Gene Milus, wheat disease expert at the University of Arkansas, believes farmers should think ahead when it comes to effective disease control and high yields for wheat. Here are some of his tips to minimize the many wheat disease problems in the Mid-South.
Q. How would you go about picking a good wheat variety to plant in Arkansas?
A. During the spring, I would look at the varieties growing in demonstration and research plots near where I farm as well as on my neighbors' farms. When the Arkansas Wheat Update (http://www.aragriculture.org/News/wheat_update/2002/HTML/August2002.asp) becomes available, check the yield, test weight, and disease reactions of varieties that looked good during the season. Look for varieties with good yield and test weight across several years and locations — these have broad adaptation and will be less sensitive to different growing conditions from one year to the next. For the varieties on my short list, I would select those at least moderately resistant to diseases that have been problems on my farm in the past.
Q. Would you plant one variety on the farm or several?
A. No single variety ranks the best for all traits. Planting several varieties allows you to customize varieties to particular fields or cropping sequences on your farm. This also spreads your risk of losses due to unusual weather or disease problems.
Q. Stripe rust has been a problem the past two to three years in Arkansas. Should farmers plant a variety resistant to stripe rust or just plan on using a fungicide?
A. If stripe rust has been a problem in your area, I would avoid varieties rated susceptible or very susceptible in the 2002 Arkansas Wheat Update. We obtained good data for stripe rust reaction on the different varieties this past season, so I feel comfortable with our ratings. There are plenty of good varieties that were resistant (R), moderately resistant (MR), or moderately susceptible (MS). If stripe rust gets going in a susceptible variety, there will be quite a bit of yield loss even with a well-timed fungicide application. Why take a risk with a susceptible variety?
Q. Sabbe is a new high-yielding wheat from the Arkansas breeding program. What disease control advice do you have for farmers growing Sabbe this year?
A. Sabbe is resistant to stripe rust, powdery mildew, Fusarium root and crown rot, and has been one of the most resistant to Septoria leaf blotch. Although it is moderately resistant to soilborne mosaic, it is susceptible to spindle streak mosaic and some yield loss should be expected in fields with the spindle streak mosaic virus. Leaf rust is the major concern with Sabbe. Leaf rust was pretty bad on Sabbe about the time it was released, but lately leaf rust has been moderate on the variety with pustules of intermediate size.
We believe that Sabbe has gene Lr 34 for leaf rust resistance. This is the only leaf rust gene that is race nonspecific, that is, it is effective against all known races of the leaf rust fungus. However, the level of Lr 34 resistance is affected by environmental conditions, so the amount of leaf rust can vary from season to season on varieties with this resistance gene.
But even if leaf rust is bad next spring, Sabbe has enough resistance so that one well-timed fungicide application should protect it.
Q. Where should wheat varieties resistant to soilborne viruses be planted?
A. Delta counties north of I-40 (northeast Arkansas) have been the hot spot for soilborne viruses in the past. However, one or both of the soilborne viruses have shown up in other areas of Arkansas. Soilborne mosaic can be very serious on wheat in Arkansas with 100 percent loss in severely affected fields. Spindle streak mosaic appears to cause significant loss only when the weather stays cool into April and symptoms persist for a long time.
Under most conditions where only spindle streak is involved, the plants will grow out of the disease and look pretty normal by harvest.
We obtained a lot of resistance data last year from our variety test in Jackson and Clay counties in Arkansas, so we know there are a lot of good varieties with resistance to both soilborne mosaic and spindle streak mosaic, so why take the chance?
Q. What is the effect of planting really early instead of within recommended planting dates with respect to disease control?
A. Compared to early plantings, later plantings have less potential to become infected with several diseases. Barley yellow dwarf (caused by a virus transmitted by aphids that fly in during the fall) and take-all (caused by a fungus that survives in wheat stubble and grassy weeds) can be managed to some extent by planting as late as practical.
Mild fall weather, like we had in 2001, is very favorable for the aphids, and this is why barley yellow dwarf was so severe this past season. Later plantings allow less time for the aphids to fly in and multiply before cold weather stops the flights, and this results in less barley yellow dwarf.
Soil temperatures are generally favorable for take-all infection during October and into early November. Later plantings will have a shorter time and a smaller root system during this period favorable for infection. Later plantings also allow more time for the take-all fungus to die before encountering wheat roots to infect. All of this can result in less take-all.
Seed decay and seedling diseases usually are problems only when plantings are done very early into warm, dry soil. On-time or later plantings generally are not affected by these problems. An apparent contradiction to these statements is that early-planted and well-established seedlings usually fair better than smaller, later-planted seedlings if the weather turns wet and cold earlier than usual. However, the effects are more likely due to the adverse conditions rather than diseases, and seed treatments likely will not help anyway. The larger plants from early plantings are just better able to withstand the harsh conditions.
Diseases such as leaf rust and stripe rust also may be more severe in early plantings because there is a longer period in the fall during which plants can be infected by spores blowing in from long distances. Avoiding fall infection would delay disease development in the spring. However, resistant varieties and foliar fungicides are better management tools for these diseases.
Q. Barley yellow dwarf caused serious losses in several parts of Arkansas last year again. How can farmers control barley yellow dwarf?
A. I consistently achieve a high level of barley yellow dwarf control in my research plots with a combination of Gaucho seed treatment and Warrior foliar insecticide applications. However, wheat growers would go broke applying these practices to their wheat fields.
My best advice for growers is to plant as late as practical to avoid most of the aphid flights. Another option is to plant the variety Roane which does have some resistance to BYD. If a grower wanted to do more, he could try Gaucho or Cruiser seed treatment or a foliar insecticide.
The seed treatments are rate-dependent — the highest labeled rate works the best — and will give about three weeks of protection.
Warrior will kill the aphids that are there at the time of application, but there is not much residual activity against aphids that may fly in a couple days later. Several well-timed applications likely will be needed for complete control.
Growing resistant varieties would be the best solution, and I am working on transferring resistance to adapted varieties through a research project funded by the Arkansas Wheat Promotion Board.
Q. Should a seed treatment fungicide be used?
A. Growers producing wheat for seed should always treat their planting seed to control pathogens that are seedborne. Although certified wheat seed is not tested for seedborne pathogens, seed growers who consistently treat their planting seed should develop a reputation for producing high-quality seed, receive few complaints about seedborne diseases, and attract repeat customers.
If loose smut or Stagonospora (Septoria) glume blotch was observed in the seed production field, a good seed treatment fungicide likely will be cost effective. Planting early into warm, dry soil favors seed decay and seedling diseases, and seed treatment would likely be beneficial under these conditions.
Q. What are the most effective seed treatment fungicides?
A. Raxil-Thiram and Dividend are the most cost effective seed treatments for the spectrum of seedborne diseases in Arkansas. At their highest labeled rates, each fungicide controls loose smut and the seedborne Stagonospora fungus that causes glume blotch. The lowest labeled rates are somewhat less effective.
These fungicides are the only practical method for controlling loose smut, and they go a long way toward controlling Stagonospora leaf and glume blotch the next spring. Both products are also active against certain seed rots, seedling diseases, and early-season foliar diseases, but except for planting too early into warm, dry soil — there is not likely to be a large difference between treated and non-treated seed for these diseases under Arkansas conditions. No seed treatments for Pythium root rot or downy mildew have given consistent results under our conditions.
So plan ahead for disease control, and make more money from your wheat crop.
Rick Cartwright is an Extension plant pathologist with the University of Arkansas Division of Agriculture.