Mycorrhizal fungi help in battle with problem weeds

A soil fungus common to nearly everywhere on the planet could hold the key to conquering some pesky weeds, among them Palmer amaranth and kochia.

Mycorrhizae fungi aid in drought resistance and nutrient absorption in plants, and are in the non-herbicidal weed-war arsenal.

Rather than load up the spray rig, a natural alternative is to equip no-till acres with fungi and the right plants to keep them alive, said Dale Strickler, of Belleville, Kansas, an agronomist for Green Cover Seed, of Bladen, Nebraska.

“The stuff needs a living root as a host,” he said.

The biggest benefit to crops is absorption of water, he said, pointing to research in Wisconsin.

“They inoculated corn (plants) with mycorrhizae fungi, shut the water off, and counted the days until they died,” Strickler said. “The inoculated plants lived 30% longer without water, and that’s a typical response.”

Another benefit is the “uptake of fertilizer,” he added. “That helps plants deal with nutrient deficiencies, just by having a big huge absorptive area.”

Mycorrhizae fungi produce spores, like seeds, that can survive several years in healthy soil, Strickler said.

“After the ground has been cropped, you keep destroying spores,” he said.

Those spores germinate and become hyphae that live inside plant roots and branch out like root extenders into soil.

“There could be a thousand times more surface area on these hyphae. They’re small and numerous and they can reach into every nook and cranny in the soil that’s too small for roots to get in,” Strickler said.

That phenomenon gives crops colonized by mycorrhizal fungi, an advantage.

“Most crops are mycorrhizal hosts, and a good chunk of weeds are not, so they don’t benefit from it,” he said. “It’s like giving firearms to the side you want to win.”

Common cash crops don’t normally produce extensive root hairs and they need the mycorrhizal fungi.

Pull up a pigweed and you will find a big taproot with fine lateral roots jutting out. These are an adaptation to grow in the absence of mycorrhizal fungi. Members of those families of weeds require nitrates to grow, he said, their favored form of nitrogen.

“Weeds exist in cropland because they are adapted to live in disturbed soil, where there is not usually a lot of fungi, and that’s why weeds compete so well in tilled conditions,” Strickler said.

Tillage takes away the advantage that crops colonized with the fungi enjoy.

“It takes quite a long time for the hyphae to produce,” he said. “In annual cropping systems the crop matures and dies before spores are produced, then we usually enter a fallow period after harvest without living roots, so the hyphae starve out and eventually we run out spores, because no new ones are being produced.”

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Pigweeds like conditions create by tilled soils, Strickler said.

“That’s an absence of fungi and an abundance of nitrogen in the nitrate form,” he said. “Tillage helps the pigweed cause.”

There tends to be less nitrate and more mycorrhizal fungi in more natural farming systems, he said.

Crops with mycorrhizae fungi “get more food and water than the weeds.

“When you till soils, you create conditions that pigweeds like. That’s an absence of fungi and an abundance of nitrate,” he said.

With the benefit of spores and hyphae, Strickler said, farm-friendly cover crops and cash crops, “can get more food and water than the weeds. No-till cover crops and inoculating with mycorrhizal fungi really shifts that battle.”

One study performed by North Dakota State University showed that a sunflower crop inoculated with mycorrhizal fungi had 54% less weed biomass than sunflowers that were not inoculated, he said.

Strickler has more proof.

“A friend of mine cleaned a bunch of pigweed seed (from his grain). He did some dirt work in a pasture and spread close to a ton of that pigweed seed on an area,” he said. “That pigweed got to an inch tall, turned yellow and died. It simply cannot grow in a native prairie system. There was still mycorrhizal fungi out there, but there was no nitrate.”

Strickler was a presenter at Soil Health U & Trade Show, Jan. 22 and 23 in Salina, Kansas.

Tim Unruh can be reached at [email protected].