Wheat Diseases to be Mindful of During 2015

Even though we have what could be considered to be a “late” start to wheat green-up, there are numerous disease issues that will likely be encountered in the coming weeks.  Scout wheat fields closely for the presence of foliar disease before applying a fungicide.  Numerous articles have been published on this blog related to the effect of a foliar fungicide in wheat production systems.  Generally speaking, a fungicide produces a more economical benefit when applied prior to disease occurring or at the earliest stages of disease development, rather than being applied as an automatic application.

For more information regarding fungicide applications as well as fungicide data sets from wheat foliar fungicide trials see:

https://www.mississippi-crops.com/2014/04/26/should-you-make-a-wheat-foliar-fungicide-application/

https://www.mississippi-crops.com/2013/03/11/fungicide-applications-in-wheat-targeting-disease-to-prevent-yield-loss/

For additional wheat topics not specifically considered below see:

https://www.mississippi-crops.com/2013/05/11/wheat-leaf-topics-and-physiological-leaf-spotting/

https://www.mississippi-crops.com/2013/04/02/important-notice-tebuconazole-label-restrictions-in-wheat/

Herbicide injury, note the roundish lesions surrounded with a halo. Thanks C. Adams for image.

Herbicide drift/injury         

Annually I observe a tremendous amount of herbicide injury on wheat.  Spray drift from adjacent fields, or even long distances, can result in spots/lesions on wheat leaves that look similar to disease or at the very least raise concern that a more prolific disease is imminent.  Depending on the herbicide product applied, either directly to the field or as drift, the lesions associated with the injury will appear quite different from the diseases outlined below and differ between products.  Generally, lesions will occur either in a small portion of the field along an edge or between trees (drift) or across an entire field.  The easiest way to verify whether disease is the culprit or herbicide injury occurred is to look for grassy weeds along the field edges or even within the field.  If the same symptoms are present on the grass blades as are observed on the wheat leaves then herbicide injury is the likely culprit.  In addition, if fungal reproductive structures (as described below for Septoria leaf blotch) are not present then herbicide injury can likely be considered to be the issue.  In the years that I have been with MSU, I have observed a good deal of herbicide injury shortly before corn planting when wheat is starting to break dormancy.  Moreover, it appears that since planting season has been substantially delayed due to prolonged cool and wet weather, I suspect over the coming weeks we may see more associated herbicide injury.  Keep in mind that small droplets can produce a fairly consistent round symptom with a halo on leaf edges as well as in the upper most canopy.  Observe if whether or not the lesions are produced on the most newly emerged leaves, so I suggest going back in a week to observe plants a second time.  Also, look for the presence of reproductive structures in the middle of lesions.  Fungal diseases, such as Septoria leaf blotch will produce fungal reproductive structures within the lesion that are generally observed without a hand lens.  In addition, keep in mind that with the forecasted cool temperatures that water droplets that freeze on the leaf for short periods can also likely produce a symptom similar to herbicide injury as well as disease.  Freeze injury can also produce injury to the leaf tip of a developing wheat plant that can easily be confused with something such as glyphosate drift.

Leaf and stripe rust comparison. Note the different pustule coloration as well as the size of the pustules present.

Leaf rust

Rarely has leaf rust amounted to enough of an issue to be associated with a yield reduction.  In general, leaf rust prefers cooler temperatures and once the environment warms up, leaf rust will generally not contribute to a yield reduction.  However, verify the response of plant wheat varieties to determine the overall disease reaction that may occur depending on the environment.  Leaf rust pustules are sporadic on leaves and generally do not have a pattern.  The sporulating pustules are generally a darker color than stripe rust pustules and more often than not the lesions are present throughout the canopy in tillering wheat, but can occur on the flag leaf itself in more advanced growth stages.  In addition, leaf rust pustules appear larger in size than those of stripe rust.  At present, leaf rust has not been observed in MS; however, as growth stages advance the occurrence of leaf rust will be more likely.  Leaf rust does not have the ability to survive in the U.S. and has to blow into the wheat production area from southern growing areas.  In general, environmental conditions that range from 59 to 77F are conducive for the development of wheat leaf rust.

