One of the more insidious maladies to infect California cotton likely will spread this season.

University of California Extension cotton specialist Bob Hutmacher and UC farm advisors have been getting more calls to evaluate fields for the presence of Race 4 fusarium wilt or FOV for short. FOV stands for Fusarium oxysporium f. sp. vasinfectum.

Hutmacher is not surprised to get calls with the expanded 2011 cotton acreage and the cool, wet post-planting weather.

Race 4 FOV is particularly ominous because unlike Race 1 FOV, Race 4 can infect cotton plants without the presence of root knot nematodes. This means that Race 4 FOV can cause damage in a wide range of soil types even when root knot nematodes are not an issue. Like other types of FOV, Race 4 is a soil-inhabiting fungus.

It can infect plants and cause a vascular wilt disease in a wide range of commercial cotton varieties, so it can be a cause for significant concern. The inoculum (spores, reproductive structures of the fungus) can be distributed by normal farming practices that move soil and plant residue around the fields, such as cultivation, land planing and irrigation.

In plant evaluations done in recent years in fields known to be infested with Race 4 FOV, UC and USDA researchers have noted that the most severely affected varieties have been Pima cottons, and conversely, the most resistant varieties identified have also been Pimas. From 12 fields originally identified by UC Extension as infected with Race 4 in 2004, UC evaluations through early 2011 now show this particular strain of wilt is present in at least four of the six major cotton producing counties in the San Joaquin Valley.

Race 4 FOV is different than the two FOV strains that have caused widespread damage to the Australian cotton industry, but the relatively fast rate of spread of the disease and potential for damage bear some similarities.

Race 4 FOV is a soil-borne wilt that can spread over time to infest an entire field. However, a grower, especially a new one, may not know he has an infested field until he sees seedling cotton plants wither and die, according to Hutmacher.

Once the disease widely infects a field and inoculum levels increase to higher levels, it will become impossible to achieve economic yields with cotton varieties with low to moderate resistance to Race 4 FOV due to plant losses, stunting and reduced yields.  At high soil inoculums levels, economic cotton production becomes harder and harder except when planting the most resistant varieties, such as Phytogen 800 Pima.

However, even this variety has shown mild plant losses and yield reductions when exposed to high Race 4 fusarium pressure. Race 4 FOV can similarly affect Acalas and non-Acala Uplands. UC and USDA experience with Uplands/Acalas in FOV screenings are that these types of cotton are broadly susceptible to and infected by Race 4 FOV when inoculum is present, and are generally less severely impacted than the susceptible Pimas. To date, however, no highly resistant Acalas or other Uplands have been identified in field Race 4 FOV screenings.

USDA-ARS geneticist Mauricio Ulloa based at the USDA field station in Shafter, Calif., is working on identification of sources of resistance to race 4 FOV in both Pima and Upland germplasm.

Planting seed pariahs

Particularly devastating is the fact Race 4 FOV renders fields pariahs as a cotton planting seed production fields. Plant pathologists are insistent that no seed be produced in known Race 4 FOV infected fields and all seed fields be tested for the presence of the fungus. It is so serious it has actually impacted the economic value of land.

And Hutmacher believes many cotton growers will unfortunately discover that fact for the first time this year.

He explained that with the expanded 2011 SJV cotton acreage, fields are being seeded to cotton after several years out of cotton, periods during which prior problem areas could be forgotten and farming practices continued to move around inoculum capable of spreading the disease. These scenarios are generating phone calls as growers search for why some cotton stands are disappearing.

If Race 4 FOV has been present in those fields, it could have been easily and unknowingly spread throughout fields with the movement of infested soil on equipment or in irrigation water, likely primary methods for its spread. Also, infested soil on farm equipment or sprinkler pipe unknowingly moved from infected fields to clean fields can also spread inoculum of Race 4 FOV.

As the weather warmed, Hutmacher’s phone started ringing with growers seeking a reason for the wilted and dying seedling cotton. The cooler weather of late may allow Race 4 FOV infected plants to survive longer only to succumb to the disease as the weather warms again. On the flip side of the cool weather, the plants could be put under additional Race 4 FOV stress that may compound impact of the disease. Either way, Hutmacher told a recent UC Cotton Workgroup meeting in Tulare, Calif., he expects to see much more Race 4 fusarium this season.

Unfortunately, other parts of the U.S. Cotton Belt may also see more of Race 4 FOV.
USDA-ARS research plant pathologist Rebecca Bennett told the cotton workgroup meeting isolates similar to the Race 4 FOV found in California have also turned up in Mississippi and Alabama.

Researchers collected 39 samples of the fusarium wilt fungus from infected cotton plants in the U.S. and compared them to known reference strains. Some samples from Alabama and Mississippi were similar to this strain (Race 4) that is particularly damaging in California. To date, however, Race 4 has not been reported as causing serious disease problems in the Southeast. However, these preliminary results document the potential for increased problems with fusarium wilt in the Southeast.

All samples from Texas were related to known strains that were previously documented. These findings alert cotton breeders to potential disease problems that may warrant expanded breeding efforts for fusarium wilt resistance.

Early identification

Hutmacher reminds growers and PCAs that the time from the seedling stage (couple of true leaves) through about first bloom stage is considered the easiest time of the year to find and identify this problem while it is still a relatively early infestation.

Identifying it early will allow the grower to initiate containment practices that can slow down the spread of the pathogen. This could include such things as cleaning as much soil as possible from equipment before moving from an infected area to a clean area.

If the disease is identified through a lab sample, make a permanent note of the locations on field maps, limit to the degree workable practices that move soil and plant debris out of infested fields into clean fields, and plant resistant varieties, suggests Hutmacher.

Race 4 fusarium symptoms can begin very early on, starting when plants only have one or two true leaves. Foliar symptoms include patchy leaf yellowing and necrosis that typically begin on the leaf margins of lower leaves, but these symptoms are not always easy to differentiate from those with other seedling diseases.

The most useful diagnostic characteristics that suggest possible FOV are: (1) the wilting of leaves and stems seen even under mild early season weather conditions; combined with (2) the dark brown stains seen in the vascular core of the tap root when you pull or dig up a plant, slice the root lengthwise and examine it. Growers should note that FOV damage is most easily seen as an early season disease with most damage visible in the seedling stage. This contrast with  the late-season (boll maturation) period more when verticillium wilt typically develops and causes the most damage.

Hutmacher said vascular staining caused by FOV also typically looks different than vascular staining seen in verticillium wilt. Race 4 FOV staining is more of a continuous, dark brown color most easily observed in the tap root, while vascular staining seen with verticillium is a more irregular, flecking stain of the vascular system.

More detailed guidelines for fusarium sampling and recommendations for containment practices are on the UC Cotton website: http://cottoninfo.ucdavis.edu.

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