2012 Wheat Disease Updates
Stripe Rust Update (May 2, 2012) and the Economic Threshold of Aphids
When spending time checking wheat during the last two days, I can see an increase of incidence of stripe rust. It is now showing up on flag leaf. This is especially evident on varieties that are showing the greatest susceptibility. These varieties include: TAM 111, Armour, Everest, TAM 112 and Jagalene. Some of the varieties that are still showing symptoms, but the development has been slower, are: Fuller, PostRock, Overley, Winterhawk, TAM 304, T-158 and Danby.
In addition, I looked at some irrigated wheat and was a bit surprised in the quick progression of the disease. This was in a susceptible variety that significantly increased in less than a week - progressed to the flag leaf and increased in severity in the mid canopy. The canopy has also been slow to dry out because of the foggy, moist, cool mornings and irrigation. This coupled with the cool overnight lows and moderate daytime temperatures.
A great deal of fungicide is being applied. It is important to note the preharvest intervals. Some of the popular fungicides being currently applied and their preharvest intervals are:
Folicur (active ingredient: Trebuconazol) - 30 days
Quilt and Quilt Xcel - full bloom
Prosaro - 30 days
Headline - full bloom
I have also been getting many questions about insecticides for aphids in wheat. Here is what I found for aphid thresholds.
Seedlings - 50 greenbugs/linear ft of row
3 to 6" wheat - 100 to 300 greenbugs/linear ft of row
6 to 10" wheat - 300 to 500 greenbugs/linear ft of row
Bird Cherry Oat Aphid: olive green colored
At boot to heading stage - 50+ per tiller
Russian Wheat Aphid: light green body
Around 20 bu/ac yield potential - 20% of tillers show symptoms and have live aphids
40 bu/ac + - 10% of tillers show symptoms and have live aphids
Stripe rust is being detected in trace levels in Sherman County. While looking at the Sherman County wheat plot, I found one leaf with a trace level of stripe rust on it. This was in the variety Garrison and this variety is very susceptible to stripe rust. This was on FL-2. It was also in the very early stages of development. The three pustules still had the glossy look and were not actively releasing spores.
Stripe rust has also been found in trace levels in Cheyenne County. This was found on TAM 111 in a protected area of the field in the canopy (not on the flag leaf). It was also in preliminary stages of development and likely just emerged from the leaf surface in the last 24 hours.
This update, combined with the one from earlier this week shows that stripe rust has been positively confirmed in Thomas, Wallace, Sherman and Cheyenne Counties. I also have a stripe rust report in Decatur County near the Nebraska line, with rust on FL-2 and FL-1 in trace levels.
TAM 111 has been resistant to stripe rust, until 2012. This year, the rust overcame the single-gene of resistance and is now susceptible to stripe rust. Other varieties that we have preliminary determined to be quite susceptible are Armour, Everest and Garrison. In addition, basically all wheat varieties have some level of susceptibility this year.
Since, stripe rust is not located on the flag leaf, there is a question on the need to treat stripe rust. There are some important items that go into making that decision. Stripe rust loves moist and cool weather and thrives in these conditions. It has been thought that stripe rust would shut down at high daytime temperatures (like over 80°.) Upon further study, the nighttime temperatures have a greater effect. After 2-3 nights, with overnight lows above 60°, the stripe rust will shut down. In addition, stripe rust can incubate in the leaf for 2-3 weeks before erupting from the upper surface of the leaf.
2012 Fungicide Chart - this has information of fungicide treatments with preharvest intervals.
After scouting fields in Thomas and Wallace Counties today, stripe rust has been found. This is being found in extremely trace levels and is not evident in every field that is walked into. See the attached pictures. No stripe rust was found on the flag leaf. All stripe rust pustules were located on one or two leaves below the flag leaf. In addition, this was nearly all located in lush wheat. I scouted this wheat first because it is wheat that would hold moisture the longest. Moisture is a key component for infection of stripe rust.
The pictures show the beginning of rust pustules on the leaf upper surface. These pustules are very newly emerged from the leaf surface, in the last 24-48 hours, and are not actively sporulating (releasing spores). The stage of releasing spores is when orange will appear on fingers after rubbing them across the leaf surface. This will happen over the next couple days. That is likely the period when the flag leaves will become infected. The stripe rust will incubate in the leaf after infection for 2-3 weeks before pustules will appear.
The weather over the next couple days will help determine control tactics on this disease. Stripe rust likes temperatures between 50 and 80°. Temperatures of 90 today will not be enough to stop the disease. It is actually night temperatures over 60° will stop stripe rust. In addition, the warm temperatures will increase stress on the wheat. Cooler temperatures and possible rain by the end of the week are favorable to stripe rust development.
In addition, 'flecks' on the leaf surface are not indicative of stripe rust. These flecks can be from a number of sources (mainly environmental) and are visible when holding the leaf up to the light. There is one variety, Hatcher, which responds to stripe rust with these flecks. However, in Hatcher, the flecks are elliptical shaped along leaf veins. The flecks that are visible now are randomly scattered on the leaf surface.
