Vasinfectum Race Research Articles

Greenhouse Screening of Cotton Varieties Against Fusarium Wilt Complex

Fusarium oxysporum f. sp. vasinfectum race 1 and 2 (FOV) and the Meloidogyne incognita (southern root-knot nematode, RKN) complex can cause severe losses in cotton, and management recommendations include using varieties that are resistant to RKN. The objective of this project was to test the response of different varieties to the Fusarium wilt complex with and without RKN resistance. Four greenhouse trials were conducted, where cultivars infested with RKN and FOV were evaluated on (i) leaf wilt symptoms rated every 3 to 4 days to calculate the area under disease progress curve (AUDPC) and (ii) visual ratings of root and stem necrosis at the end of each trial. RKN-resistant cultivars PHY 480 W3FE and DP 1747NR B2XF had statistically similar AUDPC, root necrosis, and stem necrosis as RKN-susceptible cultivars DP 1522 B2XF and DP 1646 B2XF in all trials. This demonstrated that RKN resistance does not correlate to FOV resistance, and RKN resistance alone is not sufficient to manage Fusarium wilt. Cultivar ST 4946GLB2, which carries both RKN and FOV resistance, statistically had the lowest AUDPC, root necrosis, and stem necrosis in all greenhouse trials.

Characterization of the High-Quality Genome Sequence and Virulence Factors of Fusarium oxysporum f. sp. vasinfectum Race 7.

Fusarium oxysporum f. sp. vasinfectum (Fov) is a common soilborne fungal pathogen that causes Fusarium wilt (FW) disease in cotton. Although considerable progress has been made in cotton disease-resistance breeding against FW in China, and the R gene conferring resistance to Fov race 7 (FOV) in Upland cotton (Gossypium hirsutum) has been identified, knowledge regarding the evolution of fungal pathogenicity and virulence factors in Fov remains limited. In this study, we present a reference-scale genome assembly and annotation for FOV7, created through the integration of single-molecule real-time sequencing (PacBio) and high-throughput chromosome conformation capture (Hi-C) techniques. Comparative genomics analysis revealed the presence of six supernumerary scaffolds specific to FOV7. The genes or sequences within this region can potentially serve as reliable diagnostic markers for distinguishing Fov race 7. Furthermore, we conducted an analysis of the xylem sap proteome of FOV7-infected cotton plants, leading to the identification of 19 proteins that are secreted in xylem (FovSIX). Through a pathogenicity test involving knockout mutants, we demonstrated that FovSIX16 is crucial for the full virulence of FOV7. Overall, this study sheds light on the underlying mechanisms of Fov's pathogenicity and provides valuable insights into potential management strategies for controlling FW.

Two de novo genome assemblies from pathogenic Fusarium oxysporum f. sp. vasinfectum race 4 (FOV4) isolates from California.

Fusarium oxysporum f. sp. vasinfectum race 4 (FOV4) is the most virulent cotton wilt pathogen in the United States. There is an urgent need for improved detection and diagnostics to combat the spread of FOV4. To help meet this challenge, we report the de novo assembly of two pathogenic isolates of FOV4 from California.

Pathogenic and Saprophytic Growth Rates of Fusarium oxysporum f. sp. vasinfectum Interact to Affect Variation in Inoculum Density and Interannual Infection Risk.

Fusarium wilt of cotton caused by the soilborne fungal pathogen Fusarium oxysporum f. sp. vasinfectum race 4 (FOV4) is a contemporary epidemic affecting cotton production in Far West Texas. The spatial distribution of soilborne FOV4 can be heterogeneous at small scales, and the factors that lead to this heterogeneity require investigation. Hypothetical causes include dissemination of spores through soils and variable saprophytic growth of the fungus. In the field, FOV4 DNA was quantified from soil during and after the cotton-growing season, and though the average amounts of DNA were not different between these time points, the variances of DNA across space were significantly different. Variability was higher when pathogenic growth of the fungus was expected owing to the presence of live cotton plants and lower when saprophytic growth was expected after cropping. In sterile-environment growth chamber experiments, the abundance of organic matter influenced the fungal vegetative growth rate and maximum amount as measured through quantitative PCR and the timing of the fungus' increasing its rate of spore production as measured through dilution plating. To investigate movement of spores in soils, spore mobility in experimental columns was quantified. Soil composition and organic matter abundance affected spore mobility, indicating that the timing of spore production relative to the availability of growth resources will affect the spatial spread of FOV4 and suggesting that soil properties affect the retention of conidia. The spatial spread of FOV4 through soil varies temporally and is affected by the shift between pathogenic and saprophytic growth modes of the fungus.

