Farm Talk

October 22, 2013

Approaching disease from different perspectives

Josh Coltrain
CNHI

Parsons, Kansas — What does a California field of barley from 1951 have to do with a field of Kansas wheat in 2013? The much reviled disease barley yellow dwarf (BYD) virus was named from a ferocious outbreak of the disease back in 1951 in California. The story goes that some fields of the cereal grain turned a bright yellow within a single week in April. By the end of May, symptoms of the disease were found in every barley producing county across the state.

As stated earlier, BYD is caused by a virus. This virus infects a broad range of plants including cereal grains, perennial weeds, and forage grasses. Viral diseases, in general, need a vector to carry them from plant to plant. In BYD’s case, the virus is vectored by aphids. In Kansas, the two most important vectors are greenbugs and bird cherry-oat aphids. The virus is transmitted into the plant through the aphid’s saliva.

The aphids migrate into Kansas from southern states in the fall through the spring. Once the aphid feeds on an infected plant it then becomes a carrier of the virus. The virus has no known effect on the aphids themselves. Also, direct damage from the feeding is rarely enough to cause an issue.

Two K-State researchers have recently published information concerning BYD. C. Michael Smith, K-State Entomology Professor, has been studying the aphids and William Bockus, K-State Plant Pathology Professor, has been comparing disease levels to yield loss.

In Dr. Smith’s study which started in 2012, the researchers looked at what percentage of aphids collected actually carried the BYD virus. To begin the study, they first had to develop a technique sensitive enough to detect the virus in aphids. Then they had to decided how to accurately sample the aphids since the virus is unable to be detected in dead aphids. To do this, they needed the cooperation of some county Extension agents, area agronomy specialists, consultants, and producers.

The first results have been released, and the numbers are interesting, at least to me. From southeast Kansas, there were five samples submitted, three from Wilson County, and one each from Crawford and Cherokee Counties. The three samples from Wilson county resulted in 10 percent, 20 percent, and 20 percent of the aphids in each sample carrying the BYD virus. The sample from Cherokee County included 30 percent of the aphids carrying the BYD virus. Unfortunately for the producers in Crawford County, 100 percent of the aphids in the sample submitted carried the BYD virus. Only one other sample in the state (from Butler County) reached this level.

In William (Bill) Bockus’s seven year study, Bill assigned a graduate student, Jenna Gaunce, to study how disease incidence affects yield. Counter-intuitively, literature from around the world states that the percent symptoms of BYD do not correlate to the yield loss from that disease. However, as Bockus and Ms. Gaunce have shown, it does matter in Kansas.

In their study, they purposely planted the 12 replicated cultivars two weeks early to ensure infestation of the aphids. Throughout the year, Jenna would rate the plots on a percent symptom basis which would later be compared to the yield data. The results were more staggering than they initially anticipated. The overall r-squared value was 0.29. Simply stated, this meant that across the seven years, 29 percent of the yield of the plot could attributed to the varietal response to BYD.

Using their data, they developed a linear model of Jenna’s observation of disease incidence versus the yield data. The model shows that a very susceptible variety (a 9 on the 1-9 scale) would lose 48 percent of its potential yield from BYD. Bockus also used the popular variety Everest as a comparison. Everest is a 4 on the 1-9 scale which would be considered moderately resistant and lost 18 percent of its yield due to BYD.

Unfortunately, Everest is rated as the most resistant of the 59 varieties rated by K-State Research and Extension in the publication titled Wheat Variety Disease and Insect Ratings 2013. Hopefully, with continued work from dedicated researchers, there will be some more BYD resistant varieties available to producers in the near future. If you have questions or would like more information, please call me at the office 620-724-8233, or e-mail me at jcoltrain@ksu.edu, or visit the Wildcat Extension District website at www.wildcatdistrict.ksu.edu. £