Verticillium wilt is caused by a soil fungus that is commonly found in most soils. Most plants are relatively resistant to the fungus and may never show symptoms while growing in infested soil. Certain species of woody plants like Japanese barberry are susceptible. If your barberry is only showing mild symptoms, it may continue to survive in the soil for many years with the correct care. A barberry plant that looks like it is dying should be promptly replaced with a more wilt-resistant variety.
Remove any parts of the plant that are dead or dying. Make your cut at least 1 inch into healthy plant tissue or back to the branch's point of origin. Do not prune any plant tissue that is merely wilting. This plant tissue may survive with the right cultural care. After you prune, remove any plant debris from the base of the plant and discard it.
Fertilize your barberry bush with a 10-10-10 plant fertilizer in the spring after all threat of frost has passed. Fertilized barberries are better able to resist verticillium fungi. Follow the manufacturer's instruction for the appropriate amount of fertilizer for your barberry's size, and apply the fertilizer only once annually. Don't be tempted to overfertilize, which exacerbates the symptoms of verticillium wilt.
Mature, established barberry bushes may be able to withstand drought, but they do not thrive in it. Drought-stressed barberry bushes are more susceptible to fungal invasion. Water established barberries whenever the top few inches of the soil dries out. Keep the soil around freshly planted barberries moist until the plant establishes itself and produces new growth.