Blog Archives

JenREES 7-28-19

Southern Rust in Nebraska was confirmed in Nuckolls, Thayer, and Fillmore counties last week. The lesions were typically on one leaf in an isolated portion of fields at low incidence and severity. I was recommending to watch the fields instead of spraying right away. Greatly appreciate everyone who has gotten samples to me this month and to neighboring Extension offices serving as drop off points for samples. I’ve been looking at samples since early July and honestly, common rust at times has exhibited signs similar to southern rust. At my blog site (https://jenreesources.com), I’ve posted photos showing the differences of common vs. southern rust that we’re seeing this year. Southern rust typically is orange to tan colored with tiny, clustered pustules on the upper leaf surface. Common rust has had an orange appearance to it at times with smaller lesions than the ‘typical’ brick red larger ones. However, in every southern rust sample I’ve confirmed, the pustules have not gone through the back of the leaf…there’s been indentations but nothing has produced pustules on the backside. That doesn’t mean that the fungus can’t; it just rarely does. Also, these leaves have all occurred in the mid-canopy. I realize lowest leaves of plants often have a great deal of rust on them, but it’s been common rust in leaf samples I’ve pulled and received to date. I’ve also posted photos of another disease called Physoderma brown spot on my blog. Physoderma is the disease that has purple/brown on the midribs, around leaf axils and sheaths and it also can have tiny yellow-brown spots without pustules that look a lot like southern rust on leaves of plants. It’s one that we tend to see around pollination as the fungus-like pathogen swims in water on the leaf surface and feeds off of decaying pollen.

Confirming southern rust in a few Nebraska counties thus far doesn’t mean that every field has it and we don’t know how it will progress. So our recommendation is to continue scouting and if you have a suspect sample, you can get it to me (if you’re in the area) or to our Plant and Pest Diagnostic Lab in Lincoln. You can keep updated with counties that are confirmed by checking out the Southern Rust tracking site at: https://corn.ipmpipe.org/southerncornrust/. The wide range of planting dates across Nebraska this year has resulted in a wide range of corn growth and reproductive stages in fields, some of which are still in the vegetative growth stages. Keep in mind that late planted fields are at particular risk for southern rust if it increases in development. Right now corn disease pressure in general is low. I’m anticipating that to soon change for gray leaf spot susceptible hybrids. Some growers added a fungicide in with an insecticide treatment for western bean cutworm, which made sense to save an application cost. However, in general, automatic fungicide applications when one treats before disease develops may lead to loss of full product efficacy before critical disease levels. This can also result in the need for reapplication later if the disease worsens after the previous fungicide application and residual has worn off. And, always in my mind is eventual potential for pathogen resistance…so utilizing fungicides when we need them vs. automatically applying is wise for maintaining these fungicide chemistries.

York County Fair: This week is the York County Fair! All events and details can be found at: http://www.yorkcountyfair.com/. One special addition this year is “Brownies for Bergen” on Saturday, August 3 from 5:30-7:00 p.m. The late Gene Bergen was a 4-H icon and was planning on celebrating 50 years as a York County Ag Society Member at this year’s fair before retiring. While he won’t be with us in person, the York County Ag Society, 4-H Council, and Extension Office would like to honor him and his original plans to celebrate. So please join us for brownies and ice cream and share your favorite memories and stories of Gene!


00000IMG_00000_BURST20190726202230505_COVER

Lower corn leaf that is dying early and has a great deal of rust on it. So far, this has all been common rust as pictured here in spite of smaller, clustered pustules or even orange colored pustules.

 

IMG_20190726_201958

Common rust spores are circular in shape.

00000IMG_00000_BURST20190724130934715_COVER (2)

Southern rust was confirmed on this leaf in 2019. Small, clustered, tan-orange colored pustules can be seen on this leaf.

IMG_20190726_115911

Southern rust spores are brown and oval in shape. Sharing this photo as the clear structures are spores (condia) and spore-holding structures (conidiophores) of Cercospora zeae-maydis which causes gray leaf spot (gls) in corn. Was seeing these spores/structures with only ‘specks’ of gls lesions on this leaf. With humid weather and longer leaf wetness from dews, we may see an increase in appearance of gls lesions in the next 7-10 days.

IMG_20190726_173836

Physoderma brown spot. Symptoms include both the purple/brown midrib in addition to the yellow-brown lesions on the leaf. These lesions are flat in comparison to southern rust which would have raised pustules.