A Rapid and Reliable Method for Evaluating Cotton Resistance to Fusarium Wilt Race 4 Based on Taproot Rot at the Seed Germination Stage.

Fusarium oxysporum f. sp. vasinfectum race 4 (FOV4) is a soilborne fungal pathogen threatening U.S. cotton production. The objective of this study was to develop a reliable and efficient method to evaluate cotton for FOV4 resistance based on taproot rot during seed germination through five growth chamber tests and two greenhouse tests. Seeds from eight cotton cultivars (Set 1) were germinated in a paper towel for 6 days, and taproots were inoculated with a FOV4 conidial suspension using three inoculation methods (i.e., taproot dipping, taproot wounding, and paper towel drenching), in addition to seed soaking before germination. The taproot rot-based disease incidence (DI) and disease severity rating (DSR), seed germination percentage (SGP), and plant fresh weight (PFW) were measured 7 days after inoculation. Taproot dipping was the most efficient and reliable evaluation method. The SGP and PFW were not significantly correlated with the DI and DSR, making them inappropriate to use in resistance evaluation. Pima DP 359 RF and PHY 881 RF were the most resistant with the lowest root rot. The taproot dipping method was repeated in another test and confirmed in two tests using another set of eight cultivars (Set 2). The taproot rot-based DSR at germination was significantly correlated with the DSR at the seedling stage in the greenhouse in both sets and with previous results in seedling mortality in the greenhouse and field in Set 2. The results suggest that the response to FOV4 infections at the seed germination stage is overall congruent with that at the seedling stage.

Geostatistical techniques to account for the heterogeneity of Fusarium wilt inoculum distribution in upland cotton field screening studies

AbstractVariable and uncontrollable environmental factors have a wide range of influence on crop field screening programs, with the potential to cause significant experimental errors in phenotypic data collection. These factors can be additive and negatively confound genetic studies. When field screening upland cotton for genetic resistance to Fusarium wilt caused by Fusarium oxysporum f. sp. vasinfectum race 4 (FOV4), the distribution and concentration of fungal inoculum in the field directly impact the disease severity, affecting the results of these studies. Variability among FOV4 screening fields and protocols has influenced the search for durable genetic resistance. To account for this spatial variability, rigorous use of check plots with predictable responses to FOV4 were planted throughout a screening nursery in Clint, TX, and scored for percent survival within each plot. The scores and locations were used to generate a predicted surface via kriging interpolation and conditional simulation, which estimate FOV4 inoculum pressure at every plot location in the field. These predictions created FOV4 pressure‐adjusted disease severity ratings in F3 generations of bi‐parental crosses between FOV4‐resistant and susceptible upland cotton lines. Environment‐adjusted phenotypes allow breeders to consider the environmental variance associated with the heterogenous distribution of the pathogen in the field. The techniques presented here are transferrable to any field screening program that needs to account for spatial variation of environmental factors.

Studies of Evaluation Methods for Resistance to Fusarium Wilt Race 4 (Fusarium oxysporum f. sp. vasinfectum) in Cotton: Effects of Cultivar, Planting Date, and Inoculum Density on Disease Progression.

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.

Studies of evaluation parameters for resistance to Fusarium wilt in cotton caused by Fusarium wilt race 4 (Fusarium oxysporumf. sp.vasinfectum)

AbstractFusarium wilt, caused byFusarium oxysporumf. sp.vasinfectumrace 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 (Gossypiumspp.) 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 controlled‐conditions 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.

