Genetic Diversity, Virulence, and Meloidogyne incognita Interactions of Fusarium oxysporum Isolates Causing Cotton Wilt in Georgia

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.

The Genome of Fusarium oxysporum f. sp. phaseoli Provides Insight into the Evolution of Genomes and Effectors of Fusarium oxysporum Species

Fusarium oxysporum f. sp. phaseoli, the causal agent of cowpea fusarium wilt, is a serious threat to cowpea production in China. In this study, a sample of cowpea fusarium wilt was identified as Fusarium oxysporum f. sp. phaseoli using the methods of morphological characters and molecular detection. We further reported the first genome assembly for Fusarium oxysporum f. sp. phaseoli, with 53.7 Mb genome sequence comprising 14,694 genes. Comparative genomic analysis among five Fusarium oxysporum genomes showed that four accessory chromosomes in the five Fusarium oxysporum display similar characteristics, with low sequence similarity (55.35%, vs. overall average of 81.76%), low gene density (2.18 genes/10 kb vs. 3.02 genes/Mb) and highly transposable element density (TEs) (15.01/100 kb vs. 4.89/100 kb), indicating that variable accessory chromosomes are the main source of Fusarium oxysporum evolution. We identified a total of 100 Fusarium oxysporum f. sp. phaseoli-specific effectors in the genome and found 13 specific effector genes located in large insertion or deletion regions, suggesting that insertion or deletion events can cause the emergence of species-specific effectors in Fusarium oxysporum. Our genome assembly of Fusarium oxysporum f. sp. phaseoli provides a valuable resource for the study of cowpea fusarium wilt, and the comparative genomic study of Fusarium oxysporum could contribute to the knowledge of genome and effector-associated pathogenicity evolution in Fusarium oxysporum study.

Diversity and temporal distribution of Fusarium oxysporum f. sp. vasinfectum races and genotypes as influenced by Gossypium cultivar

This study assess the population diversity and temporal variability of caused by Fusarium oxysporum f. sp. vasinfectum (FOV) races/genotypes infecting cotton cultivars with either FOV or Meloidogyne incognita resistance. All plants sampled demonstrated typical symptoms of FOV including wilting, chlorosis and necrosis of the leaves, and discoloration of the vascular tissue in the stem. A diverse population of FOV was characterized. Eight races/genotypes of FOV were collected throughout the three site years. FOV race 1 was the most predominant in all tests (AUDPC=101.1); statistically higher numbers of isolates from LA-108 (AUDPC=59.9), race 8 (AUDPC=47.5), and race 2 (AUDPC=38.6) were also found compared to other races and genotypes collected. FOV race 1, race 2, race 8, and 108 were the most virulent races identified. The genotypes MDS-12, LA-110, and LA-127/140 were found in all tests but at a low incidence, and LA-112 was only found in trace amounts. MDS-12, LA-110, LA-112, and LA-127/140 produced less disease pressure. FOV race 4 which is highly virulent and present in California and Texas was not found in Alabama. A positive correlation was observed between the accumulation of growing degree days and FOV race 1, race 2, race 8, LA-108, and LA-110. Later symptom expression influenced by seasonal heat partially mitigates damage allowing cotton to produce bolls though they may be reduced in number and lint quality. Plant resistance to the FOV as expressed in these cultivars appears to provide better protection than M. incognita resistance. PhytoGen 72, which is resistant to FOV races/genotypes had low levels of FOV infection even though it sustained a high level of M. incognita root population density. The M. incognita resistant cultivars Deltapine 1558NR B2RF and PhytoGen 480 W3FE supported a lower nematode population density, however, FOV disease incidence was not reduced. FOV races/genotypes did not vary significantly between the nematode resistant and nematode susceptible cultivars.

