Evaluating Late Season Diseases in Small Grains

Now that we have passed flowering in many areas of the state, this is the time to start to think about how to evaluate the efficacy of any fungicide treatments and determine the potential impact of any disease on final yield. This is especially important not only for Fusarium Head Blight (FHB) but also for important foliar diseases like rust and powdery mildew. In certain years, we may also observe heads damaged due to low temperatures and frost when scouting at this growth stage (Figure 1).

In the southeastern parts of PA, now is a good time to start walking the fields to make these assessments.  In central PA, wait for about one to three weeks for best symptoms to appear. We recommend scouting and assessing multiple areas of the field (minimum of 10 locations) wherein we examine 10 randomly selected plants at each location. We recommend that you use one of the following patterns: "zig-zag", "diamond-shaped", "X-shaped", or "W-shaped" to get around the field to make your assessments.

Assessing Fusarium head blight in the field: One of the challenges in assessing FHB is to take a reliable assessment of disease intensity. Three different measures are often commonly used: (1) disease incidence, (2) disease severity, and (3) FHB index. Each of these measures tells us something about the overall impact of FHB. Disease incidence is simply the number of FHB infected ears divided by the total number of ears assessed multiplied by 100 to get a percentage. For example, if you assess 100 ears and find that 50 of those have FHB symptoms, the incidence would be 50%. Disease severity is a measure of the number of spikelets that have FHB. As we highlight in Figure 3 for several different examples, it is very easy to find ears that have no FHB (disease severity = 0%) to 100% of the spikelets infected. Assessing at least 100 ears from different locations in the field should provide for a reliable estimate of the overall disease severity. Lastly, it is very common to see reported in variety and fungicide tests the FHB index, which takes into account both the disease incidence and disease severity. To calculate this value, you multiply disease incidence by disease severity, which is then divided by 100 to give a final value that ranges from 0 to 100. For example, if we had 50% disease incidence and 25% disease severity, the FHB index would be (50*25)/100 = 12.5. FHB index values of 10 or greater are often considered high. With this information, farmers can select fields to test for mycotoxins before harvest.

When planning your harvest, you may also consider the incidence value, which is the percentage of heads affected by scab. If more than 25% of your wheat heads show signs of scab:

• Use a high fan speed on your combine. This helps remove lighter, infected kernels, which typically contain the highest levels of vomitoxin.
• Segregate infected fields from clean ones. Infection levels often vary within a field or between fields, depending on factors like planting and flowering dates.

If you suspect vomitoxin contamination:

• Contact your crop insurance agent before harvesting. They can guide you on the correct procedures for sampling and testing.
• Follow their instructions carefully if an insurance adjustment is needed. Testing must usually be done prior to harvest to qualify.

Lastly, remember that even with moderately resistant varieties and fungicide applications, 100% protection against FHB cannot be achieved, particularly when weather conditions are conducive to disease development.

Diseases that could be confused with FHB are glume blotch and black chaff. However, both diseases will lead to the darkening of the glumes rather than the typical bleaching observed in FHB and will not produce the scabby/tombstone grains. For more details on these diseases, visit Crop Protection Network. 

Also leading to confusion this season may be damage to heads resulting from late freeze events, which can produce symptoms similar to those of FHB. Figures 1 and 4 show the spikelet darkening many have observed on barley in central Pennsylvania this spring. Compare this to an image of FHB symptoms on barley in Figure 5, and you will see these heads are visually identical. Both can cause water-soaking and discoloration of entire sections of the spikelet and prevent normal development of the kernel. How, then, do we determine the root cause of our head damage? One way is to take note of the timing of events in relation to the growth stage of the crop. Frost damage will likely be observable within a week of the cold event. It may also be accompanied by twisting and deformation of the heads as they continue to emerge.  FHB symptoms, on the other hand, will not be visible until about three weeks after flowering (or head emergence in the case of barley). Another clue is the fuzzy growth (mycelium) which can be seen in Figure 5. A light pink, purple, or orange color and fuzziness of the glume may develop with moist environmental conditions on FHB-affected heads. If a positive identification is desired, Pennsylvania farmers can send samples to the Penn State Plant Disease Clinic.