| April 5, 2007 |
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Leaf, Stem and Stripe Rust
Most producers are familiar with stem rust and leaf rust which can cause extensive damage. The fungi which cause these diseases, when left unchecked in favorable environmental conditions, can result in the loss of both grain yield and quality. The development of the fungi is impacted by water and temperatures. When favorable environmental conditions exist the life cycle for the spores can occur in 7 to 14 days. Frequent heavy dew, light rain, or high humidity and temperatures generally in the range of 59 to 77 degrees Fahrenheit (F) are ideal for rust development. Pustules form on the surface of the plant parts and contain thousands of microscopic spores. The pustules erupt releasing the new urediospores into the air and they are dispersed by the wind.
Urediospores germinate in rain or dew and infect leaves within 6 to 8 hours after settling on the plant surface. The infection element of the fungi is capable of penetrating into a plant directly and will enter through the plant breathing pores or wounds. Plants have many microscopic breathing pores called stomata on the surface of the leaves. This is a common spot of infestation. Once the fungal infestation element becomes established inside the plant, the invaded tissue gradually dies as the urediniospores develop. Shown in Figure 1, is the urediospores of leaf, stem and stripe rust. Rust spreads by wind blowing urediospores from plant to plant and from field to field until the crop matures.
 Leaf Rust |
 Stem Rust |
 Stripe Rust |
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Stripe rust (Puccinia striiformis)
Stripe rust is blown into our area by prevailing winds. Winds can carry rust spores for great distances. The disease can develop rapidly when moisture is not limited. Urediospores are yellow to orange in color and have an elongated shape. Urediospores lose their viability rapidly at temperatures above 59o F. Germination is best between 41o and 59o F, with limits at 32o and 68o F. Infection may occur through the winter since the mycelium remains viable to 23o F. The disease develops most rapidly between 50o and 59o F when intermittent rain or dew occurs. Many disease cycles may take place in one season, since the time between infection and sporulation under optimal conditions is about 8 days. Cool, wet falls; mild, open winters; and long, cool, wet springs favor stripe rust.
Leaf rust (Puccinia recondita)
Leaf rust is blown into our area by prevailing winds. 
Winds can carry rust spores for great distances. The disease can develop rapidly at temperatures of 52 to 72 degrees F, when moisture is not a limiting factor. The optimum temperature for sporulation and infestation is approximately 70 degrees F and infestation can occur within four hours if leaves are wet. The spores can initially infect and produce pustules where the raindrops have collected at leaf axils and tips. 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.
Leaf rust pustules (uredia) develop primarily on the upper surface of leaf blades. Pustules are round to oval, about 1/50 inch in diameter (about the size of a pinhead), and filled with thousands of orange-red spores (urediospores). These spores are easily dislodged by the wind or rubbing and accumulate as red dust on hands, clothing, and machinery. They may occur on stems but are most common on the upper surface of the leaves. Rust increases water loss from leaves so they die prematurely.
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 (Puccinia graminis)
A wave of infection usually advances northward by wind-blown spores released from infected wheat or barley plants. 
Stem rust usually enters our area in late March to mid-April depending on weather conditions and the extent of disease development on cereal crops south of us. The disease usually appears on plants in early spring.
After infection, pustules form in 7-10 days. The pustules break open, releasing masses of spores that can infect surrounding plants. Temperatures of 68 to 77 degrees F favor stem rust development. Rust spots are very small, circular or elongated, and vivid orange-red in color. Later in the year, the rust pustules darken because of the production of dark-brown-black spores which are the overwintering stage of the rust.
Infected plants usually produce fewer tillers, set fewer seeds per head and yield small shrivelled seeds with poor milling quality and food value.
Powdery Mildew
Powdery mildew has been found in some fields in the area. Few producers have made applications of a fungicide to control the fungi's development.
Losses from powdery mildew is not as extensive as severity of symptoms indicate. Powdery mildew is easy to identify. The first symptoms are light green flecks on the leaves. Very soon white cottony tufts of fungus develop. These tufts spread over the leaf and turn to a dirty gray mat. Dusty white spores are produced in great numbers and cover leaves and soil. Affected leaves turn yellow and die. Under some conditions, distinctive spots of dark green persist in yellowing leaves. The disease increases respiration and water loss and decreases chlorophyll; consequently, photosynthesis declines.