Stripe rust. Note the elongating “stripe”. In some instances pustules will not be present on the entire length of the stripe.

Stripe rust

Over the past several seasons stripe rust has overwintered within MS and surrounding states and has threatened yield; albeit on a limited basis during 2014.  The common symptoms associated with stripe rust include thin, yellow to light orange “stripes” that generally occur in a pattern along the veins on the wheat leaf.  In some cases, the “stripe” will extend beyond the pustules (see photo at left).  In situations where the stripe continues beyond the pustules, pustules may not be present throughout the entire length of the stripe.  The coloration of the sporulation that develops on the leaf within the stripes is generally yellow to light orange (see comparison photo above).  A yield reduction can occur in stripe rust susceptible varieties.  However, fungicide application, if made prior to the pre-harvest interval can reduce the yield loss potential associated with the disease.  In situations where stripe rust has overwintered in a field, and hot spots of the disease can be observed, large, yellow areas that contain infected plant material can be observed scattered within a field.  However, not all yellow areas in a field are caused by stripe rust infection and should be scouted closely since standing water or nutrient deficiencies are the two most common culprits of yellow areas in a field.  Stripe rust favors a temperature range similar to that of leaf rust; however, strains of the fungus can be observed in situations where the environment reaches 90F.

Stem rust. Note the rust pustules on the physical stem of the wheat plant.

Stem rust

Much has been made in the news media regarding stem rust and the ability of the fungal organism to overcome the varietal resistance that is commonplace within commercially available germplasm.  The majority of the wheat varieties planted in the U.S. contain some level of resistance to the stem rust fungus.  However, the resistant strains of the fungus (Ug99) have only been observed in Africa and the Middle East and as of yet are believed to not likely have the ability to spread to the western hemisphere based on wind current models.  With that in mind, stem rust is a rare disease.  I doubt that I have observed 20 plants infected with the disease in the eight years I have been in MS.  Stem rust typically prefers warm environmental conditions (between 77 and 86F) and even in years when I have observed the disease there are typically only a few scattered plants infected in the variety trial.  Keep in mind, that stem rust will generally produce pustules on parts of the plant that are not normally infected by leaf or stripe rust.  But, with that in mind, stripe rust can in rare instances produce pustules on the leaves that are wrapped around the stem as well as on the awns.  However, I have only observed one year (2012) where I could find stripe rust on the awns or on the leaf material wrapped around the stem.  Stem rust can produce pustules on the leaves, but telling those apart from leaf or stripe rust is extremely difficult.  When most obvious, stem rust will occur on the peduncle, immediately below the head, or on the exposed stem immediately below the head (see photo at left).

Powdery mildew.

Powdery mildew

Powdery mildew is generally a disease of more concern than actual yield reduction.  The symptoms associated with powdery mildew include white to yellow tufts of fungal growth on the leaf surface.  In general, powdery mildew prefers wheat leaves lower in the wheat canopy and rarely occurs in the upper canopy.  Densely planted wheat fields or wheat fields where the edges have been double-planted are generally the area where powdery mildew is observed.  However, old catfish ponds or fields that received excess nitrogen fertilization can also be observed to contain plants infected with the powdery mildew fungus.  Protecting the upper canopy, in particular the flag leaf from powdery mildew is the most important step since powdery mildew can reduce photosynthesis of infected leaf material.  However, in the years that I have been in MS I have not observed powdery mildew close to the flag leaf.  Even though fungicides are labeled for powdery mildew management rarely has the disease occurred to a level in a wheat field whereby a fungicide application was suggested to prevent yield loss.  Scout fields carefully for the presence of the disease paying attention to tree lines as well as more densely planted wheat.

Septoria leaf blotch. Note the elongated lesions with small, pepper grains contained within the leaf tissue.

