Nathan Kleczewski, Extension Specialist – Plant Pathology; email@example.com
Wheat in southern parts of Maryland and Delaware, as well as some early flowering varieties, began flowering the latter half of last week. Many fields are now breaking boot or at full head. As a general rule of thumb, expect flowering to start about 3-5 days after full head emergence, depending on temperature. Flowering occurs when 50% of the main tillers have started to produce yellow anthers from the center of the heads.
Hundreds of replicated university trials conducted over multiple states and seasons have shown that the application of Caramba, Prosaro, and Proline at flowering reduce fusarium head blight (FHB) severity by 50% AND DON by 45% relative to untreated, inoculated checks (Figure 1). That means that if you have an FHB outbreak and 20% of your heads are infected, resulting in 5 ppm DON, these products may bring down your levels to 10% severity and 2.75 ppm DON. The use of moderately resistant varieties will further reduce disease and DON, on average, 45%. In the same scenario, the MR variety would start with 11% infected tillers and 2.75 ppm DON. The application of a fungicide to this variety would potentially reduce infected tillers to 5.5% and DON to 1.23 ppm.
Figure 1. Average percent control of head bleaching (Index) and DON contamination (Vomitoxin) from 309 trials conducted over 19 years. Percent control is used to standardize the data because different years, cultivars, or locations may result in different total amounts of FHB or DON. Note that if you want to plant vegetables, Proline can be used due to its 30 day replant label.
A moderately resistant variety reduces DON compared to a susceptible check (e.g. Shirley) by roughly 45%. Some varieties I see planted this year were sold as moderately resistant based on visual ratings, but not DON. When looking at the DON, I would categorize some of these as moderately susceptible — better than a susceptible variety but not as good as moderately resistant variety. Remember, visual symptoms only predict DON about 60-70% of the time and FHB can elevate DON in healthy looking kernels. Always start your variety selection by assessing DON levels compared to a susceptible check, then rank by visual symptoms, or index.
More recent data indicate that applications 5-6 days after flowering can be just as efficacious as those at flowering. Some growers plan on applying a fungicide for FHB and are concerned about uneven emergence in the field. I suggest estimating when the earliest tillers will flower, and considering an application later in the 5-day window. This may allow the fungicide to be applied to both early and late flowering heads in the field within the 5-day window.
The scab prediction center predicts elevated levels of FHB risk FOR SUCEPTIBLE VARIETIES flowering through the end of the week. Figure 2 shows the difference in risk between a susceptible (e.g Shirley) and a moderately resistant (e.g. Jamestown) variety that was flowering on 5/4/16. Red indicates severe, yellow moderate, and green low levels of risk. Click on the Agnet box to see the data for the DEOS station nearest to your fields.
Figure 2. Difference in FHB risk between a susceptible wheat variety flowering around 5/4/16 (left) and a moderately resistant variety flowering around 5/4/16. From http://www.wheatscab.psu.edu/.
DO NOT apply QOI containing products, including premixes, to heads. Replicated university trials conducted across multiple states indicate that these products can elevate DON if FHB occurs. Note that most of these products do not even list FHB suppression on the label. They are not recommended for FHB management. See the NCERA-184 fungicide list, which I provided in last week’s WCU, for more information.
The forecast is calling for an increase in temperatures next week. Stripe rust shuts down when temperatures exceed 69°F. Powdery mildew slows as temperatures increase. Increases in temperature and low levels of relative humidity in the upper canopy can limit development of powdery mildew to lower portions of the plant, which contribute little to yield after stem elongation. Relative humidity, not water or leaf wetness, determines the level of infection by the powdery mildew pathogen in susceptible varieties. Recent cool temperatures have limited the development of our most common foliar diseases, the leaf blotch complexes. These may increase with warming temperatures provided we receive adequate amounts of rain.
In barley we are well past the point for managing diseases. However, the lesson this year, particularly for those of you contracting malting barley, is when to apply fungicides for suppressing DON. Barley begins to flower while in the boot, and continues to flower as the head emerges. It also is a closed flowering plant, unlike wheat. For these reasons we do not see significant FHB-derived yield losses in barley as the fertile anthers are not exposed to the environment to a significant degree. However, infection of the glumes by the FHB pathogen can result in elevated DON that can be a major issue for malt and overall grain quality. Consequently, fungicide applications for malting barley and barley in general need to occur when heads have just fully emerged (Figure 3). This ensures the glumes are protected by the product once the head emerges from the boot. If you see white spent anthers from the head you have missed your application window and could run into PHI issues. Remember that malting barley needs to be harvested at a higher moisture content than grain barley- be prepared.
Figure 3. When to apply fungicides to malting barley to suppress DON. Left most tiller – too early. Rightmost tiller – too late. Middle two tillers – optimal stage