the leaf blade.

Soybean diseases

Rieck-Hinz said this time of year brings on a lot of leaf diseases in soybeans which are intensified with the wet conditions.

Septoria brown spot, she said is not uncommon and is usually widespread throughout Iowa in the lower bean canopy, especially with wet conditions, but typically doesn’t need treated.

“It very rarely becomes severe where it would warrant a treatment,” she said.

Bacterial blight

Bacterial blight, Rieck-Hinz said will show up in the upper canopy of the soybean plant, typically on newer leaves. And, once again, because it is a bacterial disease, fungicide will not be beneficial in combating the disease.

“With as much water as we have had standing in the fields there are all kinds of stem and, at one time, seedling pathogens including phytophthora and rhizoctonia that has already infected the plants and there is probably not a whole lot we can do at this time,” she said. “There could be a complex of diseases out there at this point and time.”

Other weather related issues

Rieck-Hinz said is soybeans, where it has been wet; root growth has been limited in many cases as well as nodulation growth limited.

“That is why we continue to see those pale, yellowish looking soybeans,” she said. “It’s all due to lack of root growth, lack of oxygen and poor nodulation, in my opinion.”

For corn, Rieck-Hinz said there is some phenomenal looking corn typically those fields that were planted earlier and managed to get growing and was able to establish roots have that nice, dark green color.

“There are some places you see shorter corn that was delayed in planting,” she said. “And those places where they have lost a lot of nitrogen in those soils. There’s not good rooting depth, they are lacking oxygen, there are saturated soils and nitrogen loss.”

Rieck-Hinz said it might not be too late for a nitrogen application.

“If you want grain, and you don’t have your nitrogen on, then you better get some nitrogen on.”

With a later nitrogen application, a producer, Rieck-Hinz said can expected some leaf tissue burn if the fertilizer is dropped over the top or happens to land inside the whorl.

She advises if an application of a UAN solution is needed to use drop tubes to help avoid burning the leaves.

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