Genetic Diversity and Population Structure of Fusarium oxysporum f. sp. conglutinans Race 1 in Northern China Samples

Fusarium oxysporum f. sp. conglutinans (FOC), the causal agent of cabbage fusarium wilt, is a serious threat to cabbage production in northern China, and most Chinese FOC isolates were identified as FOC race 1 (FOC1). To better understand the genetic diversity of FOC1 in northern China, we collected FOC isolates from five provinces in northern China and identified them as FOC1 through pathogenicity and race test. To evaluate the genome-level diversity of FOC1, we performed a genome assembly for a FOC1 isolate (FoYQ-1) collected from Yanqing, Beijing, where cabbage fusarium wilt was first reported in China. Using resequencing data of FOC1 isolates, we conducted a genome-wide SNP (single nucleotide polymorphism) analysis to investigate the genetic diversity and population structure of FOC1 isolates in northern China. Our study indicated that Chinese FOC1 can be grouped into four populations and revealed that the genetic diversity of FOC1 were closely associated with geographical locations. Our study further suggests that genetic differentiation occurred when FOC1 spread to the northwest provinces from Beijing Province in China. The FOC1 genetic diversity based on whole-genome SNPs could deepen our understanding of FOC1 variation and provide clues for the control of cabbage fusarium wilt in China.

Interaction of Fusarium Wilt Race 4 with Root-Knot Nematode Increases Disease Severity in Cotton

Fusarium wilt, caused by Fusarium oxysporum f. sp. vasinfectum, is a severe disease of cotton (Gossypium spp.). Strains of the wilt pathogen in the United States, such as race 1, require the presence of nematodes such as southern root-knot nematode (Meloidogyne incognita) to cause appreciable disease. The exception is the race 4 strain of the wilt pathogen, which can attack cotton without concomitant infection by plant-parasitic nematodes and was first identified in California in 2001 and in Texas and New Mexico since 2017. The effects of the interaction between M. incognita and race 1 or race 4 on wilt severity and nematode reproduction on two Gossypium hirsutum cultivars, Acala 44 and FM 966, and a G. barbadense cultivar, Pima S-4, were directly compared in growth chamber assays. All three cultivars were susceptible to M. incognita. Suppression of nematode reproduction by the wilt pathogen was detected only for race 4 on all three cultivars on a per plant basis but not on a per gram root tissue basis. The control, M. incognita alone, and race 1 alone treatments caused no symptoms. Inoculation with race 1 and M. incognita caused moderate wilt symptoms in 'Acala 44' and 'FM 966' and mild symptoms in 'Pima S-4'. However, race 4 treatment caused severe wilt in 'Pima S-4' and moderate wilt severity in 'Acala 44' and 'FM 966'. The symptom severity of 'Acala 44' and 'FM 966' further increased in the presence of M. incognita. Thus, race 4 is not only capable of causing wilt in the absence of M. incognita but can also interact with the nematode to further increase disease severity. Though control of wilt caused by race 1 can be achieved mainly through breeding for nematode resistance, it will be imperative to incorporate both southern root-knot nematode and race 4 resistance to effectively control the disease should race 4 expand into southern root-knot nematode-infested fields.

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.

Detection and Characterization of Fusarium Wilt (Fusarium oxysporum f. sp. vasinfectum) Race 4 Causing Fusarium Wilt of Cotton Seedlings in New Mexico

