Distribution and Pathogenicity Differentiation of Physiological Races of Verticillium dahliae from Cotton Stems in Western China

Verticillium wilt, caused by the pathogenic fungus Verticillium dahliae, has emerged as a severe threat to cotton globally. However, little is known about the genetic diversity of this pathogen in an infected single cotton plant. In this study, we isolated three new V. dahliae strains from the disease stems of Gossypium hirsutum from the cotton field in Western China and assessed their pathogenicity to the cotton cultivar Xinnongmian-1 and its two transgenic lines, as well as two laboratory strains, VD592 and VD991. These three new V. dahliae strains were identified using DNA barcodes of tryptophan synthase (TS), actin (ACT), elongation factor 1-α (EF), and glyceraldehyde-3-phosphate dehydrogenase (GPD). Moreover, the haplotype analysis revealed that the three new races had distinct haplotypes at the TS locus. Furthermore, the results of culture features and genetic diversity of ISSR (inter-simple sequence repeat) revealed that there were separate V. dahliae strains, which were strong defoliating pathotypes belonging to race 2 type, as determined by particular DNA marker recognition. The identified strains demonstrated varied levels of pathogenicity by leaf disc and entire plant inoculation methods. Conservatively, these strains showed some pathogenicity on cotton lines, but were less pathogenic than the reference strains. The findings revealed that several strong defoliating V. dahliae pathotypes coexist on the same cotton plant. It indicats the importance of regular monitoring as an early warning system, as well as the detection and reporting of virulent pathogen strains and their effects on crop response.