Wheat Disease Update: April 20, 2013

Late infection of Barley yellow dwarf virus in wheat. Not the yellow to purple flag leaf.

First and foremost, keep in mind that fungicides should not be applied beyond flowering (Feekes 10.5.1).  I realize that a lot of applications are made at that particular growth stage for any number of reasons.  However, Fusarium head blight is the only disease with a label for such a late application timing.  For more regarding this particular disease see the information included below.

At present several diseases can be observed in the Mississippi wheat production system.  Remember, once the flag leaf emerges, keeping the flag leaf healthy is important when considering yield.  But, with that in mind, once you pass the growth stages for making a fungicide application the flag leaf can sustain a great amount of coverage before yield is significantly reduced.

Barley yellow dwarf

Some fields have started to show symptoms of late Barley yellow dwarf infection.  Late infection, at stages when the head has emerged or shortly before head emergence, will be indicated by yellowing or purpling of the flag leaf.  At this stage, no management practices are necessary.  The most difficult thing about this particular disease is the name.  The dwarfing of plants will generally only occur when infection occurs shortly after emergence of the plant.  In the years I have been in MS (now 6), I have only observed this in a single wheat field.

Leaf rust

Leaf rust has been observed at low levels in at least six counties to date.  Most situations where leaf rust have been observed have low numbers of pustules present on leaves in the lower canopy.  However, in one particular situation in southern MS a field of wheat was believed to not vernalize correctly.  The particular variety is reported as tolerant to leaf rust but since the plant has been stressed throughout the season tolerance for leaf rust has not been conferred and the infection level was observed to be high.  Scout fields for leaf rust and keep in mind in most situations the disease is present deep in the plant canopy.  Typically, even though leaf rust can be observed on an almost annual basis, leaf rust is not near as threatening to yield as stripe rust.  Specific information and photographs regarding the differences between leaf and stripe rust can be found in earlier posts (specifically: https://www.mississippi-crops.com/2013/03/30/wheat-disease-update-march-30-2013/).

Stripe rust hot spots in a field during the 2013 season. In some cases, yellow areas indicative of a hot spot can observed from the road.

Stripe rust

Stripe rust has not been as common an occurrence this season as opposed to the amount of stripe rust we observed last season (2012).  Some fields have hot spots (see attached photo) but the majority of the wheat fields in MS have not been infected by the fungus.  If you aren’t finding the disease in fields you are scouting you aren’t doing something wrong.  Generally speaking, low levels of the disease are present and detecting stripe rust in some fields may require a lot of time spent at a single location.  Presently, stripe rust has been observed in 17 counties throughout MS.  In some of those cases, a single leaf had several pustules present but in others, large hot spots have been observed in some fields even after having received a fungicide application.  In cases where hot spots persist following a fungicide application there was likely more stripe rust present than the fungicide cold effectively manage.

Septoria leaf blotch

I’ve had several calls over the past week regarding the presence of Septoria leaf blotch.  Generally speaking, Septoria leaf blotch is present low in the canopy and doesn’t affect the flag leaf.  However, I’ve seen some photographs from consultants and talked to some people on the phone that have indicated they have Septoria blotch on the flag leaf itself.

Septoria leaf blotch with a close-up of reproductive structures.

On an aside, in some cases I’ve seen photographs of small, almost circular leaf spots, with a maroon margin around a tan center.  Symptoms similar to this are not indicative of Septoria leaf blotch and are likely the result of numerous other maladies that could include herbicide drift or injury.  I’ve included a photograph of Septoria leaf blotch.  Keep in mind that the most mature lesions will produce dark, small, pepper grains that are the sexual reproductive structures of the fungus (pycnidia).  In addition, lesions are typically tan in the center and surrounded by a yellow margin.  Typically a fungicide is not necessary since the disease is normally present deep in the wheat canopy.  However, if lesions are identified on the flag leaf then a fungicide application may be necessary.  However, verify that Septoria leaf blotch is the disease present prior to applying a fungicide as there is a bacterial disease that can also look similar.  Bacterial diseases will not be controlled by a fungicide.

