Sorghum Pest Update

ROCKVILLE, Md. (DTN) -- American sorghum varieties are surprisingly well equipped to take on the sugarcane aphid, according to a Kansas State researcher.

"It's very unusual to have such a large number of commercially available sorghum lines expressing what are almost certainly completely different sources of resistance to the sugarcane aphid," KSU entomologist J.P. Michaud told DTN. "Normally when you face a new invasive aphid, everything commercially planted is susceptible."

The sugarcane aphid has been anything but predictable since its unexpected sorghum infestations in 2013, but most of the developments up until now were negative. The pest abruptly took up sorghum as a host and has spread rapidly across sorghum-producing regions of the U.S. and Mexico for four years, requiring multiple insecticide treatments and driving up producers' costs.

As researchers and companies scrambled to study and understand the pest, they made a pleasant discovery, what Michaud calls "fortuitous resistance" among sorghum varieties.

Instead of spending multiple years combing the global germplasm collections for genetic resistance and then breeding it into commercial lines, researchers have been testing commercial sorghum lines and finding that the sugarcane aphid struggles to reproduce on a number of them. Each year, more aphid-resistant hybrids have been identified, and growers have more than ever to choose from in 2017.

Michaud's most recent line-up of aphid-resistant varieties boasts 30 commercially available sorghum hybrids from a dozen different seed companies -- and that is not necessarily a comprehensive list. "The list is growing all the time," he said. "These are hybrids that have been tested by two or more independent sources." (See the list here: http://bit.ly/…)

Companies are quick to warn that the aphid-resistant sorghum varieties are not immune to aphids -- the pests simply have much more trouble reproducing on them.

"Do not expect resistant plants to be aphid-free; they will still get infested, but the aphids will not thrive," Michaud noted in his KSU article listing the resistant varieties.

Normally, the hunt for genetic resistance to a new pest leads the industry to depend on one or two sources of resistance, often stemming from a single gene or trait, Michaud noted. "Then we have strong directional selection on a pest population to evolve virulence [the ability to reproduce on a resistant plant], and the source of resistance is no good anymore. We were on that treadmill with greenbugs for 30 years."

Because these aphid-resistant sorghum varieties likely derive their resistance from completely different genetic traits, the sugarcane aphid won't be able to evolve virulence quickly, if at all, Michaud said. "We're no longer pushing all the aphids in one direction, but in a lot of different directions," he explained.

Using plant resistance has additional benefits to growers, Michaud added. Growers won't have to reach for chemical controls as quickly, which gives beneficial insect predator population a better chance to thrive and adapt to their new meal option.

That's a great added tool against aphid virulence. "Once you get natural enemies evolving to be more effective, you get lower aphid populations, which reduces the chances of new forms of aphids," Michaud explained. "They simply can't evolve as easily because there are not as many chances for genetic mutations to occur."

Chemical control options may still be necessary, and growers will have two choices there: Bayer's insecticide Sivanto and Dow AgroSciences' insecticide Transform WG -- which is only available in certain sorghum-producing states under Section 18 emergency use exemptions from the EPA.

To see if your state has acquired a Section 18 exemption for Transform, contact your local Extension office.

To see Michaud's list of independently verified aphid-resistant varieties, visit his KSU article here: http://bit.ly/…

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

Follow Emily Unglesbee on Twitter @Emily_Unglesbee

(PS/BAS)

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