The fungus Verticillium dahliae is the fungus most responsible for verticillium wilt in barberry bushes, according to plant pathologist Cynthia Ash. The fungus is a soil-borne organism and spreads systemically throughout the plant, killing it. Symptoms, which appear most frequently in summer, include curled foliage that turns yellow, brown-tipped or edged leaves, and dead branches. Barberries with advanced infections cannot be saved and should be removed. Those with less severe infections can be treated and, hopefully, saved.
Soil fumigants, such as Vapam, are somewhat effective in the control of V. dahliae. Vapam may also control certain nematodes that have a symbiotic relationship with the fungus and help it thrive. Vapam is not sold to consumers and can only be applied by a certified technician. According to agents at Cornell Cooperative Extension, fumigants may help, but there is no guarantee. Call your county cooperative extension office for a referral to a fumigant professional.
Proper and adequate fertilization of the barberry bush will help it fight verticillium that is in the soil. Barberries with early symptoms should be fertilized with a 10-10-10 fertilizer, at the rate suggested on the package. Potassium, the third number in the fertilizer formula, is particularly important to help the barberry fight an infection. Apply the fertilizer in the spring after all danger of frost has passed and then do not fertilize again for the rest of the year.
Pruning or Removing
Cut off any barberry stems and branches killed by verticillium wilt and those that are dying. Branches with recent symptoms should be left on the barberry as, according to botanists at Michigan State University Extension, they may recover the following season. Disinfect your pruning tool between each cut by dipping it into a solution of 1/2 cup of Lysol disinfectant and 2 1/2 cups of water. Do not use bleach to disinfect your tools; it causes rust. Destroy all infected barberry wood and fallen leaves. If you must remove an entire bush, take as much of the rootball as possible and remove and destroy any mulch or other materials from the soil around the removed tree.