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.

Locally severe outbreaks of Fusarium wilt of cotton (Gossypium spp.) in South Georgia raised concerns about the genotypes of the causal pathogen, Fusarium oxysporum f. sp. vasinfectum. Vegetative complementation tests and DNA sequence analysis were used to determine genetic diversity among 492 F. oxysporum f. sp. vasinfectum isolates obtained from 107 wilted plants collected from seven fields in five counties. Eight vegetative complementation groups (VCG) were found, with VCG 01117B and VCG 01121 occurring in 66% of the infected plants. The newly recognized VCG 01121 was the major VCG in Berrien County, the center of the outbreaks. All eight VCG resulted in significant increases in the percentages of wilted leaves (27 to 53%) and significant reductions in leaf weight (40 to 67%) and shoot weight (33 to 60%) after being stem punctured into Gossypium hirsutum ‘Rowden’. They caused little or no significant reductions in shoot weight and height or increases in foliar symptoms and vascular browning in a soil-infestation assay. Soil infestation with Meloidogyne incognita race 3 (root-knot nematode) alone also failed to cause significant disease. When coinoculated with M. incognita race 3, all VCG caused moderate to severe wilt. Therefore, the VCG identified in this study belong to the vascular-competent pathotype, and should pose similar threats to cotton cultivars in the presence of the root-knot nematode. Use of nematode-resistant cultivars, therefore, is probably the best approach to control the disease in Georgia.

Cotton (Gossypium hirsutum L.) germplasm line BARBREN-713 (Reg. No. GP-987, PI 671965) was developed and released by the USDA-ARS, Mississippi Agricultural and Forestry Experiment Station, Texas A&M AgriLife Research, and Cotton Incorporated in 2012. The objective of the release was to provide public and private breeders with an agronomically desirable germplasm that is resistant to both the reniform nematode (Rotylenchulus reniformis Linford and Oliveira) and the root-knot nematode [Meloidogyne incognita (Kofoid and White) Chitwood]. The line also has excellent seedling vigor in ields infested with nematodes and fungal root rot pathogens, such as Thielaviopsis basicola, Rhizoctonia solani and Fusarium spp. Resistance to reniform nematode was transferred from G. barbadense GB713 (PI 608139) and is associated primarily with the Ren 2 GB713 gene on chromosome 21. Resistance to root-knot nematode was transferred from the germplasm line M-315 RNR (PI 592514), 'LA 887', or 'Acala Nem-X' and is associated primarily with the Mi-1 gene on chromosome 11. The codominant simple sequence repeat markers, BNL 3279_105 and CIR 316_202, respectively, were closely linked to the resistance genes. A single nucleotide polymorphism marker, Gl-187401, also was developed to detect the Ren 2 GB713 gene. In controlled environment assays, BARBREN-713 suppressed reproduction of both nematodes by 90% or more. It also reduced reniform nematode populations in the ield at eight locations in four states. The line has iber quality similar to M-315 RNR but yielded more than M-315 RNR in reniform nematode-infested ields. Abbreviations: QTL, quantitative trait locus; SNP, single nucleotide polymorphism.

Cotton (Gossypium hirsutum L.) germplasm lines LONREN-1 (Reg. No. GP-977, PI 669509) and LONREN-2 (Reg. No. GP-978, PI 669510) were developed and released by the USDA-ARS, Texas Agricultural Experiment Station and Cotton Incorporated in 2007 to provide breeders with desirable germplasm resistant to the reniform nematode (Rotylenchulus reniformis Linford and Oliveira). The resistance was transferred from wild G. longicalyx via a triple-species hybrid. Crosses, backcrosses, and single plant selections were used to develop the F 2 progeny used for seed production. Resistance was followed with the codominant simple sequence repeat BNL 3279_114 marker, located 1.4 cM proximal and the phenotypic marker 'greenfuzz' (LTCOL_F) located 4.5 cM distal to the Ren lon resistance gene on chromosome 11. A single nucleotide polymorphism marker also was developed for rapid screening of large numbers of seed. The introgressed chromosome segment is smaller in LONREN-2 than in LONREN-1. Both lines reduced reniform populations by 95% in growth chamber bioassays and by 50 to 90% in ields. In the absence of nematodes in the ield, growth rate, yield, and iber quality of the lines were similar to that of the 'Fibermax 958' parent. In the presence of reniform nematodes in the ield, the lines often showed stunting and yield losses, probably due to enhanced severity of fungal seedling diseases, especially Thielaviopsis root rot. The seedling diseases in LONRENs were diminished by control measures such as fungicides, nematicides, and crop rotation with corn or sorghum, and were negligible in a second year of planting in the same ield.