Wheat disease updates are written by Dr. Bob Hunger, OSU Extension Wheat Plant Pathologist

Just a few updates from this week.  First, Mark Gregory (Southwest Area Extension Agronomist) reported the first stripe rust in Oklahoma for 2013 (see photo below).  He found this stripe rust near Paul’s Valley on an Endurance/Duster mix, and “had to look hard to find it.”

Stripe rust on wheat near Pauls Valley, OK was hard to find but was present

David Nowlin (Caddo County Extn. Educator) sent in a wheat sample (Duster) exhibiting symptoms typical of wheat streak mosaic.  If confirmed, this would be the second sample of WSM received from south central and southwestern OK.

Dr. Art Klatt (OSU Wheat Geneticist) has been this last week in Castroville, TX (far southern TX) rating breeder plots/lines for leaf and stripe rust.  He has reported that, “In Castroville there is heavy leaf rust.  We also found a little stripe rust but not enough to take readings and it was beginning to shut down due to temps. “  In Oklahoma, Dr. Klatt has reported seeing heavy powdery mildew in plots near Perkins, OK and at Stillwater, but has not yet seen any leaf or stripe rust.

Monday, 15-Apr, I and Nathalia Grachet (OSU Graduate Student) did plot work at the variety trial near Apache, OK.  Wheat there was approaching Feekes 10.  Flags leaves were out, but for most tillers, heads had not yet moved into the boot.  Freeze damage was the most striking symptoms in this trial and surrounding fields, but tan spot/septoria/stagonospora, powdery mildew, and barley yellow dwarf symptoms also could be found – but, no leaf or stripe rust.

Today (17-Apr) I checked my trials and nurseries here at Stillwater.  Wheat was at “pre” GS 10 with flag leaves fully out but the heads not yet fully moved into the boots.  Even where the boot internode was fully expanded, the head was usually just beginning to move into the boot.  I found powdery mildew low, but no rusts at Stillwater.  Spots indicative of BYDV have appeared.

Most wheat varieties are now at first hollow stem. As mentioned in previous posts, the drought has wheat development in Oklahoma a good one to two weeks behind normal. If you still have cattle on wheat pasture it is past time to remove them, as there will be a significant grain yield penalty for further grazing.

CHICKASHA

                 cm of hollow stem

Variety           03/13/13

Jagger             .

Endurance       0.5

Deliver             1.5

Pete                 0.7

Ruby Lee         3.2

Garrison           2.2

Duster              1.9

Gallagher        .

Iba                   2.2

Fuller               .

Everest           .

Jackpot            .

Doans              1.6

Greer               .

CJ                    2.2

Razor               .

Armour             .

WB-Cedar        3.7

WB-Redhawk    .

T153                .

T154                .

T158                0.9

OK08328         0.8

*Average of ten hollow stem measurements in wheat sown 09/20/12 at Chickasha, OK

Stillwater

                 cm of hollow stem

Variety            03/13/13

Jagger                 3.2

Endurance          0.7

Deliver                 1.7

Pete                    0.7

OK Bullet            1.7

OK Rising            2.1

Centerfield          1.4

Ruby Lee             .

Garrison               .

Duster                  1.6

Gallgher              1.8

Iba                       0.4

Billings                2.4

Fuller                   .

Everest                .

Jackpot                4.3

Doans                  .

Greer                   1.2

CJ                        0.4

Razor                   .

Armour                3.2

WB-Cedar            2.7

WB-Redhawk       4.1

WB-Duece CL+    .

WB-Grainfield      0.8

Winterhawk          2.0

WB-4458             .

T153                   .

T154                   .

T158                   0.6

LCS Mint             0.4

LCH08-80            0.5

LCH08 – 109        0.7

T173                   0.7

Byrd                     1.8

Brawl CL+            0.5

TAM 113             1.3

OK09915C          0.9

OK09935C          0.9

OK09634             1.8

OK09125             0.4

OK09528             0.4

OK09729             0.5

OK08328             0.3

*Average of ten hollow stem measurements in wheat sown 09/18/12 at Stillwater, OK

Onset of first hollow stem was affected by severe drought stress

First hollow stem occurs just prior to jointing and is the optimal time to remove cattle from wheat pasture. Grazing past first hollow stem can reduce wheat grain yield by as much as 5% per day and the added cattle gains are not enough to offset the value of the reduced wheat yield. Checking for first hollow stem is fairly easy.

• You must check first hollow stem in a nongrazed area of the same variety and planting date. Variety can affect date of first hollow stem by as much as three weeks and planting date can affect it even more.
• Dig or pull up a few plants and split the largest tiller longitudinally (lengthways) and measure the amount of hollow stem present below the developing grain head. You must dig plants because at this stage the developing grain head may still be below the soil surface.
• If there is 1.5 cm of hollow stem present (see picture below), it is time to remove cattle. 1.5 cm is about the same as the diameter of a dime.
• Detailed information on first hollow stem can be found at www.wheat.okstate.edu under ‘wheat management’ then ‘grazing’

We check for first hollow stem in our forage trials each year. These can be used as rules of thumb, but you will still need to check your own fields. Our Chickasha plots were sown September 20, 2012 and have had fair growing conditions. First hollow stem was measured on February 18, 2013 and the results are presented in the table below. Gallagher and Everest are just past first hollow stem and Razor is not far behind. I predict that most varieties will be past first hollow stem by March 1.

In order to maximize grain yield, we need between 500 and 900 heads/sq. yd. at harvest.  Wheat sown at 1 bu/ac will generally result in about 175 seeds/sq. yd. meaning that we will need at least three tillers per seed that go on to make a wheat head. Not just any tiller will suffice. If a tiller does not have three large, unfolded leaves by jointing, there is a good shot that it will not make a head.

One of my primary concerns right now is  tillering in many Oklahoma wheat fields will be insufficient to reach the critical 500 heads/sq. yd. number. Even though most of our wheat was sown in October, a lot of it will have a November or December (if we are lucky) germination date. A K-State study from 1995 showed that wheat emerging on November 30th produced an average of only 1.8 viable heads per plant at harvest. If we had known that our emergence date would be in November or December, we would have compensated by doubling or tripling our seeding rate.

September-sown wheat that got a good start but is now backpedaling due to a lack of moisture is a different issue. In the same study mentioned above, wheat emerging October 12 produced 1,850 tillers per plant on average. Only 476 (26%) of these tillers made it to harvest, so it is normal for early-sown wheat to slough off several tillers. This has an energy cost to the plant and is one reason early-sown wheat generally yields less than October sown wheat. We have enough tillers present in our early-sown wheat but are losing them quickly. Rain is needed quickly to stop the losses.

Agronomists are second only to lawyers in including disclaimers and qualifying statements, and there is a lot of wiggle room in the numbers I have provided. A variety that typically produces larger heads (e.g. Billings) can be on the lower end of the 500 – 900 head range and a variety with smaller heads (e.g. Duster) will need to be on the upper end.With a warm winter and rainfall, we could still hit these targets. However, to borrow some terminology from Vegas, I think there are fairly low odds that our wheat will cover the spread when it comes to tillering this year.