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Disease and Insect Considerations to Make Before Planting Wheat This Fall

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.

 

Given this disease cycle, it is easy to see several factors that determine the incidence and severity of these diseases. First, controlling volunteer wheat and other grassy weeds that serve as alternative hosts for the mite and the viruses is imperative to help limit these diseases. 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. Wheat infected in the spring also is damaged, but not as severely as wheat infected in the fall. 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.

 

A second factor linked to the severity of these mite-transmitted virus diseases is planting date. Early planting dates associated with grazing provides for a much longer time period in the fall for mites to spread to and infect seedling wheat. Planting later in the fall (after October 1 in northern OK and after October 15 in southern OK) and controlling volunteer wheat are the 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.

 

The incidence and severity of these mite-transmitted virus diseases as affected by planting date can be illustrated by the number of samples that tested positive for WSMV and HPV in 2017 compared to 2018.  In 2017, 103 wheat samples were tested by the Plant Disease and Insect Diagnostic Lab at OSU for presence of mite-transmitted viruses. Of these 103 samples, 69 (67%) tested positive for WSMV and 22 (21%) tested positive for HPV. In 2018, only 12 of 126 (10%) samples were positive for one or both of these viruses. For a number of reasons, the planting date of wheat across Oklahoma in the fall of 2017 was significantly later than the fall of 2016. I believe this helped to lower incidence and severity of the mite-transmitted viruses in Oklahoma in 2018 compared to 2017.

 

Finally, seed treatments and insecticides are not effective in controlling the mites or these mite-transmitted virus diseases. Regarding resistant varieties, 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, and the resistance is not typically an absolute resistance to the disease. Hence, severe and continuous disease pressure especially at higher temperature (greater than about 75 F) can overcome the resistance.  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.

Figure1

Figure2

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.

Figure3

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).

Figure4

Figure5

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. In 2017-2018, the incidence and severity of root rots across Oklahoma dramatically increased. This increase likely resulted from weather conditions that favored the root rots. Dryland (Fusarium) root rot was the most common root rot observed, and caused significant damage to wheat in southwestern, western, northwestern OK as well as the panhandle.

 

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).

Figure6

Seed treatments: There are several excellent reasons to plant seed wheat treated with an insecticide/fungicide seed treatment. These include:

  1. 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.
  2. Enhance seedling emergence, stand establishment and forage production by suppressing root, crown and foot rots. This was discussed above under “Root and Foot Rots.”
  3. 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.
  4. 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.
  5. 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.

 

2017-2018 Oklahoma Wheat Crop Overview

At the time of writing this report, 2018 Oklahoma wheat production is estimated to be 52.0 million bushels, which is 47% less than the 2017 production (Table 1) and 62% less than the 2016 production. The lower total grain production is the result of less wheat acres harvested across the state, primarily from abandonment due to drought or baled for hay, and the below-average yield. The 4.3 million planted acres was only down 4% compared to the previous year, but that was still 18% lower than the previous ten-year average. Number of harvested acres is estimated at 2.0 million, which is 31% less than in 2017 (Table 1), and the lowest number in the state since 1913. The statewide average yield is projected at 26 bu/ac. This is 8 bu/ac (24%) less than the 2017 state average and 3.6 bu/ac (12%) less than the previous ten-year average.

table 1

The 2017-2018 wheat growing season was a fight from start to finish for many producers across the state. The growing season got an early start due to an unusual August for Oklahoma. Temperatures were below normal, and rainfall totals were above normal for the month. This prompted producers interested in targeting fall forage to begin planting at the end of August. Planting continued to move rapidly through the Labor Day weekend, and most of the wheat during this time was sown into adequate soil moisture and emerged rapidly. Those producers who waited until after Labor Day to plant saw more unfavorable conditions as temperatures rose, and available soil moisture quickly dried up. Wheat planted during this time was “dusted-in” and finally received precipitation toward the end of the month into the beginning of October to get the seed to germinate. Wheat planting intended for grain-only was stalled during the average timeframe of early to mid-October due to these precipitation events. Once the ground dried enough, most producers were able to quickly make up time and get the crop planted, but some needed until November to finish.

 

After mid-October, the rain quit falling for the remainder of the calendar year. Crop conditions during the early part of the season were average but quickly deteriorated as the season progressed. This also led to a disappointing fall forage production and grazing season for most producers. Those who planted during late August to early September and were able to protect the crop from fall armyworm achieved good stands and had some available pasture later in the fall. However, those who waited until after Labor Day or later to plant were not as fortunate. The later planting and lack of precipitation resulted in low total fall forage production or no available pasture at all.

 

Drought conditions and average to below average temperatures persisted throughout January into February. Even for the producers who had available fall pasture, the drought conditions limited the overall number of days of grazing.

 

Some precipitation finally fell in parts of the state during late February into early March. For many fields, this was the first precipitation received since planting. Below average temperatures were observed coming out of winter, and plants broke winter dormancy later than normal. Below average temperatures persisted, resulting in slow overall growth and development during this time. The first hollow stem growth stage was reached for many varieties during the second to third week of March, which was 7 to 10 days later than normal. Unfortunately, the rain received during late February to early March was not quite enough to give any grazed wheat the boost it needed to recover well.

 

Overall growth and development continued at a slower than normal pace due to the second coldest April on record. Three separate and widespread freeze events also occurred during the first week of April, resulting in significant injury in some areas. Most wheat headed during mid- to late April because of the cooler temperatures, with this being 7 to 14 days behind normal. The prevailing thought was that this would translate into a later than normal harvest. However, the cold temperatures in April were followed by the warmest May on record. The warm temperatures and lack of rainfall advanced the crop quickly at this point, resulting in suboptimal conditions for the grain-fill period.

