Ends of Branches Die Back means Frost
Although Weigelas are generally hardy in areas where winter temperatures may dip as low as 0° F, the tender tips of their branches suffer frostbite at these temperatures and die back. This does not permanently harm the shrub. In the spring, improve the appearance of the shrub by clipping off the dead branch tips prior to bloom time.
Branches, Foliage Encrusted With Small Bumps due to Scale
Scale insects feed on plant tissues while protected by rounded, waxy shells. These shells, which may be white, yellow or brown to black, are only about 1/25 to 1/50 inch in diameter. They appear as bumps or blister-like outgrowths on weigela stems and leaves. The first sign of a scale attack is often discoloration of upper leaf surfaces, followed by leaf drop, reduced growth, and stunted shrubs. Heavy infestations may kill shrubs. Some species secrete honeydew, which coats foliage and encourages ants and sooty molds. Scale outbreaks can be triggered by pesticides used against other pests or by environmental stresses such as too much or too little water. Over fertilizing may encourage lush growth susceptible to scale attack. Avoid this by using a slow-release nitrogen fertilizer.
Handle mild scale infestations by simply scraping the telltale bumps off plant surfaces with a fingernail, or a cotton swab dipped in rubbing alcohol. Heavy infestations require spraying. Spray shrub foliage and stems with light, or "superior" horticultural oil. The oil coats plant surfaces, smothering the scale insects and their eggs.
For more information see file on Controlling Scale
Irregular Tan Spots On Upper Leaves indicates Four-lined Plant Bug
Adult four-lined plant bugs are yellowish-green with wing covers that sport four black stripes, hence their name. In their nymph stage they are orangish or reddish in color and have black dots and eventually yellow stripes as well on their wing pads. These insects feed on the leaves near the top of a weigela shrub, causing them to develop irregular bronze or tan spots. Affected leaves may eventually turn brown and die. For best results spray this insect in the nymph stage while it is feeding with encapsulated pyrethrum every 3 to 5 days until there are no more visible insects.
Cottony Masses on Plant Parts caused by Mealybugs
Mealybugs are 1/5 to 1/3 inch long, with oval, flattened bodies. They are covered with white waxy powder and adorned with short, soft spines around their edges. These insects sometimes gather in cottony white masses on weigela stems, branches and leaves, sucking sap and reducing the shrub's vigor. Infested weigela leaves look yellowish. Severely infested plants are unsightly, and they do not grow well. Close inspection may reveal cottony masses and/or busy ants on stems and leaves. The ants are attracted by the honeydew secretions from the insects' feeding sometimes encourages mold growth on the foliage. Spray shrub foliage with neem insecticide 2 or 3 times at 10-day intervals. Spray bark surfaces with heavy (Volck) horticultural oil in late winter while the shrub is dormant to kill over wintering eggs. For more information see the file on Controlling Mealy Bugs
Leaves Discolored, Growth Stunted because of Root Nematodes
Root nematodes are whitish, translucent, wormlike creatures less than 1/2 inches long. They burrow into weigela roots, their feeding gradually destroying the root system. This stunts the growth and drains the vigor of the shrub and its foliage turns reddish yellow in distress. Infested roots soon die, but the shrub struggles to form more roots above the invaded areas. Eventually they, too, become infested. The result is a stunted, knotted root system that is hard to water. Nematode attacks are most obvious in hot weather when the limp shrub foliage appears to recover poorly from the heat.
Apply "ClandoSan" a commercial nematode control that is effective and safe. Sprinkle this granular product on the soil around the affected shrub out as far as the branches spread (the drip line). Scratch it gently into the top inch or two of soil, taking care not to damage the small feeder roots close to the soil surface. Read the product label for amount to use. Adding compost or leaf mold, if it is available, to the soil around the shrub also controls nematodes by encouraging beneficial fungi in the soil. Drench the soil with liquid fish emulsion, which feeds the plants but is toxic or repellent to nematodes.
Swollen Tumor-like Growths On Stems means Crown Galls
Weigelas that develop swollen growths of various forms and sizes at their crowns (where the roots join the stems at the soil surface) or on their roots and stems have crown galls. Crown galls are abnormal tumor-like growths caused by a bacterium. Unlike any other plant galls, the growths formed by this bacterium are malignant. This is a form of plant cancer. The disease occurs worldwide. Gall bacteria are introduced into shrubs that are vulnerable due to wounds. Contaminated tools, soil, water, carry them and splashing rain. Heavily infected shrubs must be dug out and discarded in such a way as to prevent the spread of the infection. Prune out and destroy the infected canes, twigs, and stems of shrubs that show only minor infection and dip the rest in a solution of a wettable powder antibiotic such as Agri-Strep or Agrimycin. Carefully inspect new plants and nursery stock and reject any that show suspicious lumps in stems, in crowns near the old soil line, or on adjacent roots. Avoid wounding crowns and roots of established shrubs during cultivation. Disinfect garden tools in a solution of hot water and household bleach. Do not plant susceptible plants in infected soil for 5 years.
Branch Tips Turn Brown And Die Back due to Twig Blight
A blight disease caused by a fungus causes weigela branch tips to turn brown then die back until the entire branch or even the entire shrub is killed. This is different from the brown tips caused by frost that; once pruned off, do not spread. Shrubs over 5 years old are usually not seriously affected. Spray affected shrubs with copper fungicide or lime sulfur fungicide when symptoms first appear and then every 10 days in wet seasons. Avoid overhead watering that wets the foliage. Prune to increase air circulation around shrubs, taking care to sterilize pruning tools by dipping them in household bleach to prevent the disease from spreading. Because the fungus spores collect on the mulch beneath the shrubs, removing the old mulch and replacing it with fresh material may help prevent an outbreak from recurring.
If twig blight is a common problem every year, prune and destroy affected plant parts in the early spring.
For more information see file on Controlling Fungal Disease
Leaves Turn Yellow, Drop Prematurely indicates Leaf Spot
Leaf spot diseases caused by fungi occasionally attack weigela. Leaves of affected shrubs turn straw yellow or brown and are thickly dotted with small black fruiting bodies. These diseases mostly attack foliage already weakened by some other cause. Control leaf spot by shaking out all fallen and diseased leaves from the center of the bush and destroying them, and by pruning out and destroying dead branches in the center of the bush to encourage better air circulation. Spray foliage early in the spring before growth starts with anti-transpirant spray. This coating on the leaves discourages fungi. Enrich the soil around the shrub and ensure good drainage. Mulching helps prevent the disease from splashing up from the ground and infecting plants.
For more information see file on Controlling Fungal Disease
Leaves Turn Yellow and Wilt caused by Root Rot
Soil-dwelling fungi cause root and stem rot of weigela. Typically, shrub stems are attacked at or near the soil level. Foliage turns yellow, wilts, and dies. Usually root systems rot, causing plants to topple over. Remove and discard the infected shrub, or cut away affected plant parts with a clean, sharp knife or razor blade. Disinfect tools after use with household disinfectant or dip them in a solution of household bleach and water. Keep the garden clear of old plant debris and keep mulch away from stem bases. For long-term prevention, lighten heavy soil with a mixture of perilite, vermiculite or peat moss and provide good drainage. Avoid over watering. Space plants further apart to prevent crowding.