White jagged protrusions on plant stems?

Question:these killed my weigelia plant

Answers:
Are you in an area with the 17 year cicada? The female slices the horizontal branches under 1" in diameter & then deposits her eggs. These look like white protrusions and, with large numbers of deposits, they can kill the branches.

Weigela are not prone to a lot of diseases. It is possible that the white protrusions are the fruiting bodies of a fungal attack. The question is, is this a primary infection (that caused the death of your weigela) or a secondary infection that feeds on dead wood? Bacterial crown gall causes the formation of roundish, warty-looking growths on the lower stems. There is no chemical control. Remove and destroy infected plants.

Another possible explaination is 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