Corn Disease July 2015

Radio advertisements, email blasts, and other media are warning of corn diseases and the need for fungicides.  Two months of humid, wet weather has allowed for disease development.  It’s important to know what diseases truly are in your field before spraying a fungicide, particularly with today’s economics.  Here’s what we’re seeing in fields right now in the Clay, Nuckolls, Thayer, Adams county area.  Based on the diseases we’re seeing, we would recommend you scout your fields to know whether you have mostly bacterial or fungal diseases present.  Consider disease pressure, where on the plant the disease is occurring, growth stage, and economics.  We have had southern rust show up in 10 of the last 11 years I’ve been serving in this area.  If you spray a fungicide at tassel, you may not have enough residual to ward off southern rust when it appears later, potentially resulting in the need for a second application.  In our area thus far, I’m not seeing enough disease pressure in many fields to warrant a fungicide at tassel; consider delaying an application till later for economic and resistance-management reasons.  Ultimately this decision needs to be done on a field by field basis.  Please also see this UNL CropWatch article regarding fungicide application and corn growth stage.  Although I don’t have a photo of it, I’ve also seen common rust in the mid and lower portions of corn canopies thus far.

Anthracnose has been observed in fields for a good month now.  It is mostly seen on lower leaves of plants, particularly in fields where we've had high amounts of rainfall, standing water, and/or hail.  I haven't seen it moving up the plant very far yet.  There are fungicides labeled for this, but we rarely recommend fungicide treatment for anthracnose.

Anthracnose has been observed in fields for a good month now. It is mostly seen on lower leaves of plants, particularly in fields where we’ve had high amounts of rainfall, standing water, and/or hail. Lesions are tan in the center and have a wavy yellow colored margin.  I haven’t seen it moving up the plant very far yet. There are fungicides labeled for this, but we don’t often recommend fungicide treatment for anthracnose.

We saw this disease in the Saronville/Sutton area last year.  This year, we are seeing it throughout Nuckolls, Thayer, Clay, and Adams counties...and most likely others too.  Some hybrids (and have seen this across companies) are highly affected by this bacterial disease.  We have informally called it

We saw this disease in the Saronville/Sutton area last year. This year, we are seeing it throughout Nuckolls, Thayer, Clay, and Adams counties…and most likely others too. Some hybrids (and have seen this across companies) are highly affected by this bacterial disease. Neither last year, nor this year, has the pathogen causing this disease been confirmed. Plants will have vein-limited lesions in streaks on the leaves. It is often confused with gray leaf spot early on.  Take a hand-lens and looking closely, you will see wavy margins on the lesions which doesn’t usually occur with gray leaf spot.  Over time, these lesions become elongated and can coalesce with other lesions. Also looking closely with a handlens or microscope, one finds the leaf tissue eventually becomes transparent. While other diseases can look similar to it, under the microscope, no fungal spores are found other than secondary ones. Putting an infected leaf in water reveals cytoplasmic streaming in which the bacteria is escaping the leaf tissue. (The pathogen causing Goss’ wilt also does this, but this disease is not the same as Goss’ wilt). Fungicides will not cure bacterial diseases and the products advertised for targeting bacterial diseases haven’t been researched for this disease.  This is the primary disease we are seeing in our area right now-and it looks nasty in some fields!  Unfortunately there’s nothing we can do about it.  If you are considering testing one of the products for bacterial diseases, please let me know as I’d appreciate testing it with you via on-farm research.

Goss' wilt (bacterial disease) has been observed in fields as well.  I typically notice it along field edges, pivot tracks, pivot/well roads or hail-damaged fields as plant wounds in general allow an entry for bacterial pathogens.  However, research has shown that the bacterial pathogen causing Goss' wilt can enter through the leaf stomates as well.  Leaf miner damage can be observed at the top of this photo, and often, but not always, I notice this occurring together in fields.  There is nothing we can do for Goss's wilt if you have this disease in your fields right now.  There are products targeting bacterial diseases on the market.  If you're interested in trying these, please consider an on-farm research experiment to prove the efficacy to yourself and help us obtain data.

Goss’ wilt (bacterial disease) has been observed in fields as well. I typically notice it along field edges, pivot tracks, pivot/well roads or hail-damaged fields as plant wounds in general allow an entry for bacterial pathogens. However, research has shown that the bacterial pathogen causing Goss’ wilt can enter through the leaf stomates as well. Leaf miner damage can be observed at the top of this photo, and often, but not always, I notice this occurring together in fields. There is nothing we can do for Goss’s wilt if you have this disease in your fields right now. There are products targeting bacterial diseases on the market. If you’re interested in trying these, please consider an on-farm research experiment to prove the efficacy to yourself and help us obtain data.