Fusarium wilt (FW), caused by Fusarium oxysporum f. sp. vasinfectum (Atk.) W.C. Snyder & H.N. Hans (FOV), is one of the most destructive diseases of cotton (Gossypium spp.) worldwide. FOV race 4 (FOV4) is a highly virulent nominal race of this pathogen and a significant threat to cotton production in the western and southwestern USA and, potentially, the entire Cotton Belt. A field survey to identify FOV4 was performed in three southern counties of New Mexico in 619 cotton fields from 2018 to 2020. From 132 samples of cotton plants that exhibited wilt symptoms, Fusarium spp. were the most frequently isolated group of fungal species, with an isolation frequency of 57.4%. Eighty-four Fusarium spp. isolates were subsequently characterized by a DNA sequence analysis of three genes, EF-1α, PHO, and BT, encoding for translation elongation factor, phosphate permease, and β-tubulin, respectively. Forty-two isolates from 10 cotton fields were identified as FOV4 and confirmed with a positive 500-bp fragment diagnostic for FOV4. Twenty-six (62%) of the 42 FOV4 isolates were T type and the remainder (38%) were null type with and without a Tfo1 insertion in PHO, respectively. Each FOV4-infested field contained the same FOV4 genotype. Ten representative FOV4 isolates (one each from the 10 FOV4-infested fields) were evaluated for their pathogenicity on resistant Pima PHY 841 RF and susceptible Upland PHY 725 RF at 7, 14, 21, and 28 days after inoculation under temperature-controlled conditions at 21 to 22°C. Based on the disease severity rating, mortality rate, and area under the disease progress curve value, all 10 isolates were pathogenic to both cotton cultivars and differed in virulence; four isolates of the T genotype as a whole were more virulent than the six isolates of the N genotype. PHY 841 RF had significantly higher levels of resistance than PHY 725 RF to all FOV4 isolates. The results provide the first comprehensive account of the occurrence, distribution, and virulence of FOV4 in cotton production in New Mexico and will be useful for developing an effective strategy to manage FW in the state of New Mexico and the entire western and southwestern Cotton Belt.

Detection and Genotyping of Fov4 (Race 4, VCG0114), the Fusarium Wilt Pathogen of Cotton

Fusarium wilt, caused by Fusarium oxysporum f. sp. vasinfectum (Fov), is an important disease of cotton. More than 14 different genotypes as determined by VCG and sequence analyses are known to occur in the United States. Fov4 (race 4, VCG0114), originally found in India, was first detected in the United States in 2001 in California and recently in 2017 and 2019 in Texas and New Mexico, respectively. Four sub-genotypes of Fov4 have been identified, with Fov4 N, T, and MiT genotypes occurring in California, and Fov4 T and MT genotypes occurring in Texas. Unlike other genotypes of Fov in the United States, Fov4 does not require the presence of root-knot nematodes (Meloidogyne incognita) to cause severe wilt in cotton and is a major concern to US cotton growers. Fov4 can be spread through a variety of mechanisms including through infected seed. Once a field is infested, the fungus becomes endemic since there are no economically viable means to eradicate the pathogen from infested fields. Therefore, a rapid and accurate detection method is essential for early identification of infested fields and seed lots to prevent further spread of Fov4. This chapter describes multiplex and singleplex PCR diagnostics for detection of Fov4, and for detection and genotyping N, T, MiT, and MT genotypes of Fov4 from wilted cotton plants.

Acquisition and Transmission of Fusarium oxysporum f. sp. vasinfectum VCG 0114 (Race 4) by Stink Bugs

Fusarium oxysporum f. sp. vasinfectum VCG 0114 (race 4; i.e., FOV4) is an emerging pathogen that causes severe root rot and wilt of cotton. FOV4 is seed-borne, but the mode of seed invasion is uncertain. In an initial study, seeds in bolls that were puncture inoculated with FOV4 conidia when they were 25- or 30-days old became infected but remained viable. Because stink bugs can ingest and introduce bacterial and yeast pathogens into cotton bolls, we hypothesized that stink bugs may ingest and transmit FOV4. Southern green stink bugs and brown stink bugs were exposed to potato dextrose agar cultures of FOV4 and subsequently caged with cotton bolls to assess transmission potential. Both species fed on the cultures and acquired FOV4, and brown stink bugs transmitted FOV4 to cotton bolls. Thus, management of FOV4 may require management of stink bugs to mitigate the spread of the disease in 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.

