Soybean and small grain diseases

by Deborah Jeanne Sergeant

Diseases in grain can dramatically effect yield. Gary C. Bergstrom, plant pathology and plant microbe biology Extension agent with Cornell Cooperative Extension, recently presented “Soybean and Small Grain Disease Updates” as part of the recent Northwestern New York Soybean and Small Grains Congress.

Bergstrom offered a status report on the soybean diseases occurring in New York in 2021, including soybean cyst nematode, seporia brown spot, downy mildew, bacterial pustule, frogeye leaf spot, white mold, cercospora leaf blight/purple seed stain, phytophthora root and stem rot, fusarium crown and root rot, sudden death syndrome, brown stem rot, stem canker, anthracnose, pod stem blight and phomopsis seed decay.

“A lot of the typical players showed up,” Bergstrom said. “White mold was not as severe as we thought it might be. We saw quite widely distributed cercospora leaf blight/purple seed stain, soybean cyst nematode and phomopsis seed decay.”

Soybean cyst nematode was spotted in 33 of New York’s 62 counties by 2020.

“A decade ago, we weren’t finding soybean cyst nematode,” Bergstrom said. “Then we were finding it only in Cayuga County.”

Despite its spread, Bergstrom thinks farmers have been fortunate, as other areas have suffered worse damage from this pest. In the counties where it’s been spotted, its egg numbers have remained low with only a few counties reporting moderate to high numbers.

Soybean and small grain diseases

Gary Bergstrom often presents on plant disease topics for Cornell Cooperative Extension. Photo by Deborah Jeanne Sergeant

“The slogan is ‘Know your number,’ meaning how many eggs of the nematodes are in a cup of soil,” Bergstrom said. “It’s really a problem when we get into moderate to high levels of the nematode.”

On farms in the Midwest, where farmers have battled nematodes for decades, resistant seed varieties have been helpful in reducing their effects. But Bergstrom said that the PI 88788 breeding line has lost its effectiveness in resisting soybean cyst nematodes.

“A resistant variety should allow less than 10% of reproduction,” he said. “In other words, a resistant variety should stop 90% of the soybean cyst nematodes in a field from reproducing. Across the region, varieties with PI 88788 resistance aren’t hitting the mark. On some farms, one out of every two nematodes can reproduce.”

Very few varieties of commercial soybeans resist soybean cyst nematodes.

He recommended that farmers with low infestations choose a high-yielding, resistant variety and then rotate with non-host crops. For moderate to high infestations and use of resistant varieties, “then you need to use a HG type test, then choose a suitable resistant variety or rotate with a non-host crop.”

Corn, wheat and other small grains are among the crops that are not soybean cyst nematode hosts. It is yet unknown if cover crops can help reduce the number of soybean cyst nematodes.

Cercospora sojina, which causes frogeye leaf spot, has shown resistance to strobilurin fungicides. In New York’s Chautauqua County, 21% of isolates are resistant; in Herkimer County, 14% are resistant. Bergstrom added that foliar fungicides with good to very good suppression of white mold include DuPont Approach, Omega500F, Endura and Propulse.

Bergstrom said that Michael Wunsch with North Dakota State University’s Carrington Research Extension Center has improved management of white mold in soybeans by applying fungicide at the right time.

Delaying application from mid to late R1 to full R2, when the canopy is at or near closure at R1, improved efficacy by one to two bushels per acre.

“A major factor is whether the canopy is closed over at the time of the first flower/R1 application stage,” Bergstrom said. “At that R1 very first flower stage, the canopy is normally very open. They found it’s better to defer the application until the R2 stage, when the flowers are appearing on the top two nodes. Canopy closure is a big factor here.”

Fungicides should be applied once all the plants reach R2 growth stage unless the canopy closes earlier.

Fungicide droplet size also matters. “You get good efficacy with medium droplets,” Bergstrom said. “If the canopy is completely closed, you get better results with larger, coarser droplets. They have more weight and velocity to get into a closed canopy.” Research also indicates economic justification for a second application.

The timing affects fusarium head blight and deoxynivalenol (DON). Applying at anthesis ideally, and if not possible, a little later, seems to have a better effect than applying early, according to research by Ohio State.

“We see significant reduction of disease and toxin with each of these application material timings; however, the strongest reduction is with application at anthesis,” Bergstrom said. “We get decent control putting it on a little earlier, but not near what we get at anthesis or a little later.”

The same holds true for other small grains based upon Cornell’s research.

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