Physoderma brown spot typically doesn't occur in our area until tasseling (which is where we are at now in some fields).  However, I was seeing this as early as two weeks ago on some hybrids.  The pathogen causing this disease is a fungal-like pathogen that moves with water.  You will notice a purple/brown color on midribs of leaves, leaf sheaths, stalks, and tiny yellow/brown/purple spots on leaves.  This disease isn't considered yield-limiting or of significance to us in Nebraska.  Some confuse this disease with southern rust, but there are no pustules produced with Physoderma brown spot.

Physoderma brown spot typically doesn’t occur in our area until tasseling (which is where we are at now in some fields). However, I was seeing this as early as two weeks ago on some hybrids. The pathogen causing this disease is a fungal-like pathogen that moves with water. You will notice a purple/brown color on midribs of leaves, leaf sheaths, stalks, and tiny yellow/brown/purple spots on leaves. This disease isn’t considered yield-limiting or of significance to us in Nebraska. Some confuse this disease with southern rust, but there are no pustules produced with Physoderma brown spot.

The larger, rectangular lesion is typical of gray leaf spot found in some lower leaf canopies right now.  It is easy to confuse with anthracnose as both diseases appear a little different on various hybrids.  Gray leaf spot will be vein-limited while anthracnose is more blotchy in appearance.

The larger, rectangular lesion is typical of gray leaf spot found in some lower leaf canopies right now. It is easy to confuse with anthracnose as both diseases appear a little different on various hybrids.  Gray leaf spot will be vein-limited while anthracnose is more blotchy in appearance.  Comparing gray leaf spot with the unknown bacterial disease, the lesion edge will be straight for gray leaf spot and wavy for the bacterial disease.

Northern corn leaf blight is a disease we're hearing a lot about but I have yet to see it in our area.  Most have been mistakingly calling the unknown bacterial disease shown above northern corn leaf blight, but there's truly a difference as you see these photos.  Compared to gray leaf spot, northern corn leaf blight will be more cigar-shaped instead of forming a rectangle.  As this photo taken from the Mead, NE area shows, lesions are often forming in close proximity to each other on the leaves in Nebraska right now.  This disease is one to watch as it has been expanding on certain hybrids in some fields, particularly in eastern, Nebraska.  However, I still haven't seen it in our area and haven't received any confirmations of it being found in our area.

Northern corn leaf blight is a disease we’re hearing a lot about but I have yet to see it in our area. Most have been mistakingly calling the unknown bacterial disease shown above and even Goss’ wilt, northern corn leaf blight, but there’s truly a difference as you see these photos. Compared to gray leaf spot, northern corn leaf blight will be more cigar-shaped instead of forming a rectangle. As this photo taken from the Mead, NE area shows, lesions are often forming in close proximity to each other on the leaves in Nebraska right now. This disease is one to watch as it has been expanding on certain hybrids in some fields, particularly in eastern, Nebraska. However, I still haven’t seen it in our area and haven’t received any confirmations of it being found in our area at this time. (Photo courtesy of Aaron Nelson, Nelson Precision Agronomics).

Corn Disease Look-Alikes

Physoderma brown spot

Physoderma brown spot on corn. While the small, speckled lesions may look like southern rust, under hand lens or microscopic observation, there are no raised pustules as would be the case with southern rust. Also notice the brown/purple discoloration on the midrib which is also noticed on the stalk as well where the leaf color meets the stalk.

Fair week tends to be time for tasseling in corn and considerations for watering and fungicide application are being made.  Regarding diseases in corn, there has been confusion about a few diseases, particularly about a disease called physoderma brown spot which some have confused for southern rust.  The fungus causing physoderma brown spot feeds on pollen and debris on leaves and does not cause harm to the corn plants themselves.  Because the spores of this fungus move via water (it’s closely related to oomycetes), numerous lesions can appear on leaves in bands or areas where water collects.  While the lesions may look like early southern rust, there will be no pustules present and often purple colored lesions will also be observed in the midrib, leaf sheath, stalk, and outer husks.

Differentiating Rusts:

2013-07-30 09.31.26

Southern rust in corn. We currently have not seen southern rust in Nebraska in 2014. Notice how you can see raised pustules in this picture compared to the photo of physoderma brown spot above.

When differentiating between southern rust vs. common rust, there are several criteria to consider and this NebGuide is a great resource.  Typically common rust will have brick-red pustules randomly scattered on the upper and lower leaf surfaces that are larger in size.  It is common rust that we are currently seeing in our fields.

