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2015 Wheat Disease Updates

Wheat Disease Update (6/19/15)

from Jeanne Falk Jones, K-State Multi-County Agronomist

I realized that there were points that I left out on Fusarium Head Scab. One key characteristic to identifying head scab is the white heads or portions of the head being white, with orangish – pink colored fungal growth on the outside of the glume. I have attached a picture of a head that I took out of dryland Winterhawk today. It has the orange fungal growth on the glume and where the mesh hooks onto the rachis (stem).

The seeds inside these infected mesh are many times shriveled and can be covered with additional fungal growth. I attached a picture of a kernel that was out of the head in the previous picture. It looks a bit furry. As they mature, they begin to have a chalkish white appearance. These kernels are also lighter and can be blown out the back of the combine at harvest.

 head scab on kernel

There are questions about saving seed from wheat infected with head scab. The wheat can be saved, but should be definitely be cleaned. In addition, it should be treated with a fungicide seed treatment. The seed treatment is a recommended practice because the head scab actually affects wheat at two stages of growth. One is at flowering, which is what we are dealing with now. The other is in the fall at seedling emergence. Seedlings that become infected are more susceptible to seedling blight. However, seed treatments will only protect in the fall and not protect against infection at the flowering stage.

Finally, there are also concerns about the quality of the grain. Wheat infected with head scab may contain mycotoxins DON (vomitoxin) and zearalenone (an estorogen analog). Neither of these are highly toxic, but can reduce performance when fed to livestock. Incidentally, some grain elevators test for wheat infected with head scab.

Wheat Disease Update (6/18/15)

from Jeanne Falk Jones, K-State Multi-County Agronomist

I bet you thought you were done hearing from me on happenings in wheat, but not quite yet! We have a few things still showing up in wheat, even as we are getting closer to harvest. These are being found in the head...head scab and black chaff. Both of these diseases are generally found in irrigated, but I am finding them in dryland this year.

Head scab is a fungal disease that is generally found in irrigated wheat drilled into corn stalks. This is because the disease that causes fusarium stalk rot (turns the inside of the stalks pink with rot) also causes fusarium head scab. Head scab is characterized by part (or in some cases most) of the head being white. The kernels inside the white part of the head are likely beginning to shrivel or have fungus growing on them. You may also notice an faint orangish-pink color on the outside of where the kernel is being formed (glume).

The infection of this disease happens at flowering. Basically spores are floating around in the air and need moisture to infect the pollinating flowers. We had the moisture this year and apparently an abundance of head scab spores. I did not anticipate this disease showing up in places other than wheat following corn, but this year it seems to be. Yesterday, I looked at irrigated wheat following soybeans and dryland wheat in a 3 year wheat-corn-fallow rotation that both had head scab. There were corn stalks very close or adjacent to the fields that may have served as the source of the head scab inoculum.

I have attached a picture of what I found yesterday. Here is a link to an eUpdate article that discusses this disease a bit more: http://ksu.ag/1d69ZP0

 head scab

This disease is generally controlled by fungicide application. Now, you may be thinking that you sprayed fungicide for stripe rust. Many of the same fungicides that control stripe rust also work on head scab. The fungicide application needed to be applied after head emergence and prior to flowering. The fungicide only protects the parts of the plant that it is applied to because it has limited translocation (same as why we recommend treating for stripe rust after the flag leaf has emerged to protect the flag leaf).

The second disease that I am getting questions on is black chaff. This is a bacterial disease that causes the glumes around the kernels to turn a dark purplish-black color. There are also purplish-black lesions on the stem and dark colored bands/striping on the awns of the infected head. In many cases, you can also see brownish-colored streaks on the leaves...although this is difficult to see this year because of everything that has attacked the flag leaf. I have pictures of all of these symptoms attached to this email. The purpling on the heads is generally what is first noticed.

 black chaff and kernel under microscope

Since this is a bacterial disease, fungicides do not control it. In fact there is nothing that can be done with this disease. It is also a disease that likes wet environments. I have found it on both in dryland and irrigated fields. The bacteria enters the plant through openings (through wounds on the plant or even through the stomata – where gas exchange happens).

 black chaff head and stem

 

Here is a link to more in depth information from University of Nebraska on black chaff: http://www.ianrpubs.unl.edu/epublic/live/g1672/build/g1672.pdf

There are other things that can cause white or off colored heads. Freeze can cause white heads. If the heads are affected by freeze, there will be no kernels in the head. This is because freeze affects the pollination – an no pollination means no kernels. I think we may also still be seeing effects of the winter injury from the cold November temperature drop. I think this because I am finding tillers of plants that have put on heads and in some cases the heads are partially filled – but the tillers have now died. They also easily detach from the plant at the soil surface. That makes me think they were injured from the cold temperatures and rot set in (with the moisture) to finish off the tiller.