Detection, genotyping and virulence characterization of Fusarium wilt race 4 (VCG0114) causing cotton wilt in three Texas fields

AbstractFusarium wilt of cotton, caused by the soilborne fungus Fusarium oxysporum f.sp. vasinfectum race 4 (Fov4, VCG0114), is a serious disease that has adversely affected cotton production in California since 2001 and in the Upper Rio Grande Region near El Paso in the border areas between Texas and New Mexico since 2017. Fov4 was isolated from cotton stem samples collected from wilted or asymptomatic plants adjacent to the wilting plants from the El Paso area. The MT genotype was the only Fov4 genotype found in two fields in the Lower Valley of El Paso and the T genotype was the only Fov4 genotype found in one field in the Upper Valley Region of El Paso. No N, MiT or mixed genotypes of Fov4 were detected in any of these fields. Furthermore, the F. oxysporum f.sp. vasinfectum isolates were exclusively Fov4. The results support the observation that the MT genotype of Fov4 is mainly distributed in the Lower Valley of El Paso and the T genotype of Fov4 is mainly distributed in the Upper Valley Region of El Paso. Virulence of the MT genotype of Fov4 was higher than that of the T genotype. PCR methods previously developed for Fov4 specific PCR detection and multiplex Fov4 genotyping were validated by VCG analyses of the newly obtained isolates. These PCR methods were adapted for the in planta detection of Fov4, but Fov4 genotyping was limited to the detection of T, MT and MiT genotypes. It is important to continue to monitor dynamic population structure of Fov4 genotypes to prevent the spread of Fov4 and optimize disease control.

Comparative Analysis of Infection Process in Pima Cotton Differing in Resistance to Fusarium Wilt Caused by Fusarium oxysporum f. sp. vasinfectum Race 4.

Fusarium oxysporum f. sp. vasinfectum race 4 (FOV4) causes an early season cotton disease including seedling deaths. This study compared two Pima cottons (Gossypium barbadense) in the infection process of FOV4 using a confocal and a scanning electron microscope. Seedlings were grown in a hydroponic system and inoculated with a virulent local FOV4 isolate. As compared with the susceptible Pima S-7, the resistant Pima PHY 841 RF had significantly fewer conidia attached and germinated on the root surface. FOV4 penetration into the root epidermis of PHY 841 RF was delayed until 24 h postinoculation (hpi) as compared with 8 hpi in Pima S-7. In Pima S-7, hyphae progressed to the xylem through the cortex between 5 and 7 days postinoculation. However, hyphae grew much slower in the cortex with no apparent hyphae observed in the xylem of PHY 841 RF. At plant maturity, no FOV4 was detected through fungal isolation and PCR in the stem of PHY 841 RF and its resistance donor parents PHY 800 and Pima S-6, as compared with Pima S-7 and DP 744 with positive results. The results demonstrate that PHY 841 RF is resistant to FOV4, due to delayed infection, reduced fungal growth and reproduction, and prevention of the fungus from invading the xylem.

An InVitro Co-Culture System for Rapid Differential Response to Fusarium oxysporum f. sp. vasinfectum Race 4 in Three Cotton Cultivars.

Fusarium oxysporum f. sp. vasinfectum race 4 (FOV4) is a devastating fungus pathogen that causes Fusarium wilt in both domesticated cotton species, Gossypium hirsutum (Upland) and G. barbadense (Pima). Greenhouse and field-based pathogenicity assays can be a challenge because of nonuniform inoculum levels, the presence of endophytes, and varying environmental factors. Therefore, an invitro coculture system was designed to support the growth of both domesticated cotton species and FOV4 via an inert polyphenolic foam substrate with a liquid medium. A Fusarium wilt-susceptible Pima cotton cultivar, G. barbadense 'GB1031'; a highly resistant Pima cotton cultivar, G. barbadense 'DP348RF'; and a susceptible Upland cotton cultivar, G. hirsutum 'TM-1', were evaluated for 30 days during coculture with FOV4 in this foam-based system. Thirty days after inoculation, disease symptoms were more severe in both susceptible cultivars, which displayed higher percentages of foliar damage, and greater plant mortality than observed in 'DP348RF', the resistant Pima cotton cultivar. This foam-based invitro system may be useful for screening cotton germplasm for resistance to a variety of fungus pathogens and may facilitate the study of biotic interactions in domesticated cotton species under controlled environmental conditions.

Use of a Latin square design to assess experimental errors in field evaluation of cotton for resistance to Fusarium wilt Race 4

AbstractField 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.

