Four years of field, greenhouse, and laboratory tests by researchers and scientists is providing cotton growers in California's San Joaquin Valley (SJV) with critical seed information to make the best informed planting decisions in the fight against Race 4 Fusarium.

“Good germplasm is on the way,” said Bob Hutmacher, University of California Extension cotton specialist. “We're testing a wide range of germplasm from a range of seed companies interested in California Upland and Pima cotton production, plus those at the fingertips of ARS geneticists in their wide-ranging collection.”

The research results are critical as Pima cotton shines bright as a key player in the future of the SJV cotton industry. In field trials evaluated to date, Race 4 Fusarium appears to hit Pima especially hard.

The UC and the U.S. Department of Agriculture's Agricultural Research Service (USDA-ARS) have confirmed Race 4, Fusarium oxysporum f. sp. Vasinfectum, in about 36 sampled locations in Fresno, Kern, Kings, and Tulare counties.

Hutmacher released varietal test/Race 4 results during the 2007 Beltwide Cotton Conferences in New Orleans.

“We have been provided access to material from seed companies not widely planted yet but are coming through testing programs. We've been allowed to test some experimental varieties,” Hutmacher said. This includes materials from the SJV Cotton Board Testing Program as well as approved Pima and Acala variety trials.

Seed companies are hot on the trail with new materials under evaluation to minimize Race 4's impact. The screenings have included experimentals from Phytogen, Delta Pine, Hazera, and Bayer-California Planting Cotton Seed Distributors (CPCSD). Results show a range of plant performance under field and greenhouse testing in soil confirmed as Race 4 FOV infested.

Testing sites

Testing locations have included a Fresno County field trial in 2003, two grower cooperator locations in Fresno and Kern counties from '04-'06, and up to three greenhouse screenings from '03-'06 at the UC Kearney Agricultural Center near Parlier, Calif.

Yet Hutmacher said more precise testing is needed in hot spots (infected fields) and at multiple locations.

Along with Hutmacher, the Race 4 screening team includes: UC, Davis extension pathologist Mike Davis; UC Extension farm advisors Steve Wright (Tulare County) and Brian Marsh (Kern County); and USDA-ARS scientists Mauricio Ulloa and Rebecca Bennett in Shafter, Calif., and Richard Percy in Maricopa, Ariz.

Race 4 is a soil-borne disease with long surviving spores that can remain in the soil for years. It is difficult, if not impossible, to eradicate.

Pima evaluations

Of the commercial Pima materials on the market, Hutmacher said Phytogen 800 Pima is one that has repeatedly performed well in evaluations done in Race 4 FOV infested sites with lower infection rates and lower stand losses.

However, some “very hot” test sites have also shown that this variety is not immune or completely resistant. In fields known to be Race 4 infested, these screening efforts could be useful in avoiding more serious plant losses. Plant performance in the tests has been based on survival at 5-9 weeks after planting, some measures of plant stunting and growth, plus foliar damage and vascular staining ratings.

In field tests where soil inoculum levels of Race 4 were relatively high, many commercial and experimental Pima varieties were infected with a range of plant damage and stand losses.

“If Race 4 is confirmed in a field, Phytogen 800 Pima is the best commercial variety currently available,” Hutmacher said.

As screening trial summaries are compiled this winter, growers can consult with UC farm advisors and specialists for updated screening trial results for decision making in Race 4 infested fields.

Hutmacher expects other germplasm with similar good performance will be identified in further testing.

In variety evaluations conducted in infected fields, Race 4 did not severely impact Acala/Upland varieties at inoculum levels in evaluated test sites. Hutmacher said many commercial and experimental Acala and non-Acala Upland varieties tested in high inoculum test sites showed root vascular staining, an indication that they could be infected even though plant damage such as stunting and plant loss was generally quite limited.

While root vascular staining is a useful field indicator that a wilt disease such as Race 4 FOV may exist in a plant, pathology tests including DNA-type testing of the plant tissue is the only sure-fire way to confirm a diagnosis of Fusarium and whether it is the Race 4 type.

Hutmacher said foliar and root vascular damage from Race 4 can occur early in plant development seen as early as 4-5-leaf stage on through late squaring. Although described as a wilt disease, the damage is usually evident at much earlier growth stages than seen with verticillium wilt and root vascular staining which is darker in color and more continuous with Race 4 rather than the flecking or discontinuous discoloration seen with verticillium wilt.

To evaluate plants for signs of telltale vascular staining, he recommended pulling plants from the ground and using a pocketknife to slice through the taproot and look for staining.

“If you already have pathology tests to confirm the existence of Race 4 in a field or in the immediate area, then vascular staining can be a useful indicator that it might have spread,” Hutmacher said. “If a grower has never identified Race 4 in the field or nearby, it is more appropriate to consider plant sampling for a confirmation of Race 4.”

In fields confirmed with Race 4, changes in cultural practices that limit the movement of soil from infested fields can play an important role in slowing the spread of the disease, he said.