Powdery mildew is caused by the fungus Erysiphe graminis f. sp. tritici. Closely related forms cause powdery mildew on many grasses. These forms are closely adapted to their specific hosts so it is doubtful if there is much cross-infection from other crops and grasses to wheat.
The fungus survives on dead wheat plants and within infected leaves. Fall infection can occur, but the main concern is usually in the spring near the time of jointing. Lower leaves are infected first and it progresses up the plant, leaf by leaf, as long as conditions for spore production and infection persist.
Powdery mildew thrives in cool damp conditions. Optimum temperature for growth is 68 degrees fahrenheit with relative humidity near 100%, but heavy rain actually reduces it. Spores survive about 2 days at 70o F and considerably longer at cooler temperatures. This permits some long distance dispersal by wind. Some infection can occur as low as 40o F. Epidemic development stops abruptly about 75o-80o F. This often prevents severe loss since infection of the top two leaves is necessary for significant yield effect and it is usually warm enough by the time these leaves emerge to prevent extensive infection. Powdery mildew is most prevalent in low lying, thickly planted fields protected from wind and heavily fertilized with nitrogen. Only slight differences in fertility or plant density can cause drastic differences in disease severity within a field.
Since high fertility and thick planting are necessary for maximum yields, cutting back on these to control powdery mildew is not practical. There is no clear indication that cropping sequences or residue management affect powdery mildew severity.
Resistant varieties have been developed in areas where powdery mildew is more serious. Our present varieties range from moderately resistant to very susceptible with none highly resistant.
Fungicides that are highly effective against powdery mildew have been developed. But the erratic nature of powdery mildew combined with low wheat prices make regular use of fungicides uneconomical.
Other information on wheat diseases is linked.
Weed Control
In most of the wheat we have passed the stage of growth that would allow for any herbicide to be applied. Producers will have to wait until the hard dough stage before applying a harvest aid.
Cottonseed for result demonstrations have been ordered and should arrive in the next two weeks. A list of the tests to be conducted is linked..
The minimum soil temperature for planting cotton is a 10 day average of 60o F at an eight inch soil depth (take temperature at 8:00 a.m.). When soil temperatures are 65o to 70o F the cottonseed germinates and emerges quicker which helps to reduce the amount of time emerging seedlings are exposed to soil diseases. If cotton is planted at the minimum soil temperature and then watered, you will probably have a reduction in the final plant stand and health of the crop.
I have taken pictures of a couple of weeds that I was unfamiliar with in the last two weeks.
| Haresear mustard (Conringia orientalis) | ||
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| Corn gromwell (Lithospermum arvense) | ||
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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 (325) 672-6048.
On Wednesday April 11, TDA Pesticide Applicators can earn CEUs by attending the Lower Rolling Plains Agriculture Conference in Snyder. Attached is the agenda and contact information.
If you have a training where CEUs are offered please let me know.
A question asked at a Master Gardener presentation in March dealt with using the Zodiac signs to plant by. I have to admit that we used those when we castorated animals but that was about it. When I started trying to find the information concerning gardening, the science was limited but most of the discussion was the same. Since the information was compiled, I thought you might like to read what was available. Linked is information on gardening by the sign of the moon. Looks like a project opportunity for someone.
April 2, District Office, Office Conference
April 3 & 4, Taylor County, Small Grains Training--Level II
April 11, Nolan County, Lower Rolling Plains Ag Conference
April 18, District Office, Teleconference
April 19 - 26, Crystal City, Virginia--Public Issues and Leadership Development Conference
May 1 & 2, Brown County, Professional Extension Association Board Meetings
May 4, Taylor and Callahan Counties, Multi-County Wheat Tour
May 7, San Saba County, Sprayer Calibration
May 15, Concho and McCulloch Counties, Wheat Field Day and Tour
May 17, Runnels County, Wheat Tour
Sincerely,
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