Genetic Diversity, Virulence, andMeloidogyne incognitaInteractions ofFusarium oxysporumIsolates Causing Cotton Wilt in Georgia

Abstract

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.

Genetic Diversity and Population Structure of Races ofFusarium oxysporumCausing Cotton Wilt

“…Twenty distinct VCG's have been identified in FOV (Fernandez et al 1994;Davis et al 1996;Bell et al 2017), but our study identified a maximum of twelve genetically distinct groups of FOV. One possible explanation for this is that some VCG's are rare -for example, between 1994 and 2017 only one isolate belonging to the VCG known as 0111 had been identified (Fernandez et al 1994;Bell et al 2017) -and therefore were not represented among the isolates analyzed in our study. Additionally, it is possible that some FOV isolates differ at vic alleles which govern vegetative compatibility but are otherwise genetically similar, resulting in individuals of different VCGs being placed in the same DAPC group in our study.…”

“…Because individuals in the same VCG must possess several identical alleles, vegetative compatibility is thought to be a good indication of isolates' relatedness (Puhalla 1985). Additionally, Bell et al (2017) reported that individuals in the same VCG typically possess the same disease phenotype on cotton, suggesting that VCG is an ideal way to characterize FOV isolates as it is indicative of both genetic relationship and disease phenotype. Twenty distinct VCG's have been identified in FOV (Fernandez et al 1994;Davis et al 1996;Bell et al 2017), but our study identified a maximum of twelve genetically distinct groups of FOV.…”

“…Additionally, Bell et al (2017) reported that individuals in the same VCG typically possess the same disease phenotype on cotton, suggesting that VCG is an ideal way to characterize FOV isolates as it is indicative of both genetic relationship and disease phenotype. Twenty distinct VCG's have been identified in FOV (Fernandez et al 1994;Davis et al 1996;Bell et al 2017), but our study identified a maximum of twelve genetically distinct groups of FOV. One possible explanation for this is that some VCG's are rare -for example, between 1994 and 2017 only one isolate belonging to the VCG known as 0111 had been identified (Fernandez et al 1994;Bell et al 2017) -and therefore were not represented among the isolates analyzed in our study.…”

“…FOV has been studied for over a century, but there are still knowledge gaps in the pathogen’s genetic diversity ( Atkinson 1892 ). Eight pathogenic races of FOV were reported between 1958 and 1985, and although these races are widely regarded as invalid ( Armstrong and Armstrong 1958 ; Armstrong and Armstrong 1960 ; Ibrahim 1966 ; Armstrong and Armstrong 1978 ; Chen et al 1985 ; Davis et al 2006 ; Bell et al 2017 ), they are widely used in the cotton industry for characterizing FOV. In plant pathology, a race refers to a pathogen’s ability to cause disease on its host and implies a gene-for-gene interaction between host and pathogen ( Anderson et al 2010 ).…”

“…The races of FOV, however, were characterized based on isolates’ virulence not only on a differential set of cotton cultivars, but also on many non-host plants including tobacco, soybean, and okra ( Armstrong and Armstrong 1960 ; Armstrong and Armstrong 1978 ; Chen et al 1985 ). Additionally, many of the cotton cultivars that were originally used to differentiate races of FOV have not been maintained, and differential virulence reactions are not reproducible on modern cotton cultivars ( Davis et al 2006 ; Holmes et al 2009 ; Bell et al 2017 ;). Furthermore, multigene genealogies, pathogenicity tests, and other bioassays demonstrated that races 3 and 5, as well as 4 and 7, are genetically and phenotypically indistinguishable and should therefore be recognized as single groups ( Nirenberg et al 1994 ; Hering et al 1999 ; Skovgaard et al 2001 ).…”