Bacterial leaf blight showing up heavily in some hybrids.  The UNL Plant and Pest Diagnostic Lab is determining species and we will share more information in the future.  There is no control for this disease at this point.

Bacterial leaf blight showing up heavily in some hybrids. The lesions are red-brown in color, long and skinny and mostly vein-limited.  Older lesions spread outside the veins and are buff in color-sometimes they are being confused as gray leaf spot.  The UNL Plant and Pest Diagnostic Lab is determining species and we will share more information in the future. There is no control for this disease at this time of the season.

Southern rust in our area tends to have very small, raised, tan-orange pustules on the upper leaf surface of leaves in localized areas on mid-upper leaves.  These pustules are tightly clustered on the leaves. However, color and size are relative as sometimes the two diseases can look alike.  Microscopic observation is the best way to differentiate the two diseases.  Fungal spores from Puccinia sorghi causing common rust will be near perfect round circles whereas fungal spores fromPuccinia polysora will be oblong in shape.

We do have some gray leaf spot in the lower canopies and I haven’t seen much northern corn leaf blight in the fields.  But we do

have a bacterial leaf blight that is affecting quite a bit of leaf tissue on some hybrids.  These lesions are long and skinny appearing at first to be limited to the veins.  There’s been concern about these lesions being severe gray leaf spot but it’s not and there’s nothing you can do about the bacterial disease.  Please don’t mistake this bacterial disease as a fungal one and trigger a fungicide application too early.

Fungicide Application Timing

We tend to see southern rust in our part of the State each year; it’s a matter of time.  Triggering a fungicide application too early may result in no residual for when you need it if/when southern rust occurs.  Every year some producers make more than one fungicide application due to blanket applications at tassel or shortly after followed by another fungicide application when southern rust occurs later in the year.  Consider good fungal resistance management and apply fungicides when disease pressure warrants them in your fields and also consider economics for your situation for proper fungicide application timing.

Southern Rust

Several of us had been watching the USDA IPM Pipe Map for weeks.  It wasn’t showing southern rust moving and only Georgia2013-07-30 09.31.26 was really lit up.  Yet, I had heard reports in Texas and Oklahoma at one point.  Spots were evident on corn leaves when backlit-so they were bound to develop into something.

That something has showed itself to be southern rust last week as it was confirmed in 11 Nebraska Counties:  Kearney, Adams, Clay, Nuckolls, Thayer, Fillmore, Gage, Platte, Polk, York, and Boone.  I’d like to thank all the crop consultants and ag industry professionals for sharing information on what we all were seeing and for submitting samples.  2013-07-30 11.44.36

So the common question was, why didn’t the map show anything south of us?  Federal funding was no longer available for this site and scouting efforts associated with it.  Many Extension Plant Pathologists weren’t aware that the site was even still online and were sharing information via other means instead.  The map for Nebraska will continue to be updated, but for surrounding states, it is advised to consult with your local Extension Plant Pathologist.

Differentiating Rusts:

When differentiating between southern rust vs. common rust, there are several criteria to consider and this NebGuide is a great resource.  Typically common rust will have brick-red pustules randomly scattered on the upper and lower leaf surfaces that are larger in size.

For southern rust, we’re seeing very small, raised, tan-orange pustules on the upper leaf surface of leaves in localized areas on mid-upper leaves.  These pustules are tightly clustered on the leaves. Fungal spores of Puccinia sorghi, the pathogen causing common rust. However, color and size are relative as sometimes the two diseases can look alike.

Microscopic observation is the best way to differentiate the two diseases.  Fungal spores from Puccinia sorghi causing common rust will be near perfect round circles whereas fungal spores from Puccinia polysora will be oblong in shape.

Disease Progression:

With the cooler weather last week, we were unsure how the disease would progress. Southern rust likes sustained temperatures in the 80’s-90’sF with humidity and leaf wetness.  So we encourage scouting for it.

We saw how southern rust can be devastating to fields in the past regarding removing photosynthetic tissue leading to cannabalization of the stalk.  In determining a fungicide application, consider Physoderma brown spotdisease pressure in your field, stage of growth, pre-harvest intervals, and length of time for fungicide residual in addition to economics.

Look-A-likes:

Other plant samples brought in contained diseases such as physoderma brown spot (which isn’t a significant yield limiting disease of corn).  Because the spores of this fungus move via water (it’s closely related to oomycetes), numerous lesions can appear on leaves in bands or areas where water collects.  While the lesions may look like early southern rust, there will be no pustules present and often the purple colored lesions will also be observed in the midrib, leaf sheath, stalk, and outer husks.

%d bloggers like this: