Dynamic responses to Fusarium wilt (Fusarium oxysporum f. sp. vasinfectum) race 4 in two introgressed populations of Upland cotton (Gossypium hirsutum)

Abstract

Fusarium wilt (FW) caused by race 4 (FOV4) of the soil-borne fungus Fusarium oxysporum f. sp. vasinfectum causes high cotton seedling mortality under field conditions in the west and southwest US Cotton Belt. The objectives of this study were to evaluate the dynamic responses to FOV4 infections in two introgressed Upland cotton (Gossypium hirsutum) populations using an artificial inoculation and screening method. The two populations (IL-122 with 120 introgressed lines and BIL-193 with 180 backcross inbred lines) were derived from backcrossing the same recurrent Upland parent (FOV4 resistant) to two different interspecific hybrids each with a different G. barbadense parent (FOV4 susceptible). The two populations, together with their parents and other Upland lines were evaluated in three independent replicated tests for FOV4 responses at 7, 14, 21, and 28\xa0days after inoculation (DAI) under temperature-controlled conditions at 20–21\xa0°C, based on three evaluating parameters- disease incidence (DI), disease severity rating (DSR) and mortality rate (MR). Wilted seedlings were observed as the initial FW symptoms in 21% of the lines within 7 DAI, and no line had 0% DI across the three tests at 28 DAI. DI, DSR and MR were highly and significantly correlated among one another at 14, 21, and 28 DAI, and 21 DAI had the strongest correlations with 14 and 28 DAI. The results suggest that responses to FOV4 infections were overall congruent among DAI in the two cotton populations, and DSR at as early as 21 DAI can be used to assess germplasm lines for FOV4 resistance. Based on responses to FOV4 at 7, 14, 21 and 28 DAI, 17 of the 88 susceptible lines in the IL-122 population at 28 DAI showed resistant responses at 7–14 and/or 7–21 DAI, and they may provide resistant responses in early stages of FOV4 infections. Among 34 resistant (R) lines at 28 DAI, 22 lines showing susceptible (S) responses at 14 and/or 21 DAI. Similar results were observed in the BIL-193 population. The result showed that there were lines with unique responses (R or S) during different DAI and represented one of the first such studies in cotton. It provides a set of unique lines for further studies at the structural, biochemical and molecular levels and an incentive to investigate the genetic and genomic basis of the dynamic responses to FOV4 through detecting DAI-specific resistance genes or quantitative trait loci (QTL).