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.