The following is an excellent article on wheat disease scouting and fungicide use decisions from Arv Grybauskas, Field Crops Extension Plant Pathologist at the University of Maryland
Maryland Field Crop Disease News
Wheat disease observations and recommendations – May 4, 2009
Wheat is starting to head in many parts of Maryland, which gets us to a tricky decision point regarding disease management with fungicides. There are some out there who like to apply a fungicide product like Quilt at heading, regardless of actual disease pressure at that time. It’s viewed as an insurance policy against late season diseases and in some cases it brightens the straw so that those with a market for straw can have a more desirable secondary product. Fungicide applications at heading can be very effective in management of Stagonospora glume blotch (a.k.a. Septoria glume blotch) and rusts and in some cases still effective against powdery mildew. However, there is new information regarding strobilurin and strobilurin-containing fungicides that impacts this decision.
Recent research indicates that fungicides that have an active ingredient that belongs to the strobilurin class can affect the levels of the mycotoxin, Vomitoxin (deoxynivalenol or DON), which develops in scabby grain. The strobilurin only products that are registered for wheat are Headline and Quadris; and the registered products that contain strobilurins are Quilt, Stratego and Twinline. It has been known for some time that all these products when applied at flowering for scab control can result in increased levels of DON. It is for this reason that they do not have scab suppression on their labels and are not recommended. What has only recently come to light is that the application of these products at heading can also lead to increased levels of DON if scab develops. Remember the mills are actively screening for DON and may reject loads on that basis alone. This unintentional consequence of a risk of increased DON levels due to an earlier fungicide application must be considered when thinking about fungicides at heading.
The stalled front that is fluctuating between Virginia and Maryland with pulses of low pressure systems generating showers this week and last is setting us up for an outbreak of scab. It’s also conducive for all the fungal diseases but let’s first cover scab. The long stretch of wet weather is allowing the fungus that causes scab to grow and develop on previously infected crop debris. In most of our cases this means corn stubble left in the fields. Current conditions are still on the cool side of optimum so the risk of scab if wheat was flowering today is low to moderate. Actually barley may be in flower or very close in some parts of the state and those fields are at risk of a developing some scab. As the weather continues wet and gets warmer we will have increased risk of scab in small grains. The keys are source of spores, 7-day pre-flowering weather and post-flowering weather.
The highest risk of getting scab is in fields that were planted no-till or minimum till into corn stubble. There is an equally high risk of scab in back-to-back wheat or barley crops. The lowest risk is in small grains rotated after soybeans or anything other than corn or small grains. Wheat after double-cropped wheat-soybeans is also a high-risk rotation. There is no zero-risk of scab if the weather is wet and warm for an extended period around flowering but the difference in disease levels that can develop is the difference between manageable and a total loss.
There is a scab forecasting web site http://www.wheatscab.psu.edu/ that provides a map of scab risk based on the 7-day pre-flower weather. I will be providing custom commentary for Maryland at this web site. This is a great way to determine if a fungicide may be needed for scab management. The drawbacks are: you need to check the risk maps frequently as wheat approaches flowering, the forecasts are only possible for a one to three day range, and the recommended products are only available in limited supply. The recommended fungicides for scab management are Prosaro, Caramba or a mixture of Proline plus Folicur. Prosaro is a premix of Proline and Folicur. Nothing else has provided comparable and consistent disease suppression to warrant recommendation.
The bottom line for wheat fungicides at heading for the current season boils down to the following questions. First, is the site high risk for scab based on cropping history? If it is then any strobilurin-containing product is an added risk and I don’t recommend it. In fact in most high scab risk sites a fungicide decision based on scab this season will provide the most return based on my observations of other diseases. Your situation may be different. See the following info on observations and risk assessment of other diseases below. If the site is not high-risk for scab, eg. after full-season soybeans, then what are the risks of the other yield-robbing diseases?
Powdery mildew appears to have gotten a late start this season. I have not heard of any commercial fields with high levels of mildew. There are no reports of the breakdown of resistance in popular lines. In my highly susceptible mildew fungicide trial plots on 30 April there is enough disease there to expect a return from a fungicide especially since the weather is favorable now and is not expected to get hot for a while. Disease there is at 100% incidence and the flag-1 leaf has an average of 4% of the leaf area colonized. Powdery mildew will increase in the next week or two. Only susceptible to moderately susceptible varieties have the potential for enough disease to expect a return from a fungicide this season. If there are several white fluffy mildew growths on the flag-1 leaf between flag leaf emergence and boot (and that means simultaneously much more in the leaves below) then a fungicide can provide an economical return if disease favorable weather continues. If the disease has not reached the flag-1 leaf by boot stage it is unlikely that the flag leaf will be affected by disease to the extent that a fungicide will provide a return.
We have just found our first couple of rust pustules in a susceptible variety at the Wye. This weather is favorable for rust to develop and it’s early enough to be a potential problem. It is possible that we have stripe rust as well as leaf rust. Cool temperatures are optimal for stripe rust while leaf rust prefers warm temperatures. Many of our varieties are susceptible to stripe rust. This may be an isolated find but it is important that people start looking. Fungicide applications up to flower on susceptible varieties when made before significant disease development occurs can be very beneficial.
Glume blotch has been very limited. This disease is favored by driving rains and warm temperatures. We are getting some of the rainy conditions needed for the disease but temperatures are below optimum. Threshold guidelines for glume blotch are based on 25% incidence of the disease on indicator leaves. For plants that are at full flag leaf development through boot, 25 % of flag -3 leaves have to be infected to warrant a fungicide application. From boot to heading 25% of the flag-2 leaf need to be infected. I have only found lesions in some fields near the soil, approx. flag-5, and at less than 25% incidence. In another system there must be an average of 2 lesions per flag-1 leaf to warrant a fungicide application (fig 1). These levels are too low to warrant fungicide applications at this time.
Consider your scab risk before you make a wheat fungicide decision this season.
Arvydas (Arv) Grybauskas, PhD
Associate Professor and
Field Crops Extension Plant Pathologist
Department of Plant Science and Landscape Architecture
University of Maryland