ISSUE 7   June 24, 2010


Selecting a soybean variety with known tolerance to iron-deficiency chlorosis (IDC) is the best strategy to minimize the IDC symptoms and lower the risk of yield reduction in fields with a known history of IDC. This year we see many yellow soybean fields in eastern North Dakota and in the northwestern part of Minnesota. Yellowing between the veins of the trifoliolate leaves and green veins is the easiest to recognize symptom of IDC. In a severe case the leaf tissue, including the dominant growing-point, may die.

There are genetic differences among varieties in how they express IDC. Some soybean varieties are less susceptible to IDC because they are genetically bred for tolerance. One way to evaluate the tolerance of a variety to IDC is to use visual ratings at the early vegetative and flowering stages. This is the standard method used to evaluate all the varieties entered in the NDSU soybean yield trials. The different varieties are planted in replicated, side-by-side tests, on field with known IDC history and the amount of yellowing is scored. Visual ratings are made on a 1-5 scale, with 1 = no chlorosis and 5 = the most severe chlorosis. The results are reported each year in NDSUís North Dakota Soybean Performance Testing publication (A-843) and online at:

All IDC test results can be found at:

In addition to IDC scores, publication A-843 (2009 data) also reports replicated yield data of 30 varieties on IDC sites.

There is an increased interest in the farming community to explore precision planting using global positioning systems. In the near future it may be possible to plant a number of different varieties in the same field, targeting specific varieties adapted to known stress factors in the field. In order to plant the correct variety producers need to create maps indicating where plant stress occurs. In the case of IDC, the map needs to be put together during the early vegetative stages in soybean fields with IDC symptoms. This would be an ideal year to put such a map together. Based on previous research, it is known that soil factors, such as pH, salt content and calcium carbonate levels, are associated with IDC symptoms. With excess moisture and relatively cool temperatures IDC is showing up this year in many fields.

Research was conducted in 2006 and 2007 on North and South Dakota fields that had a past history of IDC. The same 18 varieties were planted on the portion of the field that had IDC conditions and another area of that same field that did not have this yield limiting condition. The objective was to determine whether the varieties with the least amount of yellowing, due to IDC symptoms, also were the highest-yielding varieties on IDC areas. A second objective was to determine whether yield could be increased by planting two different varieties on the same field. The results showed that when two varieties had the same visual IDC rating, one variety was higher yielding than the other variety on the IDC areas. For example, when two varieties were compared, each with a visual rating of 2.5 (on a scale from 1 to 5, see above), one variety yielded 42 bushels per acre and the other variety yielded 32 bushels per acre on the IDC area of the same field.

Visual ratings for IDC are useful to identify a tolerant variety. However, visual ratings for IDC, combined with yield evaluation on IDC ground, is the best way to select a variety that has IDC tolerance as well as high yield on IDC areas. Varieties that were not tolerant to IDC often were the highest-yielding variety to plant on the areas of a field that did not have IDC. This suggests that farmers can potentially increase the yield of a field by planting an IDC-tolerant variety on the IDC areas of a field and plant a different, high-yielding variety on the areas of the field that do not typically have IDC. Yields on the whole field might be increased by 2.5 to 5 bushels per acre by planting two different varieties on the same field. The varieties need to be matched for maturity so the field can be combined at the same time. This research suggests that planters that will use GPS coordinates to switch varieties and potentially seeding rates as the planter travels across the field need to be developed and marketed.

Soybean plant with IDC symptoms. The cotyledon
and unifoliolate leaves are green. The first
trifoliolate is yellow with green veins.

Ted Helms
NDSU Plant Sciences Department

R. Jay Goos and David Franzen
Soils Department

Hans Kandel
Extension Agronomist broadleaf crops



Recently, the risk for scab (Fusarium Head Blight or FHB) infection has been high in many regions of the state for small grain crops that are flowering. With more rain and humid weather in the forecast, the risk for scab development will likely be sufficiently high to justify the application of fungicides to help protect vulnerable crops. Though currently recommended fungicides do not guarantee complete scab control, they can reduce yield losses and DON levels in the harvested grain, particularly in years of moderate to high disease pressure.

The timing of fungicide application, however, is key to optimizing the effectiveness for fungicides applied for the control of FHB. Moreover, the optimum application timing varies slightly between wheat and barley. Since not all spikes within a field emerge at the same time, applications should be made when the majority of the spikes are at the optimum stage. Small, late emerging tillers should be ignored when staging a field of wheat or barley, as these tiller are often several days behind the main stems/tillers in development, and do not contribute significantly to yield.

Timing in wheat

The optimum growth stage for applying fungicide for scab control in wheat, the Feekes 10.51 stage, is the same for all classes of wheat (winter, spring and durum). The Feekes growth staging system (see table) is used as it describes development from spike emergence to graining-filling in more detail than other systems. Applying fungicide during early flowering helps protect against the fungus infection when it can do the greatest damage. The length of time from head emergence to the beginning of flowering is temperature dependent, but usually takes about three days. Experience has shown that it is better to apply fungicide too early rather than too late, but fungicides applied too early, before the head has completely emerged, are generally ineffective against scab.

Feekes scale: start of heading to early grain filling




First spikelet of head just visible


One-fourth of head emerged


One-half of head emerged


Three-fourths of head emerged


Head emergence complete


Beginning of flowering (for wheat)


Half of florets have flowered


Flowering complete

10.5. 4

Kernel watery ripe

The center spike in the following durum photo is at the ideal stage for applying fungicide in wheat. The spike on the left has emerged from the boot, but has not yet started to flower (note that there are no visible anthers extruded from the glumes). The spike on the right is past the optimum stage. Note that the anthers are bleached and dried, unlike the turgid, yellow anthers in the center spike.

Timing in barley

Flowering in barley begins just before head emergence, so barley florets are not overly susceptible to scab infection. Although scab infections do not generally impact yield significantly in barley, the fungus is able to infect the glumes and produce DON (vomitoxin), which impacts the value of the grain in the market. The optimum stage for applying fungicides to protect the glumes of barley from infection is when the spike is fully emerged from the boot (Feekes 10.5). In the accompanying photo of barley photo, the plant second from the right is at the optimum stage for spraying. The plant on the far right is may be beyond the optimum stage, if the kernel development is already in milk stage. With barley, the appearance of the first spikelet from the boot, like the first plant on the left in the photo above, is a good indication that the best stage for spraying is only a few days away.

Joel Ransom
Extension Agronomist

Marcia McMullen
Extension Plant Pathologist

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