Sources, selection and breeding of Fusarium wilt (Fusarium oxysporumf. sp.vasinfectum) race 4 (FOV4) resistance in Upland (Gossypium hirsutumL.) cotton

Diseases such as Fusarium wilt caused byFusarium oxysporumf.sp.vasinfectum(FOV), a soil borne fungal pathogen, represent expanding threats to cotton (Gossypiumspp.) production around the world. For over a decade, FOV race 4 (FOV4) has adversely impacted cotton production in California's San Joaquin Valley causing plant wilt and death. With this disease formally identified in 2019 in New Mexico and in 2017 in El Paso, TX region in proximity to the High Plains of west Texas-the largest Upland cotton producing region in the USA, the need to expand the genetic base of Upland (G. hirsutumL.) cotton and develop cultivars resistant/tolerant to FOV4 has become urgent. Our previous research in Pima (G. barbadenseL.) cotton identified high levels of resistance to FOV4 in 'Pima S-6' germplasm, and our program publicly released Pima germplasm with improved FOV4 resistance. However, the search for resistant Upland cotton has proved more challenging compared to the effort in Pima. More than 1000 Upland accessions from the USDA-ARS Cotton Germplasm Collection were evaluated for reaction to FOV4 in artificial-greenhouse conditions and in naturally infested grower fields. Less than 0.1% of the tested accessions were selected to develop highly resistant FOV4 progeny. Two sources (NM12Y1004-NM12Y1005 and SA-3208) of Asiatic breeding origin were identified with tolerance to FOV4 and used to introgress and increase resistance. Pedigree information from other parental lines used to develop progeny revealed their sources to be exotic and wild Upland germplasm. That is triple/multiple crosses included the origin of these obsolete SA cultivars 'Auburn M', 'DES 920', 'MARSPD202085', 'S.N.0503-1', PD 2165, and 'Stoneville 14', among others. A range of severity of foliar symptoms, vascular root staining, and plant mortality occurred in the infested fields based on tested accessions and observations of susceptible germplasm/cultivar-checks, indicating moderate to severe inoculum levels with the sites. Many FOV4 infected Upland cultivars typically showed fewer leaf symptoms and much lower plant mortality in early stages of the disease compared to Pima cultivars. The inheritance of FOV4 resistance/tolerance in Upland cotton ranges from recessive to intermediate, unlike in Pima cotton where resistance seems to be dominant or more complete in the host plant. Highly resistant/tolerant Upland breeding lines were developed from this breeding research effort and will be publicly released to reduce the vulnerability of the cotton industry to this pathogen.