Characterization of Current f. sp. Isolates from Cotton in the San Joaquin Valley of California and Lower Valley El Paso, Texas.

f. sp. race 4 is a causal agent of Fusarium wilt of cotton ( spp.). This study aimed to characterize the existing distribution and frequency of current field populations of f. sp. race 4 genotypes in the San Joaquin Valley (SJV) of California and Lower Valley El Paso, TX and examine representative isolates for aggressiveness during different stages of seedling development. A survey was conducted from 2017 to 2019 across 13 locations in the SJV and one location in El Paso, TX during 2018. From the SJV, isolates identified as the f. sp. race 4 T genotype were dispersed across the SJV, whereas isolates identified as the f. sp. race 4 N genotype were most frequently isolated from cotton fields in the northern county of Merced. The f. sp. race 4 isolates from the Texas location were identified as the MT genotype. A selection of representative isolates was evaluated using three inoculation assays (rolled-towel, f. sp. -infested oat seed, and root-dip inoculation) to test the isolates' abilities to produce symptoms during seedling stages of cotton development. All isolates tested were capable of producing symptoms on cotton; however, isolate aggressiveness varied within and across inoculation assays. In all assays, higher levels of disease development were observed in the moderately susceptible Pima ( L.) cultivars (DP-340 or PHY-830) when compared with the moderately tolerant Upland ( L.) cultivar (FM-2334). However, no correlation was found among the different response variables for the rolled-towel assay when compared with the root-dip and infested oat seed assays. These results suggest that different genes are involved in the resistance response during the early seedling development stage measured in the rolled-towel assay compared with the later seedling development stages measured during the root-dip inoculation and infested oat seed assays, revealing the complexity of the Fusarium wilt disease and host-plant resistance mechanisms.