 

Most wheat was mature in southwestern Oklahoma by the end of May and by the beginning of June in the central to northern parts of the state. Producers for the most part were not delayed by rainfall events, and with the dry weather during June, much of the wheat was harvested timely and quickly.

 

Overall, harvest was almost complete in the state by late June. Yields throughout Oklahoma were variable depending on location but were below average overall. Part of this variability was due to overgrazing and whether an area caught or missed a rainfall event during early spring. Field averages of 15 to 30 bu/ac were the norm across much of the state, but higher averages, even into the 50 to 60 bu/ac range, were not uncommon in some areas that received timely rainfall. Test weights throughout harvest remained at or above 60 lb/bu for early-harvested fields and did not drop much below the upper 50’s towards the end of harvest. Protein content also remained at or above acceptable levels.

 

Different insects were a concern at times during the growing season, but few were widespread or season-long outside of the fall armyworm. Unless treated, the fall armyworm devastated those producers who planted in late August into early September. Many fields had to be replanted, and some producers commented that this was the worst that they had ever observed. Unfortunately, some reports indicated the fall armyworm was still causing damage into early November. The dry weather experienced across the state through the winter provided ideal conditions for winter grain mite and brown wheat mite to thrive on wheat plants coming out of winter dormancy, and there were some reports of fields warranting control. Aphids were not really on the radar screen of most producers until mid-March, but this pest was still not the limiting factor as observed in other years. Despite the low aphid numbers, Barley Yellow Dwarf (BYD) was evident in some fields as flag leaves and heads started to emerge. While there was quite a bit of leaf purpling and yellowing associated with BYD, there was not much stunting observed, with stunting resulting from “hot spots” of aphid pressure with early-season transmission of the virus. Wheat Streak Mosaic (WSM), transmitted by the wheat curl mite, was an issue again for producers in southwestern Oklahoma, but the overall impact of WSM was not as much as the 2016-2017 crop season. Reasons for this were related to later planting and emergence of some wheat; additionally, fields which may have had WSM were abandoned due to the drought or cut and baled for hay before symptoms could be observed.

 

Diseases were at low levels overall during the season, primarily due to the drought conditions. Parts of central to southcentral Oklahoma did experience low levels of powdery mildew, leaf rust, and stripe rust. In some cases, powdery mildew could be observed high in the canopy. For the remainder of the state, it was difficult to find foliar diseases, especially during stem elongation into the grain-fill period. One disease more prominent than in years past was Fusarium foot dry (dryland root rot). Signs and symptoms of this disease appeared suddenly during early May as hot temperatures returned and as the crop progressed through grain-fill. However, symptoms of this disease can appear similar to symptoms of premature death caused by freeze, drought, and other conditions. In parts of the northwest and panhandle regions, symptoms of dryland root rot may have been confused with symptoms caused by the drought and/or freeze, whereas in others (such as the wheat variety trial at Lahoma), damage caused by the April freeze events was expressed distinctly earlier. Because of the impact that leaf rust and stripe rust have had over the past several years, producers were ready to apply a foliar fungicide to susceptible varieties, but unfavorable conditions for disease development did not warrant an application in most cases. Variety trial results from Apache and Lahoma indicated that producers in these areas were justified in not spraying, as no evidence of a positive response to a fungicide application was found. However at Chickasha where low to medium levels of leaf and stripe rust and medium to high levels of powdery mildew were present, the two fungicide applications implemented at this location contributed to protecting the yield potential for a number of varieties compared to the non-treated plots of those same varieties.

Harvest for Oklahoma Wheat About to Wrap Up

Report by the Oklahoma Wheat Commission

 

Despite the scattered rains across parts of Northern Oklahoma this week, harvest progressed in most areas. Rainfall in the Panhandle region was limited and this allowed harvesters to get a majority of the irrigated wheat out of the fields. It is reported across the Panhandle and in North Central Oklahoma that approximately 5% of the crop is left in the field to harvest. In Northern Oklahoma the issue now is muddy fields, but producers are hopeful they will get back in these areas by the end of the weekend to finish this 2018 season. Test weights declined in Northern Oklahoma this past couple weeks with all the moisture. Most elevators are reporting that since the majority of the crop was harvested before the rains, overall it is not going to impact the final averages. Grain quality for the 2018 wheat crop across Oklahoma will be favorable with high test weights and high protein contents. Test weights on average will range from 60 to 62 lbs./bu., with reports on protein running 12.5 to 13%.  While quality will be high, the amount USDA currently estimates the Oklahoma Wheat crop to be forecasted at is 52 million bushels, down 47 percent from last year. Yield per acre is expected at 26 bushels, with 2 million acres that will be harvested.  This will be the last harvest report published by the Oklahoma Wheat Commission for the 2018 season.

 

Percentages of Harvest Completed Across Oklahoma

Southwest Oklahoma 99% Complete

South Central Oklahoma 99% Complete

Central Oklahoma 99% Complete

West Central Oklahoma 99% Complete

North Central Oklahoma 95% Complete

Northwest Oklahoma 98% Complete

Northeast Oklahoma 99% Complete

Panhandle 95% Complete

Entire State of Oklahoma 98% Complete

 

For more information on winter wheat acreage, yield and production estimates in other states click on the USDA June Crop Production Report listed below.

https://www.nass.usda.gov/Statistics_by_State/Oklahoma/Publications/Oklahoma_Crop_Reports/2018/spr-crop-prod-06-2018.pdf