Forsythia shrubs help usher spring with their early-blooming, bright yellow flowers. Commonly planted species include border forsythia (Forsythia x intermedia), which grows in that grow in U.S. Department of Agriculture plant hardiness zones 5 through 8, and early forsythia (Forsythia ovata), which grows in USDA zones 5 through 7. Forsythias rarely suffer from serious disease problems, but stressed or injured shrubs sometimes attract the phomopsis fungus.
Phomopsis fungi (Phomopsis spp.) cause forsythia stem gall, a disease that often affects weakened border and early forsythia varieties. The fungal pathogens invade wounded plant tissue and cause the infected twigs and stems to develop abnormal, knobby growths, called galls. Phomopsis galls range in size from as small as a green pea to as large as 2 inches in diameter. On forsythia plants, the galls often appear in clusters near the top of the shrub. These unsightly growths mar the forsythia's naturally beautiful appearance. The galls girdle twigs and stems, weakening and killing them.
Phomopsis fungal pathogens enter shrubs suffering from pruning injuries, bark abrasions or insect infestations. Once inside the plant tissue, these virulent pathogens quickly grow within the galls they create. Phomopsis fungi don't overwinter in the soil like other fungal species. Instead, they stay active inside the galls while waiting for warm spring weather and rains to arrive. Wet conditions prompt the fungal bodies to release spores that quickly spread to other plants through water droplets, pruning tools or insects.
No chemicals can treat this disease, as of June 2013. Prevent phomopsis fungi from infecting your plant by protecting it from pruning and mechanical injuries while giving it the right care. Don't place new forsythia shrubs in spots that have previously hosted phomopsis-infected plants. Inspect your shrub before the rainy season starts and remove as much of the infected tissue as possible. Prune off twigs and stems at least 3 inches below the gall to make sure you cut into healthy wood. Mix 1 part bleach to 10 parts water and dip the pruning tools into the solution between each cut. Sanitizing your tools helps prevent spreading the fungal pathogens. Promptly place pruning debris into plastic trash bags to limit fungal spore release. Consider removing severely infected forsythia shrubs.
Taking good care of your forsythia shrub helps keep it healthy enough to survive phomopsis fungal infections. These easy-to-grow shrubs need only a spot in full sun and fertile, well-drained soil. Give your shrub about 1 inch of water every week when it hasn't rained. Mature shrubs reach between 4 and 6 feet tall with similar spreads, so plant forsythias at least 6 feet away from walls, driveways and other structures to give the roots plenty of room to spread. Forsythias need annual pruning to stop them from becoming unruly. Wait until the shrubs finish flowering in spring before thinning out one-quarter to one-third of the oldest branches. This allows more sun and air circulation to reach the interior of the shrub while encouraging the growth of younger, healthier branches.