All in all, we have some really nice wheat across the area. Lately while looking at wheat, I've realized that it wasn't that long ago I was planning for an early harvest because of dry conditions. What a difference a bit of time (and rain) makes!

 

Wheat Streak Mosaic Update (May 19, 2015)

from Jeanne Falk Jones, K-State Multi-County Agronomist

As everyone is out looking for stripe rust in their fields, wheat streak mosaic is also being found.  The wheat streak mosaic is characterized by yellowing plants, with leaves that have a mottled yellow and green appearance.  In many cases, the virus showing up in patterns that we are most accustomed to.  These include increased severity on one side of the field, where it seems all plants are infected.  The severity of the symptoms generally decrease as you move across the field, away from the source. 

The source is generally thought of as volunteer wheat because it harbors the wheat curl mite.  The wheat curl mite is a tiny insect that moves the wheat streak mosaic virus.  As it feeds on new wheat material, it transfers the virus to the plant.  There are other plants on which the wheat curl mite can live.  These include corn and several grasses found in the road ditches – including foxtails and sandbur.  These tiny mites need green material to live on and die after just a few days if they do not have the green tissue.  There was also no shortage of volunteer wheat last summer, likely because of the hail events in June and very early July.  The mites also rely on wind to move them to their next food source. 

The thing that is challenging this year is the fact that the patterns in the field do not match up with what we typically think of as wheat streak mosaic patterns.  It seems the most common pattern in the field is really no pattern at all, but randomly scattered groups of plants with the mottled green and yellow symptoms on the leaves.  These plants are also fairly easy to see because the yellowed plants stand out against the other green, healthy plants. 

Upon visiting with both an entomologist and plant pathologist, it seems the cause of the scattered plants is likely related to a heavily infested, but distant source of the wheat curl mites.  The wheat curl mite population diffused as it moved further from the main source.  With the long, warm fall the wheat curl mites would have had ample time to move around and infect wheat plants.  In addition, there is likely something that may have prevented the population from getting well established in the wheat.  It is possible that the cold snap in November may have helped suppress the mite population or prevent it from getting established in a field. 

In addition, there is a lag period between the infection of the wheat streak mosaic and the expression of the symptoms of the virus.  The virus must reach a certain level within the cell before the symptoms will show on the leaves.  The virus also replicates at different rates, based on the growth stage of the wheat plant and the temperature.  The rate of virus replication increases with increasing temperatures.  That is why it seems that the symptoms started showing up after some rain (wheat started growing) and warmer temperatures (virus increased in rate of replication).

Finally, there is nothing that can be done to treat the wheat streak mosaic virus.  Once it is in the cells of the plant, there is nothing that can be done to control the virus.  Minimizing stress on the plant is the best medicine to minimize the effects of the disease. 

 

Wheat Stripe Rust Update (5-15-15)

from Jeanne Falk Jones, K-State Multi-County Agronomist

You haven’t heard from me for a few days and there are a couple things that I need to bring you up to date on.  Stripe rust is still being found in nearly every field.  In many cases it is progressing up through the canopy.  It is beginning to be found on the flag leaf in some fields.  As I looked at wheat today, some of the pustules erupting on the flag leaf are at the blister stage (although others were a bit further along and releasing spores).  This ‘blister’ stage is just before it breaks through the cuticle of the leaf.  The pustules will be slightly raised and will be shiny.  Also, if you run your finger across them, it will not turn orange because the spores are not yet being released. 

 

The stage of these pustules correspond to the foggy rainy day last Tuesday.  This is because the time from the spore infecting the leaf to the pustule releasing spores can range from 7 to 14 days.  In addition, the 10 day forecast looks fairly conducive to the stripe rust with rain chances and fairly cool temperatures.  The stripe rust will continue to be active because we do not have warm enough temperatures in the forecast.  We need overnight lows above 60 to shut down the stripe rust.

 

So how do you make the decision to spray or not to spray fungicides?  Look at the yield potential of the field and the cost of spraying.  From some information that I received from Eric DeWolf (K-State Wheat Pathologist), if the stripe rust is in the upper canopy before/at flowering, spraying will most likely result in a 10% yield response or greater.  For 30 bushel wheat, that would likely be a response of 3 bushel or more.  If it is in the lower canopy, spraying with a fungicide will only result in the desired yield response 50-60% of the time. 

