“It can kill cotton, and you can never get rid of it … once it’s in the ground, you don’t get rid of it.”
That quote is from Dr. Mike Davis, Extension Specialist and Professor at the University of California-Davis, in reference to Race 4 of Fusarium oxysporun f. sp. vasinfectum. Or simply Race 4 of Fusarium wilt.
The Races we’ve come to know and understand in California — Races 3 and 8 — are troublesome, to be sure, but can be controlled. There is a distinct relationship between the root-knot nematode and Fusarium damage. Without the presence of root-knot nematodes, these two Races are not damaging. Control root-knot nematodes; control these races of Fusarium.
But Race 4 is just as distinctly different and it only affects cotton. Says Davis: “Race 4 does not need the interaction with the root-knot nematode to damage cotton. It enters through the roots and colonizes in the xylem — the water-conducting tissue. What it does is infect the ducts and plug them up. The plant dies.”
Though upland fields are most definitely not immune, Race 4 appears more often in Pima fields.
Who Invited You?
Although Davis says there’s no real way to determine exactly when Race 4 of Fusarium wilt arrived in the United States, he first saw it in 2002.
“We got a call from a grower who had lost a field of cotton twice,” Davis explains. “He planted his cotton and it came up and died. He thought it was seedling disease, so he planted again and it died again. He asked us to take a look and it did look like a seedling disease. But what we found was that it was Fusarium wilt and it was killing the plants outright. We had never seen that in California.”
Based on comparisons from his Fusarium culture collection, Davis determined what was found in the field was identical to a Race first identified in India. The collection’s original isolates were gathered by Dr. Jim DeVay, who is now retired from UC-Davis. Under an import permit from USDA, Davis added isolates from around the world and he now has in his collection every known Fusarium isolate.
Cotton seed is moved around the globe for breeding purposes, so it is logical to assume that Race 4 arrived in imported cotton seed.
“It could have come in on a few seed in a breeding line,” says Davis. “We just don’t know. We don’t know where the initial introduction event took place.”
Race 4 has now been identified in Tulare, Fresno and Kern Counties in California, and it spreads exponentially in a field. In the first year, it could be limited to a single, solitary plant from one contaminated seed. That lone plant would be next to impossible to see. But in that plant’s tissues, millions of spores are produced. When the crop residue is incorporated, the stem decomposes and releases the spores into the soil. So the next year, the infected area grows … and on and on, year after year.
There is no non-host plant, so there is no rotational solution. Once the infection has gotten out of hand, the only current option is to abandon cotton in that field or to plant a resistant variety.
Protecting the Seed Supply
Davis says California has 35,000 acres devoted to seed increase and that it is essential for varieties from this acreage be Race 4 free.
“The only way to prevent the pathogen from infesting a field is to plant seed that we know is free of Race 4,” says Davis.
But exactly how do you know seed is pathogen free? Unfortunately there’s not a good answer at this point. Davis is working on a detection method, but says there’s a “ways to go” before it is available.
“Right now, we can’t detect it consistently,” he explains. “Cotton seed has some inhibitors that prevent the chemical reaction that is required in the test.”
In the meantime, about all that can be done is to plant a resistant variety in fields that are known to be free of Race 4 Fusarium wilt. Start clean; stay clean.
Putting Up Resistance
If there is a silver lining around any of the Race 4 Fusarium storm clouds, it is that some varieties have indeed expressed a resistance, with the most notable being PhytoGen 800.
“In the field where the grower had lost two crops, we conducted a variety trial,” Davis says. “Bob Hutmacher, also in UC Extension, and I used 50 varieties and we had no idea if any of them would show resistance, but we had to try something. One variety — PhytoGen 800 — was about to be released commercially and it showed resistance. Immediately, the proportion of acres planted to PhytoGen 800 shot up because people didn’t want to take a chance with this pathogen.
“In fields with high populations in the soil, you will see some damage with PhytoGen 800,” he continues, “but that’s the answer in the long run — plant resistance.”
The parentage in PhytoGen 800 that expresses Race 4 resistance has been identified.
California Planting Cotton Seed Distributors (CPCSD) is working to develop a resistant variety, but that could be at least two years away, says Dr. Steve Oakley, CPCSD’s vice president, research and development.
Also, USDA’s Mauricio Ulloa, a plant breeder, is working on a resistant variety.
The Warning Flag is Up
“One of the things that concerns me is that resistant varieties are not planted in other states,” says Davis.
In other words, there is no fence around California that would prevent the eastward spread of Race 4 Fusarium wilt.
“So far, we have not found that it has spread to other states,” says Davis.
But even that comes with a qualified warning from Davis: “The bottom line is that the rest of the Cotton Belt needs to be looking out for this disease. We are already seeing infected fields move into pistachios and almonds. It is so important for other parts of the Belt to understand this problem. They do not want this pathogen.”