Diallel analysis of resistance to Fusarium wilt Race 4 in American pima cotton

AbstractFusarium oxysporum f. sp. vasinfectum Race 4 (FOV4) causes high seedling mortality in pima cotton (Gossypium barbadense L.) grown in the western and southwestern U.S. Cotton Belt. The objectives of this study were to analyze the combining abilities and genetic basis of FOV4 resistance in pima cotton using a half‐diallel mating design. Thirty‐six biparental F3 and F4 generations were derived from six commercial cultivars and three elite lines and artificially inoculated with FOV4 for screening for FOV4 resistance in four replicated tests, with two tests for each generation. Disease severity rating (DSR) at 28 d after inoculation (DAI) and the area under the disease progress curve (AUDPC) across 7, 14, 21, and 28 DAI were analyzed via Griffing's Method 2 and Hayman's diallel analysis. ‘Pima S‐6’, ‘PHY 76’, and ‘8810’ (resistant parents) were heterogeneous in their responses to FOV4 infections with 20 to 50% disease incidence and mortality rate; and they had significantly lower DSR and AUDPC with negative general combing ability effects compared with the susceptible parents. Variances caused by specific combining ability and nonadditive effects were higher than those caused by additive effects for FOV4 resistance, consistent with the estimates for broad‐sense heritability. The results indicated a predominant nonadditive effect for FOV4 resistance within these genotypes of pima cotton. A minimum of one or two genes was estimated for FOV4 resistance based on Hayman analysis. The results represented the first study applying a diallel analysis to investigate the quantitative genetic basis of FOV4 resistance in pima cotton.

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

Fusarium oxysporum f. sp. vasinfectum race 4 is a causal agent of Fusarium wilt of cotton (Gossypium spp.). This study aimed to characterize the existing distribution and frequency of current field populations of F. oxysporum f. sp. vasinfectum 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. oxysporum f. sp. vasinfectum race 4 T genotype were dispersed across the SJV, whereas isolates identified as the F. oxysporum f. sp. vasinfectum race 4 N genotype were most frequently isolated from cotton fields in the northern county of Merced. The F. oxysporum f. sp. vasinfectum 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. oxysporum f. sp. vasinfectum-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 (Gossypium barbadense L.) cultivars (DP-340 or PHY-830) when compared with the moderately tolerant Upland (G. hirsutum 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.

Importance of temperature in evaluating cotton for resistance to Fusarium wilt caused by Fusarium oxysporum f. sp. vasinfectum race 4

AbstractFusarium wilt of cotton (Gossypium spp.) is caused by a soil‐ and seed‐borne fungal pathogen Fusarium oxysporum f. sp. vasinfectum (FOV). Fusarium oxysporum f. sp. vasinfectum race 4 (FOV4) is a significant threat to cotton production in the United States. The effect of temperature and soil on disease symptoms in cultivars with a range of susceptibility to FOV4 was examined in growth chamber trials. Three upland (G. hirsutum L.) and three pima (G. barbadense L.) cultivars were planted in three types of soils (potting mix and farm soils with and without FOV4 preinfestation) and maintained at four constant temperature regimes (20, 23, 26, and 29 °C). After seedling emergence, each pot was inoculated with conidial suspensions of FOV4 and scored for resistance based on disease incidence (DI), foliar disease severity rating (DSR), and mortality rate (MR) at 7, 14, 21, and 28 d post‐inoculation (dpi). The area under the disease progress curve (AUDPC) based on DSR across the four evaluation dpi was also analyzed. Among the six cultivars tested, Pima PHY 841 RF, Pima S‐6, and the uplands Acala 1517‐08 and FM 2334GLT were the most resistant, whereas upland PHY 725 RF and Pima S‐7 were the most susceptible. The genotypic responses to FOV4 were not significantly (P > .05) affected by differences in temperature regime or soil type. Cotton seedlings manifested more severe symptoms and higher mortality when maintained at 23 °C, followed by 20/26 °C and 29 °C in descending order. The mycelial growth rate was greater at 23/26 °C, whereas plants grew slower at 20/23 °C. There were no differences (P > .05) in DI, DSR, MR, and AUDPC between the inoculated farm soil (with no FOV4 preinfestation) and the inoculated FOV4‐preinfested farm soil, suggesting the unnecessity of using the latter in artificial inoculations. However, the potting soil produced higher (P < .05) DI and DSR at 7 and 14 dpi but lower (P < .05) AUDPC. This growth chamber study demonstrated that 23 °C produced the highest DI, severity, and plant death, providing an important piece of information for screening cotton for FOV4 resistance in breeding and FOV4 disease management in cotton production.