 

I am getting the question of: If it’s on the flag leaf, is it too late to spray for stripe rust?  Not necessarily.  If nearly every flag leaf is severely infected, then it may be too late.  In most of the fields that I have been in over the last 2 days, that is not the scenario that I am seeing.  It is getting much easier to find, but is not being found on every flag leaf. 

 

By spraying a fungicide, it will not green back up the area infected with stripe rust.  Also, in my mind, I want the fungicide to work like paraquat on weeds.  Whatever it touches, I want it to control it IMMEDIATELY.  That is not necessarily the case with fungicide.  The stripe rust may continue to progress a bit after the fungicide is sprayed, however it will be suppressed.  From how I understand how a fungicide works, some of the rust pustules will be too fully developed for the fungicide to prevent it from erupting from the leaf surface.  However, it will slow it down and suppress the population.  Also, some fungicides will prevent the spores from getting into the leaf and others control the spores after they infect the leaf…this is a difference of likely less than 24 hours.  We often hear of ‘preventative’ and ‘curative’ fungicides like they are mutually exclusive modes of action, when in reality the preventatives have curative properties and visa versa.     

 

When making the decision to treat, be mindful of the preharvest intervals.  For some products, it is wheat growth stage Feekes 10.5 (beginning flowering) and for others is it 30 to 40 days prior to harvest.  In addition, I feel very comfortable with generic fungicide products.  From everything that I have seen, there seems to be no difference in the efficacy of the products or in the residual (in general 14-21 days of residual).  In addition, the generics are easier to ‘pencil out’ because of the lower prices.

 

I have attached the K-State Foliar Fungicide Efficacy Ratings publication.  This outlines the different fungicide products, their efficacy on stripe rust and the pre-harvest intervals.  I also have pictures of the progression of stripe rust on my K-State Sunflower District Agronomy page at: http://www.sunflower.k-state.edu/agronomy/wheat/stiperust.html

 

Wheat Stripe Rust Update (5-5-15)

from Jeanne Falk Jones, K-State Multi-County Agronomist

There are lots of questions surrounding stripe rust.  Is it here? How bad is it? Will it progress? Is my variety susceptible?  Do I need to spray a fungicide? So let’s try to address those questions. 

‘Is it here?’  Yes, I am finding stripe rust in basically every field that I check.  To know if you have stripe rust, look for raised orange pustules on the leaf surface.  You can run your fingers over the area to feel the slight raises on the leaf surface and your fingers will be orange from the rust spores rubbing off the surface.  Many times the stripe rust will occur in a longitudinal ‘striped’ appearance following the veins on the leaf.  However, the stripe rust can be found clear across the leaf surface.  There are pictures for helping identifying it here: http://bit.ly/1JoKJOv

‘How bad is it?’ And ‘Will it progress’ are questions that go hand in hand.  Right now the stripe rust is being found on one to three leaves below the flag leaf.  This is important because it is not yet being found on the flag leaf.  The flag leaf is important to protect because between 60 and 80% of the nutrients that go into the head, come from the flag leaf.   About a week ago, it was being found in trace levels, meaning that I had to look for quite a while to find a group of stripe rust pustules.  Now, it is getting easier to find. 

The next question is what is the future of this disease – will it get more severe?  That is really the million dollar question.  As I am writing this article it is Tuesday afternoon.  It was very foggy this morning and is now misting/raining and cool outside.  These are the conditions that stripe rust loves.  It really likes cool temperatures – with overnight lows from 40 to 60 degrees – and it takes moisture for the rust spore to infect the leaf.  The forecast is for these type of symptoms to be around for the rest of the week. 

Now, ‘Is my variety susceptible?’  It appears that the race of stripe rust is different than the last couple years.  Stripe and leaf rust are notorious for shifting the race of the disease to overcome the resistance in popular varieties.  As of now, we know Everest, Armour, TAM 111, and TAM 112 are susceptible.  However, stripe rust is also being reported on varieties with intermediate or moderately resistant reactions to the disease, including WB4458, WB-Cedar, and 1863.

How do you make a decision on treating with a fungicide?  This is really a tough question with the wide variety of potential wheat yields across the area.  Depending on the cost of fungicide and application, in some cases, it will take 2 ½ to 3 bushels of wheat to pay for the fungicide application.  In other more costly applications, it will take around 5 bushel per acre. 

Research done at K-State suggests that the yield response for stripe rust can be more than 20% when conditions favor disease development on susceptible varieties.  Fungicides applications are most likely to result in a 10% yield response or greater if stripe rust or other diseases are established on the upper leaves prior to flowering. If the disease is only present in the low to mid-canopy at these growth stages, a fungicide application will only result in the desired yield response about 50-60% of the time.  This is very dependent on the weather forecast for the next 10-14 days.

