Update on Fusarium Race 4 Varietal Evaluations in California.

Authors: HUTMACHER, ROBERT - UNIV. OF CALIF-SHAFTER; Ulloa, Mauricio; WRIGHT, STEVE - UNIV. OF CALIF-TULARE; KEELEY, MARK - UNIV. OF CALIF-SHAFTER; DAVIS, R. MICHAEL - UC-DAVIS, DAVIS, CA; MUNK, DAN - UNIV. OF CALIF-FRESNO; BANUELOS, GERARDO - UNIV. OF CALIF-TULARE; MARSH, BRIAN - UNIV. OF CALIF-SHAFTER; Percy, Richard; SMITH, LES - UNIV. OF CALIF-SHAFTER; Biggs, Monica

Submitted to: National Cotton Council Beltwide Cotton Conference
Publication Type: Proceedings
Publication Acceptance Date: 1/18/2008
Publication Date: 5/28/2008
Citation: Hutmacher, R.B., Ulloa, M., Wright, S.D., Keeley, M., Davis, R.M., Munk, D.S., Banuelos, G., Marsh, B.H., Percy, R.G., Smith, L., Biggs, M. 2008. Update on Fusarium Race 4 Varietal Evaluations in California. National Cotton Council Beltwide Cotton Conference. p. 811-812.

Interpretive Summary: Fusarium wilt is a disease of cotton that can cause stunting of plants, wilting of leaves, and in severe cases, plant death. This disease is caused by a fungus that attacks the tissues the plant uses to transport nutrients. In the San Joaquin Valley of California, Fusarium wilt has not been a serious problem because the strains of the fungus that are present usually cause crop damage only in the presence of root-knot nematodes, which are microscopic worms that attack plant roots. Between 2002 and 2007, Fusarium wilt damage was observed on cotton grown in soils inhabited by very few or no root-knot nematodes. Fusarium from these sites was subsequently identified as a new, more damaging strain (race 4). Although initial identifications of race 4 were made from extra-long staple (Pima) cottons, follow-up evaluations have confirmed infection of Upland cottons as well. In an effort to better manage this new strain of Fusarium, a wide range of cottons from seed companies and public sources were screened for resistance. Among the publicly available materials, cotton lines with a significant level of resistance were identified. Current research efforts are aimed at identifying the genetic sources of resistance so this information can be used to speed the development of Fusarium-resistant varieties for commercial use.

Technical Abstract: In recent years, differences have been noted in field situations with the fungal pathogen, Fusarium oxysporum f. sp. vas infectum (FOV), in Acala and Pima cotton in the San Joaquin Valley of California. Typically, earlier-recognized races of FOV only caused significant crop damage and yield impacts where they were found in combination with susceptible cotton varieties and at least moderate populations of the root knot nematode. A race 4 isolate of Fusarium has been identified in Pima and Acala cotton fields in the San Joaquin Valley in recent years, and this race has been shown to infect susceptible varieties, cause damage, and reproduce inoculum in soils that do not support significant root knot nematode populations. Research to date has focused on (1) developing genetic tools to more clearly identify FOV races found in California cotton, (2) evaluate plant samples in the field to verify presence of the pathogen, and (3) screen varieties under greenhouse conditions with inoculated soil, and in field sites where the FOV race 4 pathogen has been confirmed. Field screening trials of commercial and experimental cotton lines have shown that susceptible Pima varieties exhibit more severe symptoms than tested Acala and non-Acala Upland varieties. The most tolerant (lowest seedling mortality, least stunted) varieties in field screenings have also been Pima cottons. Field and greenhouse trials have confirmed that Acala and non-Acala Upland varieties can also be infected, although plant mortality and stunting are generally reduced compared with susceptible Pima varieties. Varietal differences in susceptibility to race 4 FOV have been seen in both Pima and Acala cotton entries.