Tips on how to use this section:
The symptoms of the most common problems are in the left column of the chart. In the right column are the probably causes of those symptoms. For those problems for which there is detailed information in a different part of Yardener’s Helper, the name of the problem is linked to a detailed file. For those problems for which there are no additional files, the discussion is found in the paragraphs below the chart.
|Problems of Forsythia|
|Shrub Branches Root in Soil||Natural Habit|
|Upper surfaces of lower leaves will be stippled with small yellow dots or red spots. The leaves, their stalks, and adjacent stems may be distorted or swathed in fine webbing. Check for mites on the lowest leaves of forsythias.||Mites|
|This problem (scale) appear as bumps on forsythia twigs, usually on main stems or in the crotches where leaf stems join them. As scale suck on cell juices, leaves and stems look pale and limp. They secrete a sticky honeydew which fosters the growth of an unattractive black, sooty mold on leaves.||Scale|
|Weevils feed on the edges of forsythia leaves during the day. They sometimes devour the whole leaf except for the midribs and large veins. Weevil grubs (larvae) in the soil often attack the roots, endangering the shrub.||Weevils|
|A forsythia whose roots are infested with nematodes looks sickly, wilted or stunted. Its foliage is yellowed or bronzed. It will decline slowly and die. Upon inspection the root system appears poorly developed, and even partially decayed. Roots may have knots or galls on them. Evidence of nematode activity may be most apparent in hot weather, when the shrub foliage continues to droop after the heat of the day has passed.||Root Knot Nematodes|
|Tumor-like Swellings on Trunk or Stems||Phomopsis Gall|
|Flowers and Twigs Shrivel||Twig Blight|
|Sunken Spots; Brown Spots on Leaves||Fungal Diseases|
|Base of Stems; Roots Gnawed||Rodent Injury|
Shrub Branches Rooting in Soil Is Natural for Forsythias.
The tips of forsythia's arching branches that touch the soil often take root. Either prune branch tips to prevent this, or clip off the rooted sections from the branches and encourage them to develop as new shrubs that can be transplanted elsewhere on the property.
Tumor-like Swellings on Trunk, Stems are Phomopsis Gall.
A fungus infects forsythia shrubs through wounds in the tender bark on its stems and stimulates cells to form tumor-like swellings (galls) with irregular rough surfaces there. To prevent this disease, do not plant any forsythia shrub that has suspicious visible galls on the stems. Destroy any heavily infected plants that you may have already. If only a few galls are present, cut off and destroy the affected stems. Sterilize pruning tools afterward in a solution of household bleach and hot water to avoid spreading the disease. Clean up the summer mulch after the forsythia drops its leaves and replace it with fresh for the winter.
Flowers and Twigs Shrivel Due to Twig Blight.
A dieback disease caused by a fungus occasionally attacks forsythias. The black fruiting bodies of this fungus develop either on the surface or inside the infected twigs. They first invade blossoms and flower stalks, then the twigs, killing them. Prune and destroy all dead twigs and stems. Improve air circulation around the shrub by pruning back nearby shrubs and taking out old stems from the forsythia.
For more information see the file on Controlling Fungal Disease.
Sunken Spots, Brown Spots on Leaves from Fungal Diseases.
Leaf spot diseases caused by various fungi sometimes attack forsythia. They are distinguished by yellow, brown or black dead blotches on the leaves. Often these spots come together to form larger patches of dead tissue. Anthracnose makes moist, sunken spots with fungal fruiting bodies in the center. These leaf spots may run together, resembling a blotch or blight. The dead areas border the veins or are between larger veins. Sometimes the ends of new forsythia shoots die back several inches. Pustules containing pinkish spores appear. Dieback and defoliation may occur in severe cases.
Most fungal infections are not life threatening to mature shrubs, just unattractive. Gather and destroy fallen leaves and prune away any diseased branches, even if it means cutting the entire shrub back. Thin old stems to improve air circulation. Maintain plant vigor by feeding and watering as recommended. To protect healthy leaves from infection, spray them with a garden sulfur fungicide according to label directions. Lay fresh mulch for winter. If anthracnose is a chronic problem dig up and destroy the forsythia and replace it with a different kind of shrub that is resistant to this disease. Disinfect all tools in a solution of hot water and household bleach.
For more information see the file on Controlling Fungal Diseasease.
Base of Stems, Roots Gnawed Due to Rodent Injury.
Small rodents sometimes nest in winter mulch near forsythia stems and gnaw at plant tissues. This injury reduces plant vigor and permits disease organisms to invade the shrub. Do not pile mulch against stems. Wait until the ground freezes before spreading winter mulch so that mice, voles and other rodents are forced to find other nesting areas.
For more information see file on Dealing With Mice and Dealing With Voles.