This soybean research project plays some pranks to fool Phytophthora
They’re eerie, as in spooky, and are patiently waiting in dark, earthy loam and clay. This class of fungi-like organisms with a funny name—oomycetes (oh-oh-my-seats)—is responsible for about half a billion dollars worth of soybean yield suppression in the U.S. in 2010.
It’s known as the tongue-twisting Phytophthora, a Latin name that only Daffy Duck would enjoy pronouncing, spit and all.
"Phytophthora is the most devastating disease over any other disease in the U.S., except for the effects of the soybean cyst nematode," says Allen Wrather, a plant pathologist at the University of Missouri Delta Research Center in Portageville, Mo. "Yield suppression in the U.S. last year from the fungus was 38.8 million bushels."
Wrather says the Phytophthora species sojae is becoming more of a threat because more acres are expected to be planted to soybeans in the future. Also, saturated soils cater to the increase of the disease where it is found naturally, in specific crop fields, so chronic wet spring weather doesn’t help the situation. Phytophthora is known to wipe out entire soybean fields from the
resulting root and stem rot.
Hopefully, the disease will be neutralized by a group of researchers and Extension agents armed with a $9.28 million grant from USDA’s National Institute of Food and Agriculture. The research and outreach program is coordinated by Brett Tyler of the Virginia Bioinformatics Institute at Virginia Tech and involves interdisciplinary teams at 18 institutions.
"This project isn’t as much about new discovery as it is applying our recent discoveries to make real-world improvements," Tyler says.
Seek and destroy. So how does Phytophthora sojae seek and destroy? Found naturally in the soil, it produces spores by asexual reproduction on rotten root tissues when the soil is flooded. Mobility of the spores is achieved with whiplike filaments, which allow them to swim in a corkscrew path in water, seeking host plants. They become attracted to chemicals released by soybean roots and germinating seeds. Like a magnet to steel, they make contact with the soybean root surface, where they shed the mobile filaments and form a cell wall. The spores germinate and fibrous strands penetrate into the root and begin rotting the tissue.
Tyler and his research group at Virginia Tech analyzed the genetic material in Phytophthora and found the location of a superfamily of deadly genes involved in the infection of plants. These genes produce virulent proteins that disable the immune systems of their victims as part of an
attack strategy, much as the HIV virus does in humans.
According to the research, these lethal proteins are able to easily enter a soybean plant cell across the cell membrane without any additional attack effort from the killer. These proteins also carry the "entry mechanism" that disables the cell that is necessary for the deadly fungal microbes to infect the plants.
Serving as the lead institution for this research, Virginia Tech is responsible for discovering new resistance genes, while the University of Georgia is responsible for soybean transformation.
"We know there is a protein that a soybean makes, which the fungus uses to recognize that the plant is a soybean plant. That becomes a welcome mat for this fungus," says Wayne Parrott, researcher and professor of plant pathology at the University of Georgia in Athens (UGA). "One research target is to see if we can alter that welcome mat or get rid of it: Then the fungus will not recognize that it’s on a soybean and will not attack it."
Out-of-the-box fix. Another approach that might be effective to combat the fungus involves a natural mechanism that plants have to "turn off" genes in viruses that attack them. A retired professor from UGA came up with the idea to use this same system, tweaking it so that it would work on the nematodes.
"You take one of the nematode genes that is essential for the nematode to feed on the plant and put it in the plant that uses this mechanism to stop an invading virus, but in this case, it’s nematodes," Parrott says. "When the nematode eats that, it shuts down its feeding apparatus and it will starve to death."
"This disease is damaging and hard to control," says Sam Atwell, University of Missouri Extension agronomy specialist from New Madrid, Mo. "The spores can live in the soil for many years and attack the soybean plant at any stage of its development. Phytophthora infection can occur in soil temperatures as low as 50°F, but the optimum range is between 81°F to 91°F for seedlings and young plants."
UGA’s Parrott says that university Extension outreach will be a major part of the program. "There are other disease symptoms out there in the field that can look like this one. One of the initiatives is to develop an easy test kit, something comparable to a home pregnancy test kit that farmers can use to tell what’s in their field."
|Checking root development on a 5-1 Olympus soybean variety grown in southeast Missouri are Riley James, Sam Atwell and Robert Henry.
PHOTO: John Buckner
Living with it. Since these biotech methods may be five to seven years in the making, what can be done to control Phytophthora damage in the meantime? Atwell says farmers should get the best disease trait seed package they can afford.
"We have a lot of heavy clay soils behind the levees that are prone to diseases like Phytophthora, so it’s important to know the trait package of your beans for disease tolerance," he says. Expect change as well.
Robert Henry, a New Madrid, Mo., farmer and seed dealer with 1,500 acres in beans and cotton, is on a 60-40 rotation. "We’ll drop 60% of our lineup and pick up 60% new bean varieties," he says. "The second year, we’ll drop 40% of our lineup and pick up more new varieties, so every two years, we’re almost 100% new in the lineup." Henry offers these words of caution, however: "The beans are coming so fast that you don’t really get data of disease resistance until the second year on how well they will perform."
Saturated soils can spell Phytophthora trouble. "We had 20" to 30" of rain in 10 days the last weekend of April to the first couple days of May," says Riley James, a landowner and cotton gin operator, also in New Madrid, Mo. "I’ve never seen some of these fields flooded."
Henry adds: "I can show you fields where they’ve irrigated a little heavy, and Phytophthora is eating their lunch."
At the Delta Research Center, soybean breeder Grover Shannon faces the challenge of finding flood-tolerant (and therefore Phytophthora-resistant) soybean varieties. "We have 8 million acres in the Delta of slowly drained clays, like Sharkey soils. A lot of farmers are going to irrigation because water takes the risk out of farming. However, if you’ve just irrigated and you get a 5" rain, you can get all kinds of diseases because of the soil type," he says.
Shannon is looking for "partial resistance" in a soybean variety, which means broad resistance to a lot of different Phytophthora races.
"We’re finding out that partial resistance to Phytophthora sojae—at least tolerance—is very important to flooding tolerance," Shannon says. "As more of these acres with clay soils come
under irrigation, it’s going to be even more important economically. Down here, cotton and corn get put on the good ground and beans usually get what’s left. It’s on the ‘what’s left’ ground in the Delta or anywhere else that can make a big difference to farmer’s yields."