This article was written by: Dr. Bob Hunger, Extension Wheat Pathologist, & Dr. Tom Royer, Extension Entomologist, Department of Entomology & Plant Pathology Oklahoma State University
Planting date: Much of the winter wheat in Oklahoma is sown with the intent of being used as a dual-purpose crop. In such a system, wheat is grazed by cattle from late fall through late winter/early spring and then harvested for grain in early summer. In a grain-only system, wheat is generally planted in October, but in a dual-purpose system wheat is planted in early to mid-September to maximize forage production. Planting wheat early significantly increases the likelihood that diseases and insect pests such as mite-transmitted viruses, the aphid/barley yellow dwarf complex, root and foot rots, and Hessian fly will be more prevalent and severe. For more detailed information on planting date and seed treatment considerations on wheat, see CR-7088 Effect of Planting Date and Seed Treatment on Diseases and Insect Pests of Wheat at http://wheat.okstate.edu/wheat-management/seeding/CR-7088web2012.pdf.
Mite-transmitted virus diseases: These virus diseases are transmitted by wheat curl mites (WCMs) (Figure 1), and include wheat streak mosaic (WSM), high plains disease (also called wheat mosaic), and Triticum mosaic (TrM). WCMs and these viruses survive in crops such as wheat, corn, and sorghum as well as many grassy weeds and volunteer wheat. In the fall, WCMs spread to emerging seedling wheat, feed on that seedling wheat, and transmit virus to the young wheat plants. During 2016 and 2017, these mite transmitted virus diseases (especially WSM) were severe across much of the wheat growing areas of Oklahoma as well as in many of the other Great Plains states. Weather is a determining factor in the increased incidence and severity of these virus diseases, but another major contributor is the lack of control of volunteer wheat and other grassy weeds that serve as alternative hosts for the mite and the viruses. Often, an infected field of commercial wheat is growing immediately adjacent to a field left fallow during the fall and winter (Figure 2). The fallow field contained abundant volunteer wheat and grassy weeds from which WCMs carrying Wheat streak mosaic virus (WSMV) spread into the commercial field. Wheat infected in the fall will be severely damaged the next spring. Hence, it is imperative to do yourself and your neighbors a favor by controlling volunteer wheat and grassy weeds in fields left fallow – especially, if they are adjacent to commercial wheat fields.
Seed treatments and insecticides are not effective in controlling the mites or these mite-transmitted virus diseases. However, planting later in the fall (after October 1 in northern OK and after October 15 in southern OK) and controlling volunteer wheat are two practices that can be employed to help manage these diseases. It is extremely critical that volunteer wheat is completely dead for at least two weeks prior to emergence of seedling wheat because WCMs have a life span of 7-10 days. Thus, completely killing or destroying volunteer wheat for a period of at least two weeks prior to emergence of seedling wheat will greatly reduce mite numbers in the fall. There are several winter wheat varieties that have resistance to either WSM or the curl mites, but the adaptation of these varieties to Oklahoma is limited. Additionally, the resistance is not an absolute resistance to the disease. Hence, severe and continuous disease pressure can overcome the resistance although disease effects typically is less as compared to a susceptible variety. For more information on mite-transmitted virus diseases, see OSU Fact Sheet 7328 Wheat Streak Mosaic, High Plains Disease and Triticum Mosaic: Three Virus Diseases of Wheat in Oklahoma) at: http://wheat.okstate.edu/wheat-management/diseasesinsects/EPP-7328%20three%20virus%20diseases%20of%20wheat.pdf.
Aphid/barley yellow dwarf (BYD) complex: Viruses that cause BYD are transmitted by many cereal-feeding aphids (Figure 3.). BYD infections that occur in the fall are the most severe because virus has a longer time to damage plants as compared to infections that occur in the spring. Several steps can be taken to help manage BYD. First, a later planting date (after October 1 in northern Oklahoma and after October 15 in southern Oklahoma) helps reduce the opportunity for fall infection. Second, some wheat varieties (e.g., Duster, Billings, Gallagher, Iba, Bentley, Tatanka, and Winterhawk) tolerate BYD better than other varieties; however, be aware that no wheat variety has a high level of resistance to the aphid/BYD complex. Third, control aphids that transmit the viruses that cause BYD. This can be done by applying contact insecticides to kill aphids, or by treating seed before planting with a systemic insecticide. Unfortunately, by the time contact insecticides are applied, aphids frequently have already transmitted the viruses that cause BYD. Systemic seed-treatment insecticides containing imidacloprid or thiamethoxam can control aphids during the fall after planting. This may be particularly beneficial if wheat is planted early to obtain forage. Be sure to thoroughly read the label before applying any chemical.
Hessian fly: Hessian fly (Figure 4.) infestations can occur in the fall and spring. Fall infestations arise from over-summering pupae that emerge when climate conditions become favorable. In states north of Oklahoma, a “Hessian fly free” planting date often is used to help limit fall infestations by Hessian fly. However, such a planting date does not apply in Oklahoma because Hessian fly can emerge in Oklahoma as late as December (Figure 5).
Delayed planting (after October 1 in northern Oklahoma, and after October 15 in southern Oklahoma) can help reduce the threat of Hessian fly, but a specific “fly free date” does not exist for most of Oklahoma as it does in Kansas and more northern wheat-growing states. This is because smaller, supplementary broods of adult flies emerge throughout the fall and winter. Some wheat varieties are either resistant (e.g., Duster, Gallagher, SY Flint, and LCS Wizard) or partially resistant [e.g., LCS Chrome, Everest, Ruby Lee (at cooler temperatures)] to Hessian fly infestations. Hessian fly infestations can be reduced somewhat by destroying volunteer wheat in and around the field at least two weeks prior to emergence of seedling wheat. Seed treatments that contain imidacloprid or thiamethoxam will also help reduce fly fall infestations, especially if combined with delayed planting and volunteer destruction. For more information on Hessian fly, see OSU Fact Sheet EPP-7086 Hessian fly Management in Oklahoma Winter Wheat at: http://wheat.okstate.edu/wheat-management/diseasesinsects/EPP7086hessianflyinoklahoma.pdf.