Genome Resources of Four Distinct Pathogenic Races Within Fusarium oxysporum f. sp. vasinfectum that Cause Vascular Wilt Disease of Cotton.

Whole genome sequence (WGS) based identifications are being increasingly used by regulatory and public health agencies to facilitate the detection, investigation, and control of pathogens and pests. Fusarium oxysporum f. sp. vasinfectum is a significant vascular wilt pathogen of cultivated cotton and consists of several pathogenic races that are not each other's closest phylogenetic relatives. We have developed WGS assemblies for isolates of F. oxysporum f. sp. vasinfectum race 1 (FOV1), race 4 (FOV4), race 5 (FOV5), and race 8 (FOV8) using a combination of Nanopore (MinION) and Illumina sequencing technology (Mi-Seq). This resulted in assembled contigs with more than 100× coverage for each of the F. oxysporum f. sp. vasinfectum races and estimated genome sizes of FOV1 52 Mb, FOV4 68 Mb, FOV5 68 Mb, and FOV8 55 Mb. The AUGUSTUS gene prediction program predicted 16,263 genes in FOV1, 20,259 genes in FOV4, 20,375 genes in FOV5 and 16,615 genes in FOV8. We were able to identify 525 genes unique to FOV1, 570 unique to FOV4, 1,242 unique to FOV5, and 383 unique to FOV8. We expect that these findings will help in comparative genomics and in the identification of unique genes as candidate targets for diagnostic marker and methods development to permit rapid differentiation of F. oxysporum f. sp. vasinfectum subgroups.

Pedigree selection under field conditions within Acala 1517-08 and its glandless derivatives for development of cotton resistant to Fusarium wilt caused by Fusarium oxysporum f. sp. vasinfectum race 4

The soil- and seed-borne fungus, Fusarium oxysporum f. sp. vasinfectum race 4 (FOV4), causes Fusarium wilt in cotton (Gossypium spp.) and represents a significant threat to cotton production in the US. The objectives of this study were to enhance FOV4 resistance in heterogeneous Acala 1517-08 and its derivatives through three cycles of pedigree selection in 2018, 2019 and 2020 under FOV4-infested field conditions. In 2017, in a field not infested with FOV4, 46, 12, and 23 single plants were selected for productivity from a base population of 1100–1400 plants of Acala 1517-08 and its derived glandless lines, Acala 1517-18 GLS and NM 13P1125, respectively. In 2018, the single plant selections, controls and other lines were tested in progeny-rows in an FOV4 infested field for disease responses including mortality rate (MR), root vascular staining (RVS), and disease incidence (DI), resulting in 41, 15, and 26 single plants selected from progenies of the three lines, respectively, based on RVS. These single plant selections were tested in 82 progeny-rows in the same field in 2019, and the second selection cycle based on RVS advanced 40, 16, and 37 plants selected from the progenies of the three lines, respectively, for further evaluation and the third selection cycle in a progeny test in 2020. In 2020, an average of 81–84% plants died in highly susceptible controls, Pima S-7 and Pima DP 744, due to FOV4 infections, while the resistant controls, Pima S-6, Pima PHY 841 RF and NuMex COT 17 GLS, incurred up to 19–24% mortality. As a result of the three cycles of single plant selection, a total of 12 progenies were identified to be as resistant as the resistant controls. On the progeny basis, MR between years was significantly correlated, and MR at different evaluation dates was also significantly correlated with one another and with mean RVS and DI within year. However, due to possible escapes of plants from FOV4 infections, the original RVS scores of individual plant selections were not correlated with their respective progenies; and no significant differences were detected among progeny means grouped based on RVS ratings for single plant selections. The results demonstrate that early plant mortality is a superior means of selecting against FOV4 susceptibility, while using RVS rating late in the season if excluding seedling MR may inflate and distort the resistance level of genotypes, rendering it less reliable in assessing FOV4 resistance in cotton. In addition, many single plants selected for FOV4 resistance with no RVS may be escapes, necessitating repeated progeny tests and selection. The importance of pedigree selection within germplasm, choice of resistant and susceptible checks, FOV4 inoculum density, and the use of RVS, and resistance versus tolerance in response to FOV4 infections under field conditions are discussed in detail.

