Fusarium Race 4 may be the pariah of the San Joaquin Valley cotton industry. However, growers cannot expect it to go away by ignoring it.
If they do, it could become like a parasitic cousin who moves in and never moves out and eats you out of house and home.
Mike Davis, University of California, Davis plant pathologist, said growers “did a good job of spreading it around” after the new, highly virulent strain of fusarium unknowingly was introduced into the valley. It colonized on susceptible cotton varieties and was spread primarily by soil movement within and off infected fields.
It likely came on planting seed from another part of the world. And since DNA tests have linked it to the same Race 4 found in India/Asia, the guilt is there by association. Regardless of where it came from, it is in the valley to stay.
It has been a major controversy since Davis first identified it in the 1990s. Growers who had it did not want anyone to know about it because it basically devalues the land for cotton production. In Australia where a similar virulent strain of fusarium is found, financial institutions have black-flagged entire areas as unsuitable for cotton and therefore less valuable.
Race 4 is seed-borne. Since the San Joaquin is a major producer of planting seed, containing the spread of Race 4 has become a critical issue.
Once a field is infected with fusarium, it will always be there.
Each year new fields are identified as infected, but Davis told growers and pest control advisers at a recent cotton production meeting at the West Side Research and Extension Center at Five Points, Calif., that it is not yet widespread.
Cotton producers cannot guarantee it will not infect a field of cotton, but it can be minimized. However, they must scout for it to minimize the spread of Race 4, whether they want to or not.
Bob Hutmacher, University of California Extension agronomist and cotton specialist, said if as few as one of two plants exhibit damping off-like symptoms; sample them for the fusarium tell-tale symptom of vascular staining. Call someone and get scientific analysis.
“Now when the cotton is small is when you need to look for signs of fusarium,” said Hutmacher.
If it is fusarium, identify exactly the area of the field where it was found.
“Take a small hand-held GPS unit and identify specifically where it was found,” said Hutmacher. It can be minimized using metham sodium. However, the most effective way to prevent its spread is to plant cotton varieties less susceptible to Race 4, certainly in fields identified as infected.
Race 4 affects both Acala and Pima varieties, but Pima is most susceptible to Race 4. Fortunately widely planted Phytogen 800 is highly resistant and so far is the only one that is.
Davis explained, however, that Phytogen 800 is not totally resistant to Race 4, but Race 4 fungi have difficulty colonizing on the variety and that slows the spread of the fusarium. Unlike other fusariums, it does not require the presence of nematodes to spread Race 4 as is the case with other, less virulent strains of fusarium.
Crop rotation can reduce its spread in fields. Race 4 survives on the roots of a host of crops, but it will not colonize on those roots.
While Davis could not guarantee Race 4 will not continue spreading, he did promise that if growers plant two or three crops of highly susceptible Pima or Acala varieties in fusarium-infected fields, it will become widespread.
It becomes most noticeable when there are “pickup size” areas of a field where seeding plants have died. By then it has been in the field for several years, said Davis.
The valley is a major producer of seed for the rest of the Cotton Belt; therefore, growers in other areas are more than concerned about the movement of fusarium Race 4 into their fields from planting seed. When tests were run for fusarium Race 4, none of the California strain was found.
However, tests turned up a cousin that is more virulent than Race 4.
“There have been three new fusarium Races identified in the South. One, LA 108 is highly virulent,” said Davis. “We don’t want that in California.” It, he noted, it is more virulent on one of the dominant SJV Acala varieties, Phytogen 72, than Race 4.