Genetic Diversity and Pathogenicity of Verticillium dahliae Isolates and Their Co-occurrence with Fusarium oxysporum f. sp. vasinfectum Causing Cotton Wilt in Xinjiang, China

Cotton production in Xinjiang, the largest cotton-producing area in China, has an increasingly serious disease threat from Verticillium dahliae. Eighty-five V. dahliae isolates were obtained from wilted cotton plants collected from eight counties in Xinjiang. The isolates were assessed for genotypic diversity by DNA sequence analysis and PCR molecular genotyping with specific markers for race 1, race 2, defoliating (D) pathotype, nondefoliating (ND) pathotype, and mating type idiomorph Mat1-2. Isolates belonged to lineages 1A or 2B, with three subgenotypes found in each lineage. All isolates tested positive for race 2 and Mat1-2 markers. All isolates in lineage 2B tested positive for the ND pathotype marker but only isolates in the major subgenotype in lineage 1A tested positive for the D pathotype marker. Pathogenicity assays on Gossypium hirsutum 'Acala 44' demonstrated no significant difference among subgenotypes within each lineage. Isolates in lineage 1A caused greater shoot weight reductions, percent leaf drop, and percent diseased leaves than isolates in lineage 2B. One isolate in each lineage for 1A and 2B was avirulent. Isolates in lineage 1A caused greater than 50% leaf drop and a 17-g shoot weight reduction compared with a 9% leaf drop and a 6-g shoot weight reduction by isolates in lineage 2B. Overall, 42% of the V. dahliae isolates from Xinjiang were D pathotype but the percentage varied widely among locations. Two plants had both pathotypes. Nineteen isolates of Fusarium oxysporum f. sp. vasinfectum VCG0114 (race 4) also were recovered from wilted plants in Xinjiang. Two plants had both Verticillium wilt and Fusarium wilt pathogens. Both pathogens should be considered when using or developing wilt resistant or tolerant materials for Xinjiang.

Genetic Diversity and Population Structure of Races of Fusarium oxysporum Causing Cotton Wilt

To better understand the evolution of virulence we are interested in identifying the genetic basis of this trait in pathogenic fungi and in developing tools for the rapid characterization of variation in virulence among populations associated with epidemics. Fusarium oxysporum f. sp. vasinfectum (FOV) is a haploid fungus that causes devastating outbreaks of Fusarium wilt of cotton wherever it is grown. In the United States, six nominal races and eleven genotypes of FOV have been characterized based on the translation elongation factor (EF-1α) gene and intergenic spacer region (IGS), but it is unclear how race or genotype based on these regions relates to population structure or virulence. We used genotyping-by-sequencing to identify SNPs and determine genetic diversity and population structure among 86 diverse FOV isolates. Six individuals of Fusarium oxysporum closely related to FOV were genotyped and included in some analyses. Between 193 and 354 SNPs were identified and included in the analyses depending on the pipeline and filtering criteria used. Phylogenetic trees, minimum spanning networks (MSNs), principal components analysis (PCA), and discriminant analysis of principal components (DAPC) demonstrated that races and genotypes of FOV are generally not structured by EF-1α genotype, nor are they monophyletic groups with the exception of race 4 isolates, which are distinct. Furthermore, DAPC identified between 11 and 14 genetically distinct clusters of FOV, whereas only eight EF-1α genotypes were represented among isolates; suggesting that FOV, especially isolates within the widely distributed and common race 1 genotype, is more genetically diverse than currently recognized.

Detection and Characterization of Fusarium oxysporum f. sp. vasinfectum VCG0114 (Race 4) Isolates of Diverse Geographic Origins

A highly virulent cotton wilt pathogen, Fusarium oxysporum f. sp. vasinfectum VCG0114 (race 4) was found in West Texas in 2017, after being known in California since 2001. Isolates obtained from wilted plants collected in 2017 from Texas, in 2015 from China, and during 2001 to 2014 from California and isolates from historical collections including the race 4 reference isolate were characterized by soil-infestation pathogenicity assays, DNA sequence analysis, and vegetative compatibility analysis. All obtained F. oxysporum f. sp. vasinfectum isolates belonged to VCG0114. All of these isolates, except one isolate from China, caused disease in a soil-infestation assay without nematodes. Thus, they belong to the nematode-independent pathotype. Texas isolates were significantly more virulent than were isolates from China or California on Gossypium barbadense 'Pima S-7'. Four different genotypes (N, T, MT, and MiT) were identified based on the transposable element Tfo1 insertion into the PHO gene and independent MULE or MITE insertions into the Tfo1 transposon. Some significant differences in virulence were detected among the genotypes in some locations. No differences in pathogenicity were observed between the California and China collection isolates on Pima S-7, and the virulence of the major genotypes was similar on the Gossypium hirsutum cultivar 'Stoneville 474' or the Barbren 713 germplasm line. Simple polymerase chain reaction (PCR) methods were developed to specifically determine and detect the four genotypes within VCG0114. A specific PCR method to detect all VCG0114 isolates was also developed. These methods will facilitate the timely identification of infested fields and seed lots and the elucidation of evolutionary relationships among the isolates. This should help to closely monitor the movement of the pathogen and reduce dissemination of these devastating pathogens.