Septoria leaf blotch/spot

Generally speaking, Septoria leaf blotch is present in the lowest parts of the plant canopy, especially early in the season.  In the years I have worked wheat in MS I have not observed Seprotia leaf blotch on the flag leaf.  However, over the years I have seen photographs from consultants and talked to people on the phone that have indicated they have Septoria blotch on the flag leaf.  At present, in 2015, I have only observed one field with minimal Septoria leaf blotch on a few scattered leaves.  But given the extended period of wet weather encountered this season I would scout carefully for the presence of this disease as wheat progresses.  The most mature Septoria leaf blotch lesions will produce dark, small, pepper grains that are the sexual reproductive structures of the fungus (pycnidia) within the leaf lesions.  Leaf lesions generally tend to be more irregular-shaped than circular.  Lesions are typically tan in the center and surrounded by a yellow margin that can in some instances be brown to maroon.  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 economically beneficial by protecting yield.  But, 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 (see below).  Bacterial diseases will not be controlled by a fungicide.

Tan spot of wheat. Note the yellow halo around the lesion as well as the dark spot in the center denoted by the arrow.

Tan spot

In general, tan spot can be considered a rare disease, at least in MS.  In fact, in the eight years that I have been in MS I have only observed this disease two times and both of them were in the same field in consecutive seasons.  Tan spot generally occurs in fields with a prior history of wheat production.  Symptoms associated with tan spot appear similar to paraquat injury (herbicide injury in general) as well as Septoria leaf blotch.  However, the absence of the pepper grains inside the lesion is the main difference between Septoria leaf blotch.  Moreover, tan spot lesions tend to be tan in coloration, with a yellow halo around the lesion.  The lesions are generally diamond or oval-shaped and can be as long as ½ inch; however, not all diamond-shaped lesions are the result of tan spot as I have observed herbicide injury to produce similar symptoms depending on the environment at the time of application or product used.  The most distinguishing characteristic is when the leaf is held up to the light.  A small, dark spot is normally present in the center of the lesion.  The pattern developed by the small dark spot in the center with the tan lesion surrounded by a yellow halo produces an “eyespot” appearance.  Keep in mind that paraquat injury can look similar.  However, the lack of yellow halo as well as dark spot in the middle of a paraquat-induced lesion should help tell the two apart.

Bacterial leaf streak of wheat.

Bacterial leaf streak/black chaff

In the recent past, when wheat heading stages have occurred during periods of cool, wet weather, bacterial leaf streak has been observed throughout the wheat crop.  When the bacterium infects leaf tissue the disease is recognized as bacterial leaf streak.  However, if the bacterium infects wheat heads the disease is called black chaff.  In general the two diseases occur following freezing temperatures with excessive moisture after the head has emerged from the boot.  Once infection occurs the diseases can be easily confused with Septoria leaf blotch or even stripe rust if the lesions on leaves are not carefully observed.  Leaf lesions will range in size from a half an inch long to several inches in length when they coalesce, but this will generally depend on whether or not the conducive environment has occurred for an extended period of time.  Infected heads will appear to have a purplish to black discoloration.  In the past I have observed situations where wheat stands with variable plant growth stages have appeared to contain more black chaff due to the emergence of the head at the time of the proper environment for disease to develop.  Once infection occurs no treatment is necessary as the disease is caused by a bacterium and a foliar fungicide will not provide disease management of a bacterial disease.

Fusarium head blight (FHB or scab) can occur on a part of a wheat head and not the entire head. A pink hue, caused by fungal sporulation, is oftentimes indicative of FHB. However, note that other issues can result in white heads as well.