This 2012 Fungicide Chart has information on fungicide treatments, timing, rates and preharvest intervals. In addition, all fungicides have 2-3 weeks of residual activity. This depends on the stripe rust pressure and environmental conditions.
I will be scouting in Rawlins, Cheyenne and Sherman Counties tomorrow (Wednesday, April 25) and will send out another report tomorrow evening/Thursday morning. If you have any questions, please let me know.
Threat of Stripe Rust for Wheat (April 13, 2012)
From Jeanne Falk, K-State Multi-County Agronomist:
Stripe rust has been found in trace levels in southern Scott County. This is the closest stripe rust to the Sunflower District. The threat level for stripe rust in the next week is moderately low.
Moderate temperatures and heavy dew in the morning are two components needed for infection of stripe rust. Both of these components will be in place this week and can increase the risk for infection of stripe rust. The final component is the stripe rust innoculum. Since the presence of disease continues to move closer, the risk is increasing. Since there are only trace levels showing up in fields shows, the risk for an explosion of stripe rust (similar to 2010) is low at the current time.
Also remember that the flag leaf contributes over 75% of the nutrients for the filling of the wheat head. Do not apply prior to at least 2/3 flag leaf emergence. Fungicides have limited translocation in the plant. This means the fungicide can move a limited amount (less than 1/2 inch) in the leaf. It can not move leaf to leaf.
Please read Erick's information below about the change in the stripe rust to attack different varieties.
From Eric DeWolf, K-State Wheat Pathologist:
The wheat in Kansas is now heading and beginning to flower in Southeast and South Central Kansas. Wheat in central KS is now at the boot stage and will likely begin to head soon. In fact, it is likely that some fields are already beginning to head out in central region of the state. As we move north and west in the state, the wheat is moving toward flag leaf emergence to boot stages of development.
My own scouting and reports from other KSU agronomists and agents indicates that stripe is generally at low levels in many fields throughout central Kansas. The stripe rust was generally limited to the F-2 and F-1 leaves and less than 1% incidence. This afternoon; however, I have received reports that stripe rust has now moved to the flag leaf and the severity of disease has increased dramatically in some fields. Here are some specific reports: Stripe rust was reported on the flag leaf in Montgomery, Labette, Crawford and Wilson counties (Southeast, KS). Stripe rust was reported on the upper canopy (F-1 and Flag) with a noted increase in the incidence in many fields this past week in Saline, McPherson, Harvey, Reno and Ellsworth county (central KS). Stripe rust was observed on the flag leaf in Sedgwick county (South central, KS) with severe stripe rust developing in a field of Armour wheat in the southeast portion of this county. I have also seen stripe rust in north central KS including Cloud, Mitchell counties. The disease appears to be limited to the F-2 and F-1 leaves currently in north central KS.
Varieties with the Jagger based pedigree that have Yr17 are being affected by stripe rust. Varieties such as Everest, Armour, and TAM111 are also being affected by stripe rust this year. This strongly suggests that the stripe rust population has changed to overcome these sources of genetic resistance.
My assessment of the situation is that stripe rust is widely established in central Kansas this year. The weather has been conducive for disease development and the weather forecast appears to favor continued development. The disease is still at low levels in many fields, however, the severity of disease will likely increase dramatically in the next 10 days. I think there is high risk of severe yield loss to stripe rust for wheat in at least the eastern 2/3 of Kansas. Based on my current information I believe there is at least a moderate risk of severe disease in western KS. I will attempt to get more information about western Kansas next week.
Threat of Stripe Rust for Wheat (April 6, 2012)
Stripe rust is a challenging wheat disease because it thrives in the same environment that wheat thrives. These conditions include moderate temperatures with moisture.
The following is a report from Erick DeWolf, K-State wheat pathologist. "I continue to get reports of low levels of stripe and leaf rust in Kansas. Reports to date have come from primarily south central and central regions of the state. Counties where stripe rust has been reported include: Harper, Kingman, Sedgwick, Pratt, Reno (south central); McPherson, Saline (Central). The wheat in these areas of the state ranges from flag leaf emergence to heading, which is approximately 2-3 weeks ahead of normal crop development. Weather conditions this week have been favorable for continued spread and development of rust diseases."
With the storms that moved through the area in the last couple weeks, they circulated in a counter-clockwise direction. This could move rust spores from central Kansas to western Kansas. This is a bit of a deviation from the normal south to north movement of the rust spores on the wind currents. There have also been reports of trace levels of rust in the panhandles of Texas and Oklahoma. These are usually the areas that are watched for the movement of stripe rust into northwest Kansas.
Finally, this is just an early warning and is not a cause for alarm. It is just important that everyone know where we are with this disease. Our wheat is not in the stage of growth for fungicide applications to protect from rust. Wheat is normally treated when the flag leaf is at least 2/3 emerged. This is because fungicides move only on a limited basis within a leaf and will not move from leaf to leaf in the plant.
If you have any questions, please call me at the K-State Extension Office or at the Experiment Station in Colby at (785) 462-6281.