Root and foot rots: These are caused by fungi and include several diseases such as dryland (Fusarium) root rot, Rhizoctonia root rot (sharp eyespot), common root rot, take-all, and eyespot (strawbreaker) (Figure 6). During the late spring of 2016 and 2017, several samples of wheat were received that were diagnosed as being affected by take all and other root rots. This could indicate a greater incidence of wheat root rots in 2017-18, but the incidence and severity of root rots is highly dependent on weather conditions so it is impossible to predict their incidence and severity this early.
Controlling root and foot rots is difficult. There are no resistant varieties, and although fungicide seed treatments with activity toward the root and foot rots are available, their activity usually involves early-season control or suppression rather than control at a consistently high level throughout the season. Often, there also are different “levels” of activity related to different treatment rates, so again, CAREFULLY read the label of any seed treatment to be sure activity against the diseases and/or insects of concern are indicated, and be certain that the seed treatment(s) is being used at the rate indicated on the label for activity against those diseases and/or insects. Later planting (after October 1 in northern Oklahoma and after October 15 in southern Oklahoma) also can help reduce the incidence and severity of root rots, but planting later will not entirely eliminate the presence or effects of root rots. If you have a field with a history of severe root rot, consider planting that field as late as possible or plan to use it in a “graze-out” fashion if that is consistent with your overall plan. For some root rots, there are specific factors that contribute to disease incidence and severity. For example, a high soil pH (>6.5) greatly favors disease development of the root rot called take-all. OSU soil test recommendations factor in this phenomenon by reducing lime recommendations when continuous wheat is the intended crop. Another practice that can help limit take-all and some of the other root rots is the elimination of residue. However, elimination of residue by tillage or burning does not seem to affect the incidence or severity of eyespot (strawbreaker).
Seed treatments: There are several excellent reasons to plant seed wheat treated with an insecticide/fungicide seed treatment. These include:
- Control of bunts and smuts, including common bunt (also called stinking smut) and loose smut. The similarity of these names can be confusing. All affect the grain of wheat, but whereas common bunt and flag smut spores carryover on seed or in the soil, loose smut carries over in the seed. Seed treatments are highly effective in controlling all the bunts/smuts. If common bunt (stinking smut) was observed in a field and that field is to be planted again with wheat, then planting certified wheat seed treated with a fungicide effective against common bunt is strongly recommended. If either common bunt or loose smut was observed in a field, grain harvested from that field should not be used as seed the next year. However, if grain harvested from such a field must be used as seed wheat, treatment of that seed at a high rate of a systemic or a systemic + contact seed treatment effective against common bunt and loose smut is strongly recommended. For more information on common bunt & loose smut, see: http://www.entoplp.okstate.edu/ddd/hosts/wheat.htm and consult the “2017 OSU Extension Agents’ Handbook of Insect, Plant Disease, and Weed Control (OCES publication E-832),” and/or contact your County Extension Educator.
- Enhance seedling emergence, stand establishment and forage production by suppressing root, crown and foot rots. This was discussed above under “Root and Foot Rots.”
- Early season control of the aphid/BYDV complex. This can be achieved by using a seed treatment containing an insecticide. Be sure that the treatment includes an insecticide labeled for control of aphids.
- Control fall foliar diseases including leaf rust and powdery mildew. Seed treatments are effective in controlling foliar diseases (especially leaf rust and powdery mildew) in the fall, which may reduce the inoculum level of these diseases in the spring. However, this control should be viewed as an added benefit and not necessarily as a sole reason to use a seed treatment.
- Suppression of early emerged Hessian fly. Research suggests that some suppression can be achieved, but an insecticide seed treatment has little residual activity past the seedling stage.
Authored by: David Marburger and Bob Hunger
Wheat streak mosaic (WSM) was a significant disease issue for a number of producers in Oklahoma this past year (Figure 1). This disease, caused by wheat streak mosaic virus, is transmitted by the wheat curl mite (Figure 2). This mite can survive from one wheat growing season to the next on a number of different grass species, but primarily on volunteer wheat. Some like to refer to these host weeds or volunteer wheat during the summer fallow period as the “green bridge.” One of the best ways to manage this disease is controlling the wheat curl mite by eliminating the green bridge, and this can be done with tillage or a herbicide application. Since the life span of the wheat curl mite is about two weeks, volunteer wheat needs to be dead (i.e., no green tissue) for at least two weeks prior to the emergence of the newly planted wheat crop. If you spray glyphosate two weeks prior to emergence of the new wheat crop, for example, this will likely not control the wheat curl mite as the volunteer wheat will not be dead (no green tissue) for that minimum two week period.
Figure 1. Symptoms of wheat streak mosaic.
Figure 2. Wheat curl mites on a leaf surface at approximately 30-40x magnification. Source: Texas A&M.
With the current wheat prices, rain that has fallen, and rain in the forecast (trying not to jinx it here), producers especially in southern Oklahoma are going to be ready to plant by the end of August into early September to target more fall forage. In that scenario, volunteer wheat needs to be controlled right now in order to have that plant tissue dead for the minimum two week period prior to the emergence of the new wheat crop. For producers who will be planting a little later than this, there is a little bigger window for controlling that volunteer wheat yet, but it is something to keep at the top of your to-do list.
More information on wheat streak mosaic and wheat curl mites can be found in the OSU Fact Sheet EPP-7328 Three Virus Diseases of Oklahoma.