Soft red winter wheat is grown on about 1 million acres in Arkansas each year. The environment, especially in the spring, favors numerous diseases which can be caused by fungi, bacteria or viruses.
Shifts in varieties planted or new cultural practices like no-till seedbeds — which favors tan spot — can increase or decrease specific diseases in any given year.
In 2002, major diseases in Arkansas included barley yellow dwarf, stripe rust and soilborne viruses. Less-important diseases included Septoria leaf blotch, bacterial streak, leaf rust, take-all, head scab, loose smut, and glume blotch.
Minimizing diseases in the Arkansas wheat crop involves three types of control — planting resistant varieties, use of cultural practices unfavorable for certain diseases, and applying foliar fungicides during the spring. Effective disease control increases yields and quality of wheat.
The following diseases were noted in Arkansas during the 2001-02 season.
Soilborne virus diseases, especially soilborne wheat mosaic (SBWM), were important in northeast and east central Arkansas during 2002. Some fields suffered substantial yield loss because stunting symptoms persisted into April.
Fields confirmed to have SBWM in 2002 should be planted with resistant varieties in the future.
While wheat spindle streak mosaic (WSSM) causes less yield loss, susceptible and very susceptible varieties should not be planted in fields where this disease has become common.
In the spring, wheat plants from fields with soilborne virus symptoms should be taken to a county Extension office to be sent to the Cooperative Extension Service Plant Disease Clinic for analysis and confirmation of the virus involved.
Fields in northeast Arkansas should be planted with varieties resistant to both viruses if field history is unknown.
Stripe rust was serious in much of eastern and southwest Arkansas during the spring of 2002. First noted in mid-March to late March, the disease rapidly developed in fields with highly susceptible varieties. Dry conditions in mid-April slowed the epidemic and, combined with the use of foliar fungicides, prevented major losses in most fields.
From now on, Arkansas growers should consider stripe rust resistance when choosing a wheat variety to plant. A fungicide application should be budgeted for varieties rated susceptible to very susceptible. Losses of 50 percent or greater are possible under the right conditions for stripe rust.
Tan spot caused heavy damage to a few fields in at least five counties, all on no-till wheat which strongly favors the buildup of the tan spot fungus over time. There is very limited information on resistance to tan spot among soft red winter wheats, but we are starting to address this.
The main control for tan spot is the use of fungicides prior to heavy damage to the upper two leaves. Scouting of no-till fields for tan spot, regardless of variety, is currently recommended. A fungicide application at swollen boot (Feekes GS 10) may be beneficial if tan spot lesions are easily found on the lower leaves of most plants prior to this stage.
Leaf rust developed late during the spring of 2002 and did not cause any serious damage. Resistant varieties should still be considered as a major control option although the rust fungus adapts to resistant varieties over time.
Refer to the latest disease reactions in state Extension publications on varieties when selecting a resistant variety. Fungicides should be budgeted for those varieties rated susceptible.
Bacterial streak and black chaff were noted in southeast Arkansas during 2002 but did not cause widespread losses. If you grow seed for your own farm use, do not use seed harvested from fields with noticeable bacterial streak or black chaff in 2002. The disease is seedborne but current seed treatments do not control bacterial streak or black chaff.
Loose smut and glume blotch were noted in some fields again in 2002. These fields were generally planted with “on-farm” seed that was not treated with recommended seed treatment fungicides. Both diseases are seedborne and can be controlled by either Raxil or Dividend seed treatment fungicides. Commercial seed is much less likely to have these problems because seed growers routinely use these seed treatments.
Never use seed harvested from fields with noticeable loose smut or glume blotch and be sure to treat the seed as above.
Glume blotch can be controlled in the spring with preventative foliar fungicide applications during the boot stage, but it cannot be effectively scouted for.
Foliar fungicides are not effective on loose smut.
Fusarium head scab was noted in some fields during 2002 but at a low level. There are no highly-resistant varieties.
Planting several wheat varieties that flower and mature at different times during the spring may minimize scab risk for the farm. Planting several different varieties to spread risk is generally a good idea anyway. There are no effective fungicides for scab.
Septoria leaf blotch attacks all varieties, but those rated moderately resistant are less affected than others.
Foliar fungicides are available to help reduce losses, and scouting can save unneeded applications and money.
Septoria leaf blotch is common in most wheat fields each spring but does not cause serious losses unless consistent rainfall occurs during late March and April. This has not been the case in recent years as dry April weather has been more common in Arkansas.
Powdery mildew was not common in 2002. Because it is rarely widespread in the state, most variety reactions to powdery mildew remain unknown under Arkansas conditions. Foliar fungicides in the spring are the most effective control option, though rarely needed.
Take-all disease was observed in many long-term wheat fields during 2002, although it developed very late in the spring, resulting in less obvious symptoms than normal.
Resistant varieties are not available and fungicides are not effective. Rotation out of wheat for at least one year is the most effective management option for irrigated fields, and rice is the best rotation crop in the summer.
Continuous wheat with summer fallow is the best option for dryland fields.
For more information on take-all, consult the new Fact Sheet FSA 7526 Management of Take-All Disease of Wheat in Arkansas available from the local county Extension offices in Arkansas or on the Internet at http://www.uaex.edu.
Fusarium root and crown rot was not widely observed during 2002. This disease is more common in years with very dry conditions during grain fill, not the case in 2002.
Variety reactions in the latest Extension wheat publication were made in 2001 when the disease was more of a problem.
Like take-all, fusarium root and crown rot symptoms include premature ripening of infected plants resulting in low yield and shriveled grain. Infected plants have brown roots and crowns with a pinkish interior when cut open. There will be black streaks on the lower stem at the base of the plant.
Barley yellow dwarf was destructive in Arkansas during 2001-02. The mild autumn and winter resulted in more widespread infection than normal. Resistant varieties are not available and fall insecticide treatments to kill aphids that vector the virus have not proven economical.
Planting as late as practical can reduce fall infections that cause the greatest yield loss.
Selection of a high-yielding, disease-resistant wheat variety is the most important factor for successful wheat production in Arkansas due to the many economic wheat diseases during the season and the high cost of fungicides. Many modern wheat varieties have good resistance to one or more of these diseases.
However, no variety has complete resistance to all major diseases in the state.
Since several diseases may occur each spring, resistant varieties alone may not always offer total protection but usually result in lower input cost and greater profit since fungicides can be avoided most of the time.
In some cases, resistant varieties are the only practical control option, for example, to control soilborne viruses. Disease ratings in the new Arkansas Extension wheat variety publication is based on the most recent field observations made in University of Arkansas research and variety testing plots, but may also include observations from prior years or commercial wheat fields.
Seed treatment and foliar fungicides available in Arkansas are included each year in the MP154 Arkansas Plant Disease Control Products Guide, available through local Arkansas Extension offices and on the Internet at http://www.uaex.edu.
E.A. Milus, associate professor of plant pathology, University of Arkansas. R.D. Cartwright, Arkansas Extension plant pathologist.