Managing Fusarium oxysporum f. sp. vasinfectum Race 4 with beneficial microorganisms including entomopathogenic fungi

Fusarium spp. are dangerous pathogens of several commercially important crops around the world. Among non-chemical alternatives, beneficial microorganisms offer a potential control option for Fusarium and other soilborne pathogens. A greenhouse study was conducted using cotton as a model plant and F. oxysporum f. sp. vasinfectum Race 4 (FOV Race 4) as the pathogen to evaluate the efficacy of commercial formulations of microbial and botanical biopesticides. Streptomyces lydicus (Actinovate® AG), Bacillus amyloliquefaciens (Stargus®), giant knotweed extract (Regalia®), entomopathogenic fungi - Beauveria bassiana (BotaniGard® ES), Isaria fumosorosea (PFR-97™ 20% WDG), and Metarhizium brunneum (M. anisopliae; Met52® EC) were applied to FOV Race 4 infested soil in three treatment regimens, and the growth of susceptible Pima cotton seedling growth and health was monitored for 5 weeks. Treatments were generally effective against antagonizing FOV Race 4 especially when applied multiple times at a lower rate or once at a higher rate. Entomopathogenic fungi were as effective as other products that are exclusively used for disease management. Beauveria bassiana provided superior control for a long period compared to other treatments. This study reports the efficacy of non-chemical alternatives against FOV Race 4 and the potential of using entomopathogenic fungi for disease management in addition to their normal role of arthropod pest management.

Quantitative Trait Loci Mapping of Multiple Independent Loci for Resistance to Fusarium oxysporum f. sp. vasinfectum Races 1 and 4 in an Interspecific Cotton Population.

Fusarium wilt, caused by the soilborne fungal pathogen Fusarium oxysporum f. sp. vasinfectum, is a vascular disease of cotton (Gossypium spp.). F. oxysporum f. sp. vasinfectum race 1 (FOV1) causes major plant injury and yield loss in G. hirsutum cultivars with coinfection with root-knot nematode (Meloidogyne incognita), while F. oxysporum f. sp. vasinfectum race 4 (FOV4) causes plant damage without nematode coinfection in G. hirsutum and in G. barbadense cultivars. Quantitative trait loci (QTL) analysis of the interspecific cross G. barbadense Pima S-7 × G. hirsutum Acala NemX revealed separate multiple loci determining resistance to FOV1 and FOV4, confirming that race specificity occurs in F. oxysporum f. sp. vasinfectum. Based on the area under the disease progress stairs, six major QTLs on chromosomes (Chrs) 1, 2, 12, 15 (2), and 21 contributing 7 to 15% to FOV1 resistance and two major QTLs on Chrs 14 and 17 contributing 12 to 33% to FOV4 resistance were identified. Minor-effect QTLs contributing to resistance to both FOV1 and FOV4 were also identified. These results define and establish a pathosystem of race-specific resistance under polygenic control. This research also validates the importance of previously reported markers and chromosome regions and adds new information for the location of F. oxysporum f. sp. vasinfectum resistance genes. Some F8 recombinant inbred lines have resistance to both FOV1 and FOV4 and also to root-knot nematode, providing multiple resistance sources for breeding.

Detoxification of Fusaric Acid by the Soil Microbe Mucor rouxii.

Fusarium oxysporum f. sp. vasinfectum race 4 (VCG0114), which causes root rot and wilt of cotton (Gossypium hirsutum and G. barbadense), has been identified recently for the first time in the western hemisphere in certain fields in the San Joaquin Valley of California. This pathotype produces copious quantities of the plant toxin fusaric acid (5-butyl-2-pyridinecarboxylic acid) compared to other isolates of F. oxysporum f. sp. vasinfectum (Fov) that are indigenous to the United States. Fusaric acid is toxic to cotton plants and may help the pathogen compete with other microbes in the soil. We found that a laboratory strain of the fungus Mucor rouxii converts fusaric acid into a newly identified compound, 8-hydroxyfusaric acid. The latter compound is significantly less phytotoxic to cotton than the parent compound. On the basis of bioassays of hydroxylated analogues of fusaric acid, hydroxylation of the butyl side chain of fusaric acid may affect a general detoxification of fusaric acid. Genes that control this hydroxylation may be useful in developing biocontrol agents to manage Fov.