Fusarium Head Blight Visual Severity Scale

Jessica S. Engle, Patrick E. Lipps, and Dennis Mills

Fusarium head blight (FHB) of wheat has become an increasing problem in the U.S. Corn Belt. FHB is the result of infection by the residue borne fungus Fusarium graminearum and can result in excessive yield loss and mycotoxin contamination. Diseased kernels are often shriveled and lower in weight, and are lost with the chaff during combining. Mycotoxins such as vomitoxin (deoxynivalenol or DON) produced by the fungus in grain and straw are detrimental to animal and human health. Currently, there are no wheat varieties with complete resistance to F. graminearum, only those with a moderate level of resistance.

Accessing the level of FHB in wheat is difficult. Disease incidence and severity should be used to evaluate the response of varieties. Disease incidence is calculated as the proportion of heads with symptoms out of a randomly chosen predetermined number of heads in a sample. Disease severity is determined by visually estimating the percentage of diseased spikelets on a head and calculated by averaging (including zero scores) the severities of all heads (Stack 1995). The currently used scale for visually assessing the percentage of diseased spikelets was developed by North Dakota State University Extension Service and uses an awned variety with about 13-14 spikelets per head (Stack 1995). In the mid-west, both awned and non-awned varieties are grown that have 13-20 spikelets per head in the field. A spikelet is a discrete group of 3-5 flowers (florets) on the head. Estimating the proportion of florets on heads having different numbers of florets reduces the accuracy of disease estimates.

Differences between an awned variety and an non-awned variety can lead to disease assessment problems. These differences lead to the construction of a scale using an non-awned variety. When an awned variety has premature spikelet death, the awn also dies and the appearance of the diseased tissue may lead the rater to overestimate disease severity. A non-awned wheat head will usually be more linear in shape compared to awned types.

A: White chaffed cultivar with symptoms of FHB.

B: Red chaffed cultivar with symptoms of FHB.

C: Red chaffed cultivar with symptoms of FHB on the lower portion of the head and symptoms of Stagonospora glume blotch on the upper portion of the head.

D: White chaffed cultivar with symptoms of Stagonospora glume blotch.

E: Close up of head with brown lesions of Stagonospora glume blotch near the tips of glumes.

Not only are there problems with evaluating disease on awned and non-awned varieties, but there are also evaluation difficulties between white and red chaffed varieties. On white chaffed varieties, the white glumes create a stark contrast in the field compared to the uninfected, green tissues. Heads of diseased red chaffed varieties appear much darker than those with white chaff. Stagonospora glume blotch can also be present on infected heads and has a similar coloring on red chaffed varieties. Stagonospora glume blotch can be distinguished by the darker brown colored lesions that are present near the tips of the glumes.

The assessment scale presented here utilizes heads with white chaff to assist in estimating the proportion of diseased spikelets on a head. Heads were of several genotypes grown in the greenhouse. When rating heads, compare the head to a similar representative picture and record the corresponding percentage of disease. To reduce sampling error and improve mean estimates, multiple raters may be employed, although correlation between evaluators has been high.

0% 1% 5% 9% 14% 14% 24% 25% 35% 35% 50% 66% 69% 89% 100%

Acknowledgments: Ms. Audrey L. Johnston for editorial comments.

References:

Stack, R. and McMullen, M. 1995. A visual scale to estimate severity of Fusarium head blight of wheat. (http://www.ext.nodak.edu/extpub/plantsci/smgrains/pp1095w.htm)