|March 30, 2001
Two types of rust affect wheat in Texas. Stem and leaf rusts which cause extensive damage and are recognized by their reddish pustules that appear on the stems or the leaves. The fungi which cause these diseases, when left unchecked in favorable conditions for development, can result in the loss of both grain yield and quality. Stem rust is a parasitic fungus which enters the leaves, stems or spike through the stomata. In 6 to 10 days the fungi begins reproducing and appears as red elongated pustules on the surface of the plant parts. These pustules contain thousands of microscopic spores which are carried by the wind to nearby plants. There they are germinated in the rain or dew and cause new infections.
Leaf rust pustules (uredia) develop primarily on the upper surface of leaf blades. Pustules are round to oval, about 1/50 inch in diameter, light red, and filled with spores (urediospores). These spores are easily dislodged by the wind or rubbing and accumulate as red dust on hands, clothing, and machinery. Rust increases water loss from leaves so they die prematurely.
The optimum temperature for sporulation and infestation is approximately 70 degrees fahrenheit (F) and infestation can occur within four hours if leaves are wet. New pustules erupt within 7 to 10 days. Rust development is slowed drastically with cooler temperatures or a shorter leaf wetting period. At 60 degrees F a wetting period of 8 to 10 hours is necessary. Very wet conditions reduce spore dispersal. Epidemics generally develop when favorable moisture exists and temperatures are above 60 degrees F.
Since the leaf rust loss results from the early kill of the upper two wheat leaves, the faster an epidemic develops the greater the loss. Loss can be roughly predicted by matching severity on the flag leaf with growth stage. If the flag leaf is severely damaged prior to the milk stage, the approximate loss in yield is 25 to 40 percent. If the disease damages the flag leaf between the soft and hard dough stages of the developing wheat, yield losses are usually less than 10 percent.
Stem rust pustules (uredia) develop on leaves, especially on leaf sheaths, necks, glumes and awns. These pustules are 1/16 to 1/4 inch, much larger than leaf rust, dark red in color, considerably longer than wide, and are often evident on both the upper and lower leaf surfaces. Pustules raise and break the epidermis making them rough to touch. As the wheat matures the pustules turn black and remain rough.
Stem rust greatly increases water loss from leaves and stems so they die prematurely. Shriveled, light weight grain results. Wheat is its most common host but will attack barley, rye, oats, goat grass and wild barley. With the use of early maturing wheat varieties and resistant varieties stem rust has been reduced to a minor disease.
The optimum temperature for sporulation and infestation is approximately 75 degrees fahrenheit (F) and infestation can occur within four hours if leaves are wet. New pustules erupt within 7 to 10 days. Rust development is slowed drastically with cooler temperatures or a shorter leaf wetting period.
Under severe disease situations, the use of chemical control in conjunction with cultivar resistance may prolong the useful life of that particular cultivar. Several fungicides may be used to control diseases that have reached economic threshold levels.
The mancozeb-containing fungicides (Dithane, Manzate, and Penncozeb) are protectants. They must be in place prior to infection to achieve control. Mancozeb does not enter the plant, but provides a protective barrier to the disease causing organism.
Since most of our wheat has leaf rust already the fungicides of choice would be Stratego, Tilt or Quadris, which are systemic fungicides. These fungicides enter the plant and move systemically to some degree within the conductive tissues. They may be protective, eradicant, or both. The eradicative properties can stop disease development even though infestations have already occurred; however, symptoms remain if present at the time of the fungicide application. When control of a disease complex is needed, a mixture of a protectant and an eradicant fungicide may be required.
NOTE: With the current price of wheat it may be difficult to justify the application of fungicides, even with potential yield decreases caused by the rust infestations.
|Product name||Company||Use Rates
|Stratego||Bayer||10 ounces||$11.72||Can be applied until ligule of flag leaf emergences. Do not graze for 30 days or hay for 45 days when the 10 oz. rate is used.|
|Tilt||Syngenta||4 ounces||$10.08||Can be applied until ligule of flag leaf emergences. Do not graze or hay.|
|Quadris||Syngenta||6.2 to 10.8 ounces||$14.68 to $25.57||Can be applied until late head emergence. Do not graze. There is a 14 day haying restriction.||NOTE: Be sure to follow label directions. Use on approved crops to control listed pests that have been identified above economic threshold levels.|
Small Grain Hay
One option that producers will consider is harvesting their small grain as hay. Quality is high if the small grain forage is harvested in the correct growth stage. For oats that can be delayed until the soft dough stage. For the other small grains the cutting of hay needs to occur before head emergence. Baling quality small grain hay is a challenge due to the potential of mold development. During early spring, heavy dues and cool temperatures delay baling for seven days or more. Long range weather forecasts of warm cloudless days will be an important consideration before swathing small grain forage.
From early reports, producers should be encouraged to conduct soil tests. Nutrient levels have been lower than expected. An application of nitrogen at planting time may make a noticeable difference in the initial growth of the plant.
Shown are four weeds that are providing color to our crops and barditches at this time.
Sisymbrium irio L.
On Monday, April 9, there will be a training conducted at Abilene for producers needing to obtain a Private Applicators License. For more details and to register for the meeting call Gary Bomar at (915) 672-6048.
If you have a training where CEUs are offered please let me know.