Knock-out of theVdPHB1 gene in V. dahliae decreases its pathogenicity. (A) Disease symptoms in cotton plants at 7, 14 and 21  DPI with Vd991 and Vdphb1 mutants. The pathogen inoculation was performed using the root-dipping method. The experiment was repeated three times. (B) DI of cotton plants at 21  DPI with Vd991 and Vdphb1 mutants. The DI was evaluated with three replicates of 36 plants. (C) Vascular discoloration of cotton stems at 21  DPI with Vd991 and Vdphb1 mutants. Upper, transverse section; lower, longitudinal section. (D) Relative transcript levels of immune-related genes in cotton at 21  DPI with Vd991 and Vdphb1 mutants. Total RNAs were extracted from cotton roots after infected with Vd991 and Vdphb1 mutant. Gene expression levels were normalized using the cotton ubiquitin gene as the control according to the 2^−ΔΔCt method.