Fusarium head blight (scab)

A tremendous amount of information has been disseminated in the Farm Press and on the internet regarding the importance of Fusarium head blight, or scab.  Frankly, scab is a much greater concern in the upper Midwest.  The biggest concern related to the presence of scab has to do with the toxin that is associated with the fungal infection.  The toxin is sometimes referred to as a vomitoxin and is more specifically called deoxynivalenol (DON).  The toxin itself is similar to aflatoxin and can result in dockage at the elevator or denial of a load of grain based on the concentration of the toxin present in the harvested grain.  However, the environment can dictate whether or not this particular disease occurs and there have been years in the recent past when Arkansas has observed substantial scab throughout the state so it is not a disease to ignore.  Environmental conditions at the time of flowering (Feekes 10.5) generally determines whether or not scab will occur.  Rain received at the time of flowering can increase the likelihood of scab occurring, but there are additional environmental conditions necessary for the disease to occur.  Scab can result in sterile wheat heads or heads that will appear white.  But, the issue with this particular symptom of the disease is that there are several other issues that can result in white heads including freeze injury, glyphosate drift/injury, or even something as simple as a rodent feeding on the main stem at the base of the soil line can result in white heads.  When environmental conditions remain conducive for extended periods of time, the fungus that causes scab can sporulate on the heads.  When sporulation of the fungus occurs the head will develop a pink discoloration as a result of the sporulation.  The best way to determine the outlook for scab is to follow the predictive model that can be located at: http://www.wheatscab.psu.edu/.  The model will become active on the website on April 1, just in time for MS wheat farmers to start determining if scab will be a threat during 2015 or not.  Fungicides are effective management tools to reduce the risk of scab.  However, a fungicide application needs to be made immediately prior to flowering so as to reduce the risk of infection.  But, a word of caution should be made regarding the predictive model mentioned above.  Mississippi has a low number of weather stations that feed data into the predictive model.  Essentially gaps in the monitored environment exist and can result in either a false positive (disease being predicted in an area where it will not occur) or false negatives (disease not being predicted in an area where the disease is later observed).  In the years since I started in MS, I have observed more scab and heard more reports of scab from the northwestern part of the state around Tunica, MS.  In addition, be mindful that some of the better products to manage scab do not contain strobilurin (quinone outside inhibitor, QoI) fungicides since some research has indicated that the QoI fungicides, applied at specific growth stages, can in fact result in an increased chance of developing the toxin associated with scab (DON).  Moreover, there are restrictions on the use of fungicides that contain tebuconazole.  A total of 4 fl oz/A of tebuconazole can be applied to wheat in a single year.  Therefore, fungicide applications with a stand-alone tebuconazole product (e.g., Monsoon) or prothioconazole + tebuconazole (Prosaro) should be monitored to reduce the risk of applying too much tebuconazole in a single season.  For additional information on this situation see: https://www.mississippi-crops.com/2013/04/02/important-notice-tebuconazole-label-restrictions-in-wheat/

Volunteer wheat plant likely infected with Barley yellow dwarf virus. Note the overall yellow appearance of the plant next to green “new” wheat plants.

Barley yellow dwarf virus (BYDV or BYD)

A tremendous amount of confusion surrounds Barley yellow dwarf virus (BYDV or BYD).  The name itself is indicative of the symptoms of the disease that are generally observed if infection by the virus occurs shortly after the wheat plant emerges through the soil surface.  The symptoms associated with BYD are generally confused with other, more commonly occurring maladies such as nutrient deficiencies or even standing water.  BYD is more commonly observed after the flag leaf has emerged.  However, the earliest infection of BYD, which typically occur in the fall, can result in stunted, more yellow plants.  In fields where volunteer wheat is present, BYD can more regularly be observed in a few scattered plants.  Aphids (bird cherry oat aphid among others) move BYD from field to field; however, not all plants that contain aphids will present symptoms of the disease.   In the eight years that I have lived in MS I have only observed three entire plants in stages of development prior to head emergence where I thought BYD was the culprit.  However, once the head emerges, late-season infections can result in yellow to purple flag leaves that are indicative of a late infection (post Feekes 10) due to aphid feeding.  Once the disease is observed at this stage it is not only too late to make an insecticide application, but other remedies to affect the wheat are not available.  Generally speaking unless the virus infection occurs early in the stages of wheat development the potential for yield loss associated with BYD is extremely low.

The more often observed symptom associated with BYDV, purpling flag leaves. Ranges of color can develop on the flag leaf from yellow to purple to almost orange.