Production Blog

Are You Seeing Spots?

Pam Smith
By  Pam Smith , Crops Technology Editor
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Some stalks are turning up with polka dots and other interesting symptoms this season after the prolonged wet weather earlier in the year. Some are economic problems and others are not. (DTN photo by Pamela Smith)

AUBURN, Ill. (DTN) -- Are you finding cornstalks dotted with discolorations that look like a preschool class armed with magic markers went wild? Black, brown and purplish polka dots found on the stalk could be purple leaf sheath or Physoderma brown spot.

Field days are a great time to stalk the stalks. Last week I attended a Monsanto event near Auburn, Illinois. It was hotter than blue blazes, but Lance Tarochione, technical agronomist for Asgrow/DeKalb, showed us some curious corn disorders to make sure it wasn't just the heat causing us to see spots.

Purple leaf sheath (sometimes called yeast spot) lesions develop when tassel pieces or pollen fall between the collar of the leaf and the stalk. The staining present underneath the leaf rarely penetrates the stalk -- peel the leaf back and scrape and you'll find the stained areas doesn't go much below the surface.

Tarochione said fungicides won't help this purple spotting and it's not an economic problem. "It's merely yeasts and sometimes saprophytic fungi gathering to feed on what the yeast have done," he noted. "It's not harmful to the corn plant and can be found in most hybrids."

Physoderma brown spot is a legitimate "disease" -- although Tarochione classifies it as mostly a curiosity in central Illinois. It's caused by the chytridiomycete fungus, Physoderma maydis (syn. P. zeae-maydis), which is closely related to the oomycete or water mold fungi, such as the downy mildews.

"Typically, Physoderma has distinct black dots that are nearly perfectly round, do not run together and are about the size of a BB. They often wrap around the stalk somewhat like the stripes on a barber pole. I believe this is due to the infection occurring in the whorl earlier in the season," Tarochione said.

"Fungicide treatments may have helped control it this year, but we're not really sure if those are even applied at the right timing for this disease." Tarochione said the disease is displaying randomly in the fields he's observed.

"It's not a disease we typically think of being economic and I've never made a recommendation because of it. However, I do want people to be aware of it, because it looks kind of weird and we're getting lots of questions about it, and so growers don't mistake it for more than it is, we've been talking about it," he told DTN.

However, last week Iowa State University plant pathologist Alison Robertson issued a news release indicating the disease has been spotted with increasing regularity and apparently with more severity in Iowa cornfields. She observed that the disease shows up two ways -- in the leaf and the stalk. Read more here: http://bit.ly/…

"The stalk rot usually doesn't show leaf symptoms," Robertson said. "It looks like a beautiful plant, and you don't notice it until the plant falls over and breaks." That breakage could occur during high winds or a rainstorm or it could occur during harvest, she said. Many of the infected plants won't yield any crop at all. Plants with severe brown spot may not form ears, and, with stalk rot, fallen plants may not be picked up by a combine.

Robertson explained that three factors must be present for any disease to take hold: a susceptible host, a pathogen and the right environmental conditions for the pathogen to flourish.

"Excessive rain during the early vegetative growth of the crop is a major part of the equation that's been in place the last couple years," she said. Robertson said little research has explored ways to combat Physoderma, and there's little farmers can do this year if they've discovered it in their fields. The sporangia that give rise to the disease can survive for three to seven years in a field, so she recommends that farmers rotate corn out of an infected field in following years. Changing seed varieties to a less susceptible hybrid can also help, she said.

Robertson is conducting trials to learn more about Physoderma, which could lead to a better understanding of how to fight off the disease. "Right now, we don't really know when or how to apply fungicides or how other management techniques might affect the disease," she said. "I'm hoping we can fill in some of those knowledge gaps."

So let's connect the dots. If you're seeing purple leaf sheath, don't sweat the small stuff. Finding small amounts of Physoderma shouldn't pose much of a problem. Continue monitoring stalks though and flag those fields showing stalk damage for timely harvest and then, plan to rotate/search for less susceptible hybrids.

For an accurate diagnosis, sample and send to your state plant or diagnostic clinic.

Read more at:

http://bit.ly/…

http://bit.ly/…

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