Corn Disease Update: June 18, 2016

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist June 18, 2016 07:00

Corn Disease Update: June 18, 2016

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Common rust has been more regularly observed this season.

Common rust has been more regularly observed this season.

Two words can describe the corn growing season to this point: clean and quiet. Most of the corn I have looked at over the past couple of weeks has been free of disease, for the most part.  In addition, most of the calls I have received have been about common rust, whether or not southern rust has been observed in the state, or what I thought about automatic fungicide applications.  I will cover each of these below.

Common rust

Due in large part to cooler temperatures until the last week, more common rust has been observed in this corn crop than any of the past nine years I have been in MS. In general, common rust can occur from the bottom of the canopy to the uppermost leaves.  More often than not, the pustules can form a slight pattern, appear to produce more maroon-colored spores, and the pustules can be substantially larger than those formed by southern rust.  In addition, old common rust lesions can appear similar to other lesion-forming diseases given a period of time after the pustule ruptures.  At this point in the season, and with the entrance of warmer (more normal temperatures for this time of the year) temperatures, common rust has almost played out.  The disease is not going to spread and cover the plant.  In fact, the pustules that are observed now are the result of infection that occurred several weeks ago.

Keep in mind that pustules of common rust can be observed on both sides of the leaf. However, with that particular statement that does not mean that will always be the case and there have been instances when southern rust has been observed on the underside of the leaf, but that was a rare instance.  Observing spores of the fungus and comparing the shape and size is the best way to determine which rust is present.  However, common rust tends to be observed near the base of the leaf and can have a tendency to appear in lines or a general pattern across the leaf.  But, over the last week I have received calls regarding common rust near the leaf tips.  The pattern of pustules and tendency to form near the stalk or base of the leaf are both a result of the timing of infection.  In most cases, infection occurred when the leaf was still in the whorl and the pustules observed now are the result of infection that occurred several weeks ago.  Common rust pustules in the lower part of the canopy can appear smaller in size and the coloration can tend to be closer to southern rust.  I have always felt this was a result of shade and a more conducive environment.

My biggest suggestion about common rust is to let the environment manage the disease and to not bother to make a fungicide application to prevent spread. In the years since I arrived in MS this disease has not reduced yield.

Northern corn leaf blight

To date I have not received a telephone call regarding NCLB and have only observed a single field with lesions south of Natchez at the Adams/Wilkinson county line on Highway 61. I do realize that some years this disease will look bad in some fields.  However, I am almost convinced that this particular disease does not like our environment (warmer daytime temperatures) and appears to struggle in our production system.  Lesions can expand at night, in part due to cooler temperatures, but the corn crop tends to outrun yield loss.  Be mindful that foliar fertilizer applications with some products can produce lesions similar to NCLB.

Corn standability is not general impacted by southern rust infection as indicated in this photo 18 days post treatment in a field situation with southern rust infection at the time of fungicide application.

Corn standability is not general impacted by southern rust infection as indicated in this photo 18 days post treatment in a field situation with southern rust infection at the time of fungicide application.

Southern rust

As of Friday, June 17, 2016, southern corn rust has only been confirmed in Georgia and Louisiana. Two parishes, one in northeast LA have reported small outbreaks of southern rust.  Outside of those two states southern rust has not been observed in MS; however, I have heard rumors that southern rust has been observed in certain parts of the state.  Keep in mind that when a lot of common rust covers the leaf the first reaction is to call the disease southern rust.  If you think you have some southern rust or would like someone to confirm the presence of the disease in your corn please do not hesitate to call.

To stay abreast of the southern rust situation in the United States please refer to:

http://scr.ipmpipe.org/cgi-bin/sbr/public.cgi

The map at this website is similar to the one that has been used for soybean rust. Areas in MS have been scouted over the past several weeks and so far no southern rust has been observed.  Early in the season one of the best plants to look at tends to be sweet corn since there is absolutely no resistance to southern rust and the growing season tends to be shorter in sweet corn.

One last thing about southern rust. Towards the end of the corn growing season many people will make claims that “corn will lodge in the presence of southern rust.”  In addition, I tend to receive numerous questions regarding whether or not corn will die in the presence of southern rust.  Last year I was able to make some late (dent) fungicide applications.  I made observations 18 days post-application and the corn had not lodged as a result of southern rust (see photo).  Many people in the corn industry have stated that for southern rust to produce a situation where severe lodging could result from infection, the infection would likely need to occur prior to tassel (VT) so the plant had enough time to cannibalize itself.  In the nine years I have been in MS I have not observed this situation, nor have any of my colleagues in states from LA to MN.

Automatic fungicide applications

Based on years of research trials conducted at many locations in MS the MSU-ES still suggests to wait until a yield-limiting disease is observed in field corn rather than making an automatic fungicide application. Based on the general age of the corn crop in western MS most of the corn crop is beyond the tassel growth stage.  However, corn in east MS ranges from V7/V8 through some earlier growth stages.  Save a fungicide application for if and when southern corn rust might threaten this season.  But, with that said, do not make an application unless southern rust has been confirmed in your corn and base the application on:

-expected yield

-growth stage at observation of the disease

-number of weeks until black layer

-irrigation method

-overhead irrigation could increase disease spread within a field as compared to

furrow irrigation

Save your money, this is going to be a long season and with the reports of southern rust from the region, southern rust has been confirmed at least a month earlier in LA. I am not making this statement to alarm anyone, but as a realistic response since commodity prices are lower than they have been and fungicides provide a good yield-loss prevention tool when needed rather than providing an automatic yield increase in the absence of disease.  In my almost 10 years in MS I have observed fungicides to provide a good yield response only when disease was present.  I base that statement on well over 2,000 plot observations since 2007.

 

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist June 18, 2016 07:00
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