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Allen, T.

Wheat Stripe Rust Detected in Mississippi

Current situation

Wheat leaf rust compared to stripe rust.

Wheat leaf rust compared to stripe rust.

As expected, stripe rust was detected in two wheat fields near Greenwood, MS by a Jimmy Sander’s representative on Monday (2/25/2013).  The positive identification of the disease occurred in the laboratory later that morning.  The mild winter temperatures and excessive rainfall has deposited spores into our production area.  Rusts, of numerous crops, are an annual occurrence in MS but the fungus that causes stripe rust, as well as leaf and stem rust of wheat, must blow into our production system from an overwintering source every year.  Historically, and specifically considering the observations from the 2012 wheat crop we are a little more than three weeks later than the first observation of stripe rust from 2012 (January 30, 2012).  Mississippi marks the fourth state this season that has detected wheat stripe rust (AR, LA, MS, TX).

The two wheat fields in question were between Feekes stages 6 and 7.  The easiest way to tell the two stages apart from one another has to do with the number of visible nodes.  Plants at Feekes 6 only have a single node visible, while plants that have reached Feekes 7 have two visible nodes.  Refer to some specific publications regarding growth staging wheat plants that can be obtained from numerous internet sources.  One of the best guides that I use regularly includes a diagram for each growth stage as well as a description and can be located at:

http://varietytesting.tamu.edu/wheat/docs/mime-5.pdf

Or, refer to one of the diagrams available on the MS Crop Situation Blog:

http://www.mississippi-crops.com/2012/03/12/identifying-wheat-growth-stages-using-the-feekes-scale/

Detecting stripe rust early in the season can be challenging.  Sporulating pustules are typically observed on leaves deeper in the canopy.  As the wheat plant progresses and the boot emerges (Feekes 10) and the subsequent heading stages begin (Feekes 10.1) stripe rust is more regularly observed on flag leaves.  Scouting for the disease during early Feekes stages (prior to 10.1) can be difficult simply due to the short stature of the wheat plants.  Crouching in the field and moving plants around to observe the leaves obscured in the lower canopy is one of the best methods of detection.

Determining the different rusts of wheat from one another

Stem rust of wheat, note rust pustules on the physical stem (or neck) and base of the head.

Stem rust of wheat, note rust pustules on the physical stem (or neck) and base of the head.

At the field level, determining the specific rust on a wheat plant prior to boot stages can be difficult.  Remember, wheat can be infected with three different rusts: leaf rust, stem rust, and stripe rust (or yellow rust).  All three of the rust diseases differ from one another but can be confused since they all three can occur on the leaves.  However, with that said, I have not personally observed stem rust on leaf tissue and have only observed the disease once in the six years that I’ve lived in MS and that was at a single location during 2012.  At this time I mention stem rust for a couple of reasons.  Firstly, stem rust has been a big story in the news for the past several years.  Keep in mind, the stem rust they are referring to, the “Ug99” strain, is only present in other parts of the world at this time.  Stem rust caused by the Ug99 strain of the fungus has not been detected in the United States.  Secondly, the majority of the wheat varieties planted in MS have some form of resistance/tolerance to stem rust so observing stem rust is a rare occurrence.  The location during 2012 that had observable stem rust was in an experimental variety.  Thirdly, stem rust is typically more of a concern during periods of warmer temperatures that tend to be higher prior to physiological maturity.  Based on the environment since January it is likely we will observe stem rust again this year.  As a final comment, stem rust is more of a concern in parts of the U.S. where the alternative plant host of the fungus, barberry, is present.  Be mindful, that in addition to producing sporulating lesions on the stem, stem rust can also produce lesions on the head and awns in extremely rare situations.  I have included some photos of stem rust.  In addition, there are some comments below since stripe rust is one of the diseases that will appear to occur on the stem of the plant.  Additional images of stem rust are included in the gallery and can be found across the internet.

Leaf rust versus stripe rust

Stripe rust of wheat.  Note aggregated pustules with spore masses.  In addition, note the slight formation of a "stripe" in the leaf tissue.

Stripe rust of wheat. Note aggregated pustules with spore masses. In addition, note the slight formation of a “stripe” in the leaf tissue.

Telling leaf and stripe rust apart from one another is most important during the season.  Generally speaking, leaf rust produces scattered pustules on leaves that oftentimes appear larger when compared with stripe rust pustules and the spore mass exuding from the shredded leaf material is more orange or russet when compared to stripe rust spore masses.  Stripe rust pustules typically occur in some sort of a pattern.  Early in the season, stripe rust pustules will be aggregated in a small area on the leaf and may not form the characteristic “stripe” that is oftentimes observed on the flag leaf.  In some cases, the stripe itself will not be covered with sporulating pustules but will extend beyond the pustules and turn the leaf tissue in that surrounding area yellow.  Compare the photos included.

In addition, one thing to keep in mind, just because rust becomes visible on what could be considered the plant “stem” understand that not all rust that occurs on the leaf tissue or whorl that is wrapped around the stem is stem rust.  I have observed stripe rust on the leaf whorl that wraps around the stem.  But if you remove the portion of the leaf from around the stem the rust only occurs on the leaf whorl itself and does not penetrate to the stem tissue.  Moreover, I have also observed stripe rust on parts of the head.  However, that was under extreme stripe rust pressure that occurred during 2012 and only at one location.  Conversely, stem rust will produce pustules on the stem itself and peduncle immediately below the head (see photo above).  In addition, the pustules that form on the stem as a result of the stem rust fungus appear to shred the stem material.

Adult plant resistance

One of the confusing subjects regarding early detection of strip rust has to do with whether or not a particular variety is resistant.  Early in the season, regardless of the specific resistance of most wheat varieties, plants can become infected by the stripe rust fungus.  As plants age, if they are resistant, the “adult plant resistance” will take over and the plant will no longer be susceptible to the fungus.  In particular, since we have had an abnormal winter, and most wheat fields appear stressed, this has likely contributed to some varieties becoming infected that are normally tolerant to the disease even as younger wheat plants.

Management alternatives

Management alternatives are limited if stripe rust infection has been observed in a field.  Fungicides are the only method to prevent yield loss but there are some caveats to that particular statement.  Research trials suggest that early fungicide applications may not translate into a yield benefit and a second fungicide application may be necessary depending on the environment encountered for the rest of the season.  In addition, susceptible wheat varieties typically respond differently to a fungicide application than do resistant varieties.  If adult plant resistance will take over, and the rust fungus has not been altered genetically – meaning the same race of the fungus is present now as when the variety was developed – then any infection that has occurred to date will likely not result in a yield reduction.  Keep in mind, with the excessive rainfall that we have had this winter in some particular situations the wheat plant may not be responding to nutrition in a normal fashion, especially if fields are waterlogged, so stress could play a compounding role in disease incidence and progression.  Moreover, prior to making a fungicide application keep in mind that a fungicide may not provide a benefit unless the plant has been provided the proper fertility.  Fungicides provide the most consistent economic benefit when applied in the presence of disease and will not account for a lack of nutrition.  Do not substitute a fungicide application for fertility.  In addition, waiting to see what sort of yield potential a particular wheat field could achieve is likely better than a “spray first” attitude.

 

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