Physoderma brown spot on corn

Physoderma brown spot on corn. Photo by Doug Jardine, K-State Research and Extension.

It’s never too early to prepare for disease prevention. During the Kansas Corn Management School in Parsons, Kansas State University plant pathologist Doug Jardine named his top diseases to watch in the New Year.

Here to Stay Southern Rust

Southern rust and two varieties of stalk rot held peak yield-reducer status in 2019 and will continue to prove difficult to manage in 2020.

“Southern rust is a disease that does not survive in the winters here,” Jardine said. “Typically, spores move in from Cuba, Florida and along the Texas-Mexico border.”

Almost all of the southern rust that comes into southeast Kansas originates in the Mississippi Delta region and then comes up through Arkansas into the state. Jardine said watching for reports of southern rust in other states can be a critical way to monitor the disease because, once reports appear as far south as Mississippi, it may be only a few weeks before problems emerge in Kansas.

“Southern rust spores are blown in from the south during big spring thunderstorms or if a tropical storm pushes onto the Gulf Coast,” Jardine said. “Spores will wash out in the rain and land on leaves of late vegetative stage corn or early reproductive stage corn.”

The disease life-cycle for southern rust lasts about 10 days and a single spore can become billions of spores in a matter of a few weeks.

“It can be a very explosive disease when we have temperature in the upper 80s to low 90s and high humidity or rainfall,” Jardine said. “With the right conditions, it can have a significant impact.”

Southern rust has evolved over time. Where 15 to 20 years ago crop scouts would have predicted southern rust around the beginning of August, in 2019 the disease arrived significantly later with all counties in Kansas effected by Labor Day, Jardine said.

Identifying southern rust during a crop visit can be fairly easy, especially in the late stages of the disease.

“Usually you get massive amounts of pustules on the upper side of the leaf,” Jardine said. “They’re very dense, typically circular and they may have a yellow halo around the edges.”

Common rust is not easily confused with southern rust in scouting because it is present in cooler weather from May to early June. Jardine said common rust is not typically treated because it shuts down quickly in the presence of higher temperatures.

When facing southern rust, it can be beneficial to treat the disease to prevent yield loss, especially in years with weather conditions similar to 2019.

“In southeast Kansas trial plots, there was a very distinct negative correlation between the amount of rust present and the yield,” Jardine said. “Especially this year when corn was planted late, we received lots of calls in September saying that there was rust and producers were not sure if they should spray or not.”

Stalk Rot

“For all of the other diseases we talk about when it comes to corn, year in and year out our greatest losses continue to come from stalk rots,” Jardine said. “There are two primary stalk rots in this part of the state — charcoal and Fusarium stalk rot.”

Stalk rots at severe stages will cause lodging in the plants, leading to inefficient harvests and yield loss. In less obvious cases, stalk rot can be just as detrimental to yield with few symptoms.

“Even at harvest on a healthy corn plant, those basal internodes should still be green,” Jardine said. “If your stalks look brown then that corn plant probably died several weeks earlier and it has stalk rot in it.”

When spliced open, the inner stalk of the corn plant should be fairly white with a high degree of integrity, Jardine said. Affected plants would have disintegration of the inner pith tissue where only the water conducting tissues remain behind because they are more resistant to breakdown.

The disintegration of root tissue associated with stalk rot causes plants to fall or have reduced standing strength.

“Most people when they think about stalk rot, think about lodging,” Jardine said. “You can have a field with a high level of stalk rot but if you have a hybrid variety with good standability characteristics, it doesn’t lodge but you’ve still lost money.”

Stalk rot can also mean smaller ears, and smaller ears spread over the field means yield loss, Jardine said. When plants lose leaf area, it makes the plant more susceptible to stalk rots because the plant begins drawing nutrients from the base of the plant rather than the leaves, expediting the breakdown of the stalk tissues.

“It’s really important to go out and inspect fields late in the season for stalk rot,” Jardine said. “The stalk coloration is really important, but most of us when we evaluate lodging do a push test.”

During a push test, scouts will push the corn plant to 45-degree angle and then let go. If the plant snaps back up, it’s deemed healthy, and if it lists or lodges, stalk rot is present.

As for distinguishing between the two most common types of stalk rot, charcoal rot looks the same but with black flecks in the tissue. The best conditions for each type are also indicators. If the weather is wet at the beginning of the year, dry in the middle, wet later, then the conditions favor Fusarium stalk rot. If the weather is hot and dry throughout the season, then charcoal rot is the most likely stalk rot present.

On the Horizon

“Physoderma brown spot kind of blew up on us throughout the United States not just here in Kansas,” Jardine said. “The question we’re asking ourselves is if it will be manageable with fungicides.”

Physoderma brown spot has been less of a problem in southeast Kansas than it has been in other parts of the state. However, Jardine said it’s smart to scout and prepare for the disease should it be identified locally.

“There’s two things you’re looking for with Physoderma brown spot — yellow bands up the leaf and brown spots down the mid rib, “ Jardines said. “The brown spots are the source of the disease’s name and are very characteristic.”

Spots can cause breakage at the node in more severe cases, Jardine said. The banding associated with the disease is related to the way the fungus grows. Physoderma brown spot needs light to grow and is active during the day and inactive at night.

“Later in the season at the base of the leaf sheet if you rub your thumb across that region, it will come back covered with the spores of the fungus,” Jardine said. “Each one of those spores has thousands of swimming spores inside it that will be released when the main spore breaks open.”

Water is necessary to cause the spores to break open but when they do it can reproduce very quickly and overwinter from two to seven years. For this reason, it can be especially detrimental to irrigated fields. While tar spot has not been scouted south of Interstate 70 yet, Jardine said it is another disease to keep an eye on.

“Tar spot, as the name describes, looks like someone took a brush with tar on it and flung it at the leaves,” Jardine said. “There are raised, black circular lesions on the upper side of the leaf.”

Jardine will retire from K-State on Feb. 28 ending a 35-year long career educating producers about plant pathology across the state. This presentation at the Jan. 8 Corn School in Parsons was one of the final stops on his “farewell tour.” 

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