Treating Diseases that Harm Your Weigela

While healthy weigela shrubs are resistant to most diseases, there are a few that can have severe negative effects on their appearance. These include twig blight and leaf spot, along with crown gall and others. Learn more below about how to recognize, treat and prevent outbreaks of disease in your flowering weigela.

Twig Blight

For the fungus that causes twig blight, look for branch tips that go brown long before early frost, and die off slowly. Eventually the entire plant will be affected, so try to catch twig blight in its earliest stages. Twig blight most often attacks young weigela with less than 5 years of garden growth.

  • Treat twig blight by spraying the affected areas with either a copper or lime sulfur-based fungicide formula when you first spot the symptoms.
  • Use it again every week to 10 days during periods of rain and wind. While treating for twig blight, water the weigela only at the roots.
  • Avoid watering the leaves and canes.
  • Prune out twigs and canes that have died back due to twig blight and destroy them, then discard with trash. This pruning will improve air and light around the roots, helping prevent further outbreaks.
  • Rake up and dispose of winter mulch early in the spring to stop mold spores from introducing the fungus to the shrub.

Leaf Spot

Leaf spot disease is also due to a fungus, spread by mold spores. It is highly preventable by ensuring effective drainage of the soil under your weigela, and pruning to thin out the shrubs. This will give the roots and canes more access to the oxygen and nitrogen in the air. If the leaves of your weigela turn pale yellow or dull brown, and become covered with black swelling spots, your plant has leaf spot.

  • Use an anti-transpirant leaf spray on the shrubs in early spring to prevent leaf spot.
  • Clear dead leaves and canes out every year in the late summer so the fungus has no suitable medium to grow in.
  • Add a high-nitrogen fertilizer to the soil to encourage healthy leaf growth.
  • When pruning away dead and diseased canes, remember to wipe off your shears between each cut with a clean cloth and either rubbing alcohol or 3 percent hydrogen peroxide. Note: ammonia and chlorine cleaners will damage steel tools.

Crown Galls

These are a form of plant cancer. Lumpy growths appear at the crown of infected weigela, where the stems protrude from the ground at the roots, and are malignant. They will spread to kill the entire plant. The bacterium that causes crown galls can be carried through soil, water, and improperly cleaned tools.

  • Crown gall bacteria enter plants through pruning wounds, so take great care when you prune your weigela to use sharp tools, and make the smallest possible cuts.
  • Treat weigela with mild crown gall infection by cutting back all the diseased canes, and spread a solution of powder antibiotic, such as Agrimycin, around the roots and crowns.
  • Dig up and destroy severely infected plants, to avoid spreading this malignant infection.