If you are planning to spray a fungicide, make sure the flag leaf has emerged.  Fungicides will move around within a leaf, but will not move from leaf to leaf.  So, to ensure the flag leaf is protected, make sure the flag leaf is exposed to the fungicide application.  In addition, there are lots of products that are available to farmers that have very good to excellent efficacy ratings.  There are some important differences in cost to consider. The cost of a fungicide treatment (product + application costs) generally ranges between $10-$25 per acre. Products containing the active ingredient tebuconazole, or propiconazole are often the least-cost options and have high efficacy ratings. 

 

Wheat Rust Update (5-2-15)

from Jeanne Falk Jones, K-State Multi-County Agronomist

I have been checking fields all of this last week and have been getting reports in from others checking fields.  So, the short answer is that stripe rust is being found throughout the area – HOWEVER – in trace levels.  I have found it in Thomas, Rawlins, Cheyenne, Sherman and Wallace Counties.  In many fields, it is not easy to find.  For example, I spent over 30 minutes in a circle of irrigated wheat to find only three clusters of stripe rust pustules.    

 

In addition, the rust is being found in the lower part of the leaf canopy.  It is on 2 to 4 leaves below the flag leaf.  This is important because the flag leaf (or even one below the flag leaf) has not been affected.  In some cases, the rust pustules look like they are just under the surface of the leaf (shiny appearance) and have not yet erupted.  In other cases, the pustules are further through their development and have released the spores (more fuzzy appearance).

 

The conditions that are conducive to the development of stripe rust include moist conditions and temperatures between 50 and 60 degrees.  The recent rainfall events and moist, foggy days have favored rust development.  In addition, it seems that thick irrigated wheat would have the best situation for the development of stripe rust because in many cases the canopy is very slow to dry out after being watered or a rain shower.  The irrigated wheat is the wheat that I have been checking the most…mostly because I think that is where I will find the stripe rust first.

 

If we get to a place to start treating rust (which at this point is a big IF), it is very important to wait until the flag leaf has fully emerged.  The reason for this is because the fungicide will move around in the leaf that it is applied to, but will not move from one leaf to another leaf.  It is important to protect the flag leaf.

 

All in all, I think we will be able to draw more conclusions as we move into next week.  In all reality, if stripe rust was moved north with the storm system two weeks ago, we would just now be starting to see the symptoms because it can take up to 2 weeks for the pustules to show up after the spores have infected a cell.  In addition, we still have time to make any decisions about treatment, IF we arrive at the treatment threshold.  Many fungicides must be applied just prior to flowering. 

Also, there is a K-State Agronomy eUpdate from Erick DeWolf, wheat pathologist, that was released on Friday, May 1: http://ksu.ag/1EC5WmN

 

Wheat Rust Update (4/18/15)

from Jeanne Falk Jones, K-State Multi-County Agronomist

I have been getting a few questions about leaf and stripe rust and the likelihood that we will find it in northwest Kansas.  Follow the links for maps for stripe ( http://www.arcgis.com/home/webmap/viewer.html?webmap=aa8bcfd29fe94fd1ad397992ae76c3a1&extent=-130.5965,13.5965,-75.8407,54.784 ) and leaf ( http://www.arcgis.com/home/webmap/viewer.html?webmap=2c390053d2ca4cb3a2f333091bcdf671&extent=-125.2482,13.2906,-70.4924,54.6022 ) rust observations and a report from yesterday from the USDA ( http://www.ars.usda.gov/SP2UserFiles/ad_hoc/36400500Cerealrustbulletins/15CRB2.pdf ) . 

Up to this point, stripe rust has only been found in some nursery plots in the Manhattan area in Kansas.  Leaf rust has been observed in northeast and south central Kansas.  In addition, for Oklahoma and Texas, the observations nearly all are in central or the eastern parts of the state. 

Since earlier this spring, the predictions have taken the tracks for both stripe and leaf rust to the east of northwest Kansas.  And so far, that seems to be the case.  However, there is concern with this last weather system that is moving though the area.  The way it has set and spun makes me wonder if rust spores will be moved north with the southerly winds.  This would likely take the rust into south central or southeastern Kansas (not northwest Kansas)  However, we will not know for at least two weeks if that was the case.    

Additionally, here is the link to the report for the state of Kansas from Erick DeWolf in K-State Agronomy’s eUpdate newsletter: https://webapp.agron.ksu.edu/agr_social/eu_article.throck?article_id=530