By Pierce Paul, Katelyn Willyerd and Dennis Mills, Ohio State University Extension
This past fall, most of our wheat was planted at the recommended time due to early soybean harvest, however, dry conditions in late-fall and early-winter, prevented adequate growth and tiller development going into to dormancy. This could potentially result is higher-than-usual winter kill and poor stands. So, some growers are understandably concerned about this year’s wheat crop. However, it is still too early to tell what will happen.
One positive is the fact that the crop has had very good snow cover during most of the winter. What happens over the next few months or so will be critical. Rapid and repeated freezing and thawing could cause heaving and damage to the crop, but a more gradual transition could see the crop going into the spring in decent shape. Tillering will resume in the spike, making up for poor fall growth. So, we will have to observe the weather and the crop over the next few months before making an assessment.
Questions have been raised about the effects of ice on the crop in areas where the snow has melted and the water later becomes frozen. Being a cold season grass, the wheat plant can tolerate fairly harsh weather conditions. It is programmed for this by keeping the growing point (the crown) below the ground until conditions are warm in the spring. However, extremely cold conditions could still cause damage to the plant. Once the leaves are hardened (which occurs in the fall), they can tolerate temperatures between 0 and 10 F. Younger leaves are more tolerant to cold conditions than older leaves. Roots may be killed by temperatures between 23 and 26 F, but these are usually protected from such temperature by being below the soil line. The plant is only killed outright by low temperatures if the crown is damaged. So, essentially, unless the crown is killed, the plant will survive freezing conditions.
For varieties adapted to cold conditions, such as those grown in Ohio, the crown can withstand temperatures as low as -9 to -11 F. The existing snow cover will provide the necessary insulation to prevent temperatures capable of killing the crown. Since most of our wheat was able to harden in the fall before the snow cover, the crown is less vulnerable to damages if the snow melts and extremely cold temperatures occur shortly after.
There are also concerns about head scab going into this year after wheat faced serious setbacks from the disease in 2010. Over the last few years we have observed several new trends in the timing of foliar fungicide applications in wheat, and in 2010, we conducted several field trials at multiple locations to evaluate the efficacy and benefit, if any, of these new fungicide programs. Our current recommendations are to apply a foliar fungicide at flag leaf emergence or heading to control foliar diseases, if a susceptible variety is planted and weather conditions are favorable for disease development.
For head scab management, we recommend applying a triazole at flowering, along with planting the most resistant variety and rotating wheat with soybean. These recommendations are based on years of research that consistently show that they are the most effective and economical fungicide programs for disease management in wheat. However, recently, some growers have expressed interest in several new foliar fungicide application programs, including applications at green-up, split applications, and applications at flowering.
Quite often, the primary purpose of these new programs is to “preserve” or “protect” yield and not necessarily to manage diseases. Before presenting results from our 2010 trials, let us first recap how the yield is made in wheat in an effort to better understand the rationale behind current management recommendations.
Growth and development of the wheat plant can be divided into four distinct phases: tillering, jointing or stem extension, heading, and ripening. The main yield components of the crop are head-bearing tillers per acre, spikelets per head, kernels per head, and kernel size. These components are determined at different stages in the development of the plant, therefore the crop needs to be managed during each of the four stages to achieve the best yield and minimize yield and quality losses.
Seeding rate, planting depth, fertilizer application, planting date, and weed and seedling disease control are all important for determining the number of tillers per acre, and as such, are commonly managed at planting or during the tillering stage of crop development. The head begins to develop and the number of potential spikelets per head and head size are determined during the late tillering and early jointing growth stages (Feekes 5 and 6). Nitrogen application is important at this stage of crop development since it can affect the number of kernels per head.
Stresses such as severe drought at Feekes 5/6 may also reduce the potential number of kernels per head. However, the number kernels that ultimately develop per head and size of these kernels depend on pollination and grain fill. The flag leaf (the uppermost leaf of the plant) contributes about 75% of the compounds needed for grain fill. As a result, it is very important to protect the flag leaf from damage caused by foliar diseases and insects, since these may substantially reduce grain yield and quality if the damage occurs before grain fill is complete.
Before the 2009-2010 growing season, most of the early (before flag leaf emergence) and late (after heading) foliar fungicide application programs had not been tested in Ohio with currently available fungicides. However, results from previous studies showed that the greatest benefits from foliar fungicide applications were obtained when applications were made between Feekes 8 and 10. This is largely because most of our major foliar diseases, with the exception of powdery mildew, usually develop and reach the flag leaf after Feekes 8-9. In 2010, we evaluated the effects of single, split, and double applications of several triazole (Prosaro, Caramba and Folicur), strobilurin (Headline), and combination (Twinline, Quilt, and Stratego) fungicides on powdery mildew, Stagonospora, head scab, and grain yield.
Applications were made at green-up, flag leaf emergence, boot, and flowering. Among the single application programs, applications made at flag leaf emergence or boot did better than green-up applications in terms of foliar disease control and yield. A single application of a triazole at flowering provided the best control of head scab. Among the programs with double or split applications, we observed the best results with those treatments that included an application at full rate at Feekes 8-9.
A single full-rate application at this growth stage did just as well or better than the grean-up+flag leaf or the flag leaf+flowering applications. Comparing a single application at flag leaf emergence with a single application at flowering, all of the tested fungicides resulted in better control of powdery mildew when applied at flag leaf emergence than when applied at flowering, and comparable levels of Stagonospora control were achieved with the two programs.
This is largely because powdery mildew develops early and as such applications made at flowering are generally too late to provide the best control of this disease. Stagonospora, on the other hand, usually develops later in the season, and in a wet growing season like 2010, foliar fungicides may still provide very good control when applied at flowering. In fact, because of the high levels of powdery mildew, Stagonospora, and head scab we had in 2010, the fungicide programs that provided the best overall control of all three diseases and resulted in the highest yield gain were those with a triazole applied at flag leaf emergence followed by a second application at flowering.
However, it is rarely ever beneficial to make two foliar fungicide applications in wheat in Ohio. The yield gain is generally not sufficient to offset the cost of two applications. If foliar diseases are a concern, then one well-timed application between Feekes 8 and 10 should be sufficient to control powdery mildew, Stagonospora, Septoria, and leaf rust. If head scab is of concern, a well-timed application at flowering will provide the best control of scab, while at same time provide protection against the late development of Stagonospora and rust. In general, conditions favorable for scab development are also favorable for Stagonospora.
In summary, here is what we observed in our trials and what we recommend:
1. For foliar disease control, fungicides provided the best results when applied between Feekes 8 (flag leaf emergence) and Feekes 10 (boot).
2. Applications made at Feekes 8 or 10 did better than applications made at green-up or split applications in terms of disease control or yield response.
3. If the variety is susceptible and weather conditions are highly favorable throughout the growing season, an application at full rate at Feekes 8 or 9 followed by a second at flowering may be beneficial, if the value of the crop is high.
4. Fungicides are generally not beneficial when resistant varieties are planted.
5. For adequate head scab control, fungicides should always be applied at flowering or as close as possible to this growth state.
Fungicides with strobilurin chemistry are not recommended for scab control since they have been associated with increased levels of vomitoxin in the grain.