No abstract

Field evaluation of cotton for resistance to Fusarium oxysporum f. sp. vasinfectum Race 4 (FOV4) is important in breeding. In this study, an 8 × 8 Latin square design was arranged in two FOV4‐infested fields each with two trials to evaluate eight cotton (Gossypium hirstum) cultivars based on mortality rate (MR) and root vascular staining (RVS). For a comparison, two trials of the same design were grown in the greenhouse. Consistent with the mapping pattern of MR in the Texas field, the analysis of variance confirmed the existence of effects due to tiers not planting rows in the Texas location only. Significant variation due to cultivars was consistently detected in the two trials of both locations. The repeatability was confirmed by significant correlation coefficients between most tiers (replications) and between the two trials in each field. The greenhouse test had the highest mean MR (72–78%) and lowest coefficients of variation (CV; 17–28%), followed by the Texas field with a lower MR (36–57%) and similar CV (22–29%); and the New Mexico test had the highest CV (59–73%) and the lowest MR (13–18%). The MR between the two fields and between the fields and the greenhouse were significantly correlated. ‘Pima S‐7’ had the highest MR (79–93%), whereas ‘FM 2334GLT’ had the lowest (15–35%), consistent with RVS at crop maturity. Among randomized complete block designs (RCBD) with three to seven replications, the mean squared error and CV were the lowest in RCBD with three replications.

Fusarium wilt caused by Fusarium oxysporum f. sp. vasinfectum race 4 (FOV4) is an early season disease causing root rot, seedling wilt, and death. To develop an appropriate field evaluation method for resistance to FOV4 in cotton breeding, the objectives of this study were to investigate the effects of cultivar, planting date, and inoculum density on disease progression in 2020–2021. Results showed that the usual local mid-April planting had the lowest disease severity (DSR) or mortality rate (MR) in 2020 and 2021. DSR or MR increased at the late April and early May plantings in both years and reached the highest at the early May planting in 2020, while MR in 2021 was followed by a decrease in the late May planting and reached the highest in the mid-June planting. Local daily low temperatures between mid-April and mid-June were favorable for FOV4 infections, whereas daily high temperatures at 35°C or higher suppressed wilt severity. When seedlings at the 2-true leaf stage were inoculated with 104, 105, 106, and 107 conidia ml−1 per plant in 2020, DSR was low but a linear relationship between inoculum density and DSR was observed. When a FOV4-infested soil supplemented with artificial inoculation was used, disease progression in three moderately susceptible or moderately resistant cultivars followed a linear model, while it followed a quadratic model in the highly susceptible Pima S-7 cultivar only. Among the other three cultivars, FM 2334GLT had the lowest DSR or MR except for one planting date in both years, followed by PHY 725 RF and Pima PHY 881 RF in ascending order, which were consistent with the difference in regression coefficients of the linear models. This study demonstrates that disease progression curves due to FOV4 can be used to compare responses to FOV4 infections among cotton genotypes in cotton breeding and genetic studies, regardless of planting date and inoculation method.

Fusarium wilt, caused by Fusarium oxysporum f. sp. vasinfectum race 4 (FOV4), is an early season vascular disease with symptoms including seedling wilt and mortality. The objectives of this study were to investigate the relationship between mortality rate (MR) and disease severity rating (DSR) based on root vascular staining (RVS)-DSR and stem vascular discoloration (SVD)-DSR in the field and the relationship of MR in controlled conditions with MR and DSR in the field. Results from two field tests and two greenhouse tests each with a Latin square design for eight cultivars of cotton (Gossypium spp.) and two field progeny-row tests were analyzed. In the field, MR at the square stage was significantly correlated with both DSR (RVS) and DSR (SVD) at crop maturity, and they were also significantly correlated with DSR based on both MR and RVS-DSR (MR+RVS) and both MR and SVD-DSR (MR+SVD). The results show that DSR (MR+RVS) was the most accurate in assessing FOV4 resistance, and DSR (SVD) and DSR (MR+SVD) can be used in combination with DSR (RVS) and DSR (MR+RVS), or as the substitutes for RVS. Mortality rate in the greenhouse was significantly correlated with MR and DSR (MR+RVS) and DSR (MR+SVD) in the field, indicating consistency between evaluations under both conditions. Furthermore, foliar symptoms-based MR and DSR under controlledconditions were significantly correlated, with coefficients of variation at 20 to 30% in other related studies. The results confirm the validity of using the two parameters in greenhouse evaluations for FOV4 resistance in cotton.

Fusarium wilt caused by soil-borne fungus Fusarium oxysporum f. sp. vasinfectum race 4 (FOV4) is an early season disease causing root rot, and seedling wilt and death. However, it was unknown if cotton (Gossypium spp.) growth stages and root wounding affected genotypic responses to FOV4. The objectives of this study were to investigate the effects of cotton growth stage, cultivar, and root wounding on FOV4associated disease development at 7-, 14-, 21-, and 28-d post inoculation through five replicated tests in controlled environmental conditions with temperatures at 21 to 23 ˚C. Seedlings inoculated at the cotyledon stage incurred the highest disease incidence (DI), disease severity rating (DSR), and mortality (MR), followed by seedlings inoculated at the 1-, 3-, and 5-true leaf stages in descending order. Root wounding at the cotyledon stage followed by immediate inoculation with FOV4 inoculum made all the tested cultivars and lines highly susceptible. Among the six cultivars tested, FM 2334GLT and Pima DP 359 RF were the most resistant, Pima S-7 was highly susceptible, and Acala 1517-08, Acala 1517-18 GLS, and Pima PHY 881 RF were susceptible. The importance of artificial inoculation at a true leaf stage with or without root wounding to identify resistance that may be undetected under field conditions was discussed.