Fusarium head blight

Over the past few seasons I have fielded a tremendous number of questions regarding Fusarium head blight (FHB).  Sometimes the disease will be referred to as scab.  Historically speaking we have not had an issue with FHB in Mississippi; however, there was one year during the 1990s where FHB was a problem in the Mid-south but it has not been a recurring disease.  FHB requires an incredibly specific environment and is typically more of an issue when rainfall occurs during the time of flowering.  At present, most of the wheat in MS is not flowering and within the past week I have only observed a limited number of fields in the Delta that contained flowering wheat.

An internet based risk map tool for FHB is available at: http://www.wheatscab.psu.edu/ .  Basic information regarding the model and a user guide can be located by clicking on the tabs on the left hand side of the web page.  Generally speaking our risk for the occurrence of this particular disease has been low this season.  The information used to develop this particular predictive model are considered to be better than 75% effective at determining where FHB will likely occur as a result of specific environmental conditions that are necessary for inoculum production and plant infection.  However, one major issue is not considered on the website.  Crop growth stage (phenology) is not considered so be mindful that even though the risk of the disease could be high, if you are well before, or well after flowering (in certain circumstances) then you are out of the window of risk.  In addition, if you are going to rely on this particular website make sure you click on the “winter wheat” tab and not the “spring wheat” tab.  The particular “blight risk” will be completely different in the spring wheat situation.

Single white wheat spike that likely resulted from freeze injury rather than a disease such as FHB.

Keep in mind that even though FHB can produce white wheat heads as a result of infection, there are several other issues in our production system that can result in white heads.  Freeze damage, glyphosate drift, insect injury, or even rodent feeding at the base of the wheat plant can all lead to white heads.  Don’t jump to conclusions when scouting a field of wheat.  In the years I’ve been present in MS I’ve only come across the disease with any regularity in a single wheat field in east MS.  Moreover, wheat heads affected by FHB will generally be white and pink as the fungus that causes FHB produces pink to salmon colored fruiting bodies on the wheat head itself.

For management purposes, even though fungicides are labeled for application to manage FHB applying a fungicide may be only slightly better than planting a FHB-resistant variety and applying a fungicide.  But, with that said, I am not suggesting that anyone in MS needs to plant a FHB resistant wheat variety unless their particular farm has had a history of the disease or they have had continuous corn planted in a particular field that will be used for wheat production since corn is also a host of the fungi that causes FHB.  Fungicide ratings that have been provided by the NCERA-184 (wheat disease committee) made up of plant pathologist with wheat responsibilities from throughout the U.S. suggest that the fungicides labeled for use to manage FHB are all rated “good”.  No single fungicide product provides outstanding management of FHB.  In addition, research from the Midwest suggests that applying a fungicide that contains a strobilurin compound at flowering in a year when FHB can be a high risk can tend to increase the mycotoxin produced by the fungus.

Fungicide response and test weight

Several weeks ago I made a blog post regarding the results from fungicide trials conducted in Stoneville, MS during the 2011/2012 wheat season (http://www.mississippi-crops.com/2013/03/11/fungicide-applications-in-wheat-targeting-disease-to-prevent-yield-loss/).  Generally speaking the greatest yield response occurred when disease, in this case stripe rust, was present in plots.  Fungicides should be used to manage foliar diseases caused by fungi.  Results from university-based fungicide screening programs suggest that expecting “plant health” in the absence of disease following a fungicide application have produced sporadic results on this particular topic.  In addition, susceptible varieties with observable symptoms of disease likely benefit the most from a fungicide application.  More simply put, if a variety is tolerant to a disease such as stripe rust let the genetics of the plant manage the disease since applying a fungicide will likely not produce an added benefit.  Fungicide trials conducted last year utilized Georgia Gore, a public wheat variety that is susceptible to leaf and stripe rust.  In my program I tend to focus on the efficacy of the fungicides in the presence of disease.

Regardless of the year I hear people ask if a fungicide application will benefit test weight.  Last year in particular, at least in the trials I conducted in Stoneville, test weight was not affected by a fungicide application unless disease was present (see above link).  In addition, some test weight affects, specifically minor reductions in test weight compared to the non-treated, were the result of higher fungicide application rates.