Detection and Characterization of Fusarium oxysporum f. sp. vasinfectum VCG0114 (Race 4) Isolates of Diverse Geographic Origins

A highly virulent cotton wilt pathogen, Fusarium oxysporum f. sp. vasinfectum VCG0114 (race 4) was found in West Texas in 2017, after being known in California since 2001. Isolates obtained from wilted plants collected in 2017 from Texas, in 2015 from China, and during 2001 to 2014 from California and isolates from historical collections including the race 4 reference isolate were characterized by soil-infestation pathogenicity assays, DNA sequence analysis, and vegetative compatibility analysis. All obtained F. oxysporum f. sp. vasinfectum isolates belonged to VCG0114. All of these isolates, except one isolate from China, caused disease in a soil-infestation assay without nematodes. Thus, they belong to the nematode-independent pathotype. Texas isolates were significantly more virulent than were isolates from China or California on Gossypium barbadense 'Pima S-7'. Four different genotypes (N, T, MT, and MiT) were identified based on the transposable element Tfo1 insertion into the PHO gene and independent MULE or MITE insertions into the Tfo1 transposon. Some significant differences in virulence were detected among the genotypes in some locations. No differences in pathogenicity were observed between the California and China collection isolates on Pima S-7, and the virulence of the major genotypes was similar on the Gossypium hirsutum cultivar 'Stoneville 474' or the Barbren 713 germplasm line. Simple polymerase chain reaction (PCR) methods were developed to specifically determine and detect the four genotypes within VCG0114. A specific PCR method to detect all VCG0114 isolates was also developed. These methods will facilitate the timely identification of infested fields and seed lots and the elucidation of evolutionary relationships among the isolates. This should help to closely monitor the movement of the pathogen and reduce dissemination of these devastating pathogens.

Fusarium wilt of cotton may commonly result from the interaction of Fusarium oxysporum f. sp. vasinfectum with Belonolaimus longicaudatus

The interaction between Fusarium oxysporum f. sp. vasinfectum (Fov) and Meloidogyne incognita (root-knot nematode) resulting in Fusarium wilt (FW) of cotton is well-known. Although Belonolaimus longicaudatus (sting nematode) can also interact with Fov and cause FW, it has long been believed that virtually all of the FW in Georgia is caused by the interaction of Fov with M. incognita. In recent years, FW has been reported more frequently in Georgia, which suggests that something affecting the disease complex may have changed. In 2015 and 2016, a survey of 27 Georgia cotton fields in 10 counties was conducted. At least 10 soil and stem samples per field were collected from individual plants showing symptoms of FW to quantify plant-parasitic nematode levels and identify Fov races. Fov race 1 was identified in all samples in 2015, but one sample also had the LA110 genotype and another sample also had the LA108 genotype. In 2016, all Fov races and genotypes found in 2015 were present, however, MDS-12 and LA127/140 also were found. Meloidogyne incognita was present in 18% of fields in 2015 and 40% in 2016, whereas B. longicaudatus was present in all fields in 2015 and 75% of fields in 2016. Regardless of whether they occurred separately or together, M. incognita and B. longicaudatus were present, respectively, in 18% and 55% of individual samples in 2015 and 40% and 51% in 2016. However, M. incognita without B. longicaudatus was found in 7% of samples in 2015 and 34% in 2016, whereas B. longicaudatus without M. incognita was found in 45% of samples in 2015 and 44% in 2016. We conclude that Fov race 1 continues to be the dominant race in Georgia and many instances of FW in Georgia may be due to Fov interacting with B. longicaudatus and not M. incognita as previously believed.

The interaction between Fusarium oxysporum f. sp. vasinfectum (Fov) and Meloidogyne incognita (root-knot nematode) resulting in Fusarium wilt (FW) of cotton is well-known. Although Belonolaimus longicaudatus (sting nematode) can also interact with Fov and cause FW, it has long been believed that virtually all of the FW in Georgia is caused by the interaction of Fov with M. incognita. In recent years, FW has been reported more frequently in Georgia, which suggests that something affecting the disease complex may have changed. In 2015 and 2016, a survey of 27 Georgia cotton fields in 10 counties was conducted. At least 10 soil and stem samples per field were collected from individual plants showing symptoms of FW to quantify plant-parasitic nematode levels and identify Fov races. Fov race 1 was identified in all samples in 2015, but one sample also had the LA110 genotype and another sample also had the LA108 genotype. In 2016, all Fov races and genotypes found in 2015 were present, however, MDS–12 and LA127/140 also were found. Meloidogyne incognita was present in 18% of fields in 2015 and 40% in 2016, whereas B. longicaudatus was present in all fields in 2015 and 75% of fields in 2016. Regardless of whether they occurred separately or together, M. incognita and B. longicaudatus were present, respectively, in 18% and 55% of individual samples in 2015 and 40% and 51% in 2016. However, M. incognita without B. longicaudatus was found in 7% of samples in 2015 and 34% in 2016, whereas B. longicaudatus without M. incognita was found in 45% of samples in 2015 and 44% in 2016. We conclude that Fov race 1 continues to be the dominant race in Georgia and many instances of FW in Georgia may be due to Fov interacting with B. longicaudatus and not M. incognita as previously believed.

Retention of Resistance to Fusarium oxysporum f. sp. niveum in Cucurbit Rootstocks Infected by Meloidogyne incognita

Interspecific hybrid squash (Cucurbita maxima × C. moschata 'Strong Tosa') and bottle gourd (Lagenaria siceraria 'Macis') rootstocks are resistant to Fusarium oxysporum f. sp. niveum but susceptible to Meloidogyne incognita (Southern root-knot nematode). Coinfection of Early Prolific Straightneck summer squash (C. pepo) with root-knot nematode and F. oxysporum f. sp. niveum has been reported to increase susceptibility to Fusarium wilt. The objectives of this study were to determine whether such an interaction occurred between M. incognita and F. oxysporum f. sp. niveum races 1 and 2 on Strong Tosa, Macis, and watermelon cultivars Fascination (resistant to race 1) and Tri-X 313 (susceptible to both races). Hosts were inoculated in a greenhouse with one of four pathogen treatments: F. oxysporum f. sp. niveum, M. incognita, both pathogens, or neither pathogen. Galling was present on ≥10% of the root systems of 90% of the plants inoculated with M. incognita. Bottle gourd had less galling than interspecific hybrid squash. Plants not inoculated with F. oxysporum f. sp. niveum did not wilt. Four weeks after inoculation, incidence and severity of Fusarium wilt and recovery of F. oxysporum did not differ for any hosts inoculated with F. oxysporum f. sp. niveum alone and F. oxysporum f. sp. niveum plus M. incognita (host-treatment interactions not significant). In general, Early Prolific Straightneck grouped with the F. oxysporum f. sp. niveum-resistant rootstocks when inoculated with F. oxysporum f. sp. niveum race 2 and with the susceptible watermelon when inoculated with race 1, regardless of inoculation with M. incognita. Recovery of F. oxysporum from stems of inoculated watermelon was greater than recovery from the other three hosts, regardless of nematode inoculation. In conclusion, our experiments do not support the hypothesis that resistance to F. oxysporum f. sp. niveum in cucurbit rootstocks or resistant watermelon cultivars would be compromised when M. incognita infects the roots.