Kay,

Thanks for your question and sorry to hear your lilacs have this problem.

You do want to remove any infected shrubs.  Controlled burning would be the most effective - you would want to consult your town's fire department for guidelines.  If you have a transfer station that accepts garden waste that would also be an option.  Short of those, you just want to move the plant debris as far away as you can on your property from the lilac area.  The pathogen causing the disease does have other host plants (eg: elms) so there is no way to completely rid it from your yard, unfortunately.  You do want to be sure you disinfect any tools you have used on the diseased plants before using them elsewhere (and especially on any new lilacs.)

The organism is spread by insects with piercing-sucking mouthparts, such as leafhoppers.  Controlling weeds, which can provide habitat for the insects can reduce the likelihood of future problems.  If you only have a few shrubs you can monitor for the insects and spray insecticidal soap if you see a significant number but be aware that this is not a selective treatment and other insects will also be killed.  

When you replant (and you can replant into the existing area), there are some more resistant varieties you can choose.  Common lilac (Syringa vulgaris) and hybrids of common lilac and Syringa relexa or Syringa villosa are more tolerant of the disease.  Providing the plants with optimal conditions to maintain overall health can also help (appropriate fertilization - consider a soil test; right amount of watering; good location; etc).

Kay

I did some focused research on the causative agent of lilac witches broom and it appears this specific phytoplasma mostly (exclusively?) causes a condition in ash trees called ash yellows.  I have not found any reference to the lilac phytoplasma being an issue in many other shrubs, although there are mentions of 'aster yellows' that may be caused by a similar phytoplasma in asters and same/other variants that can be a problem in fruit trees, dogwoods, willows, locusts, tomatoes, clovers, etc.  Also, as the development of 'witches broom' is a general plant response to an injury or infection, it can have different causes (eg: fungi) in different plant families.

This may be more than you want to know, but the explanation for this lack of clarity may be due to the difficulty in culturing the phytoplasma bacteria.  Here is a good explanation I found:

"Phytoplasmas are single-celled organisms descended from bacteria, but they have lost the cell wall. They are considered mollicutes (“soft skin”), and able to change shape in response to their environment. Phytoplasmas are obligate parasites that only live in the phloem cells of plants or in their insect vectors. They cannot be grown in pure culture (many bacteria can be grown in culture). Due to the inability to grow them without a host, phytoplasmas are not given Latin names (genus and species). Instead, they are referred to as ‘Candidatus Phytoplasma’.

Phytoplasmas are transmitted by leafhoppers, planthoppers, or psyllids; they reproduce in their vectors. Over 1000 plant species are known to be infected by phytoplasmas, including: apples, cherries, potatoes, tomatoes, strawberries, and clovers. Phytoplasma diseases were assumed to be caused by viruses until the late 1960’s, when newer detection capabilities were developed.

Symptoms of phytoplasma infections include two main types: yellows and witches’ brooms. Yellows symptoms are due to phloem dysfunction and changes to resource allocation. Plants may have a general unhealthiness, foliage may appear chlorotic (yellow), wilting may occur, stunting of plants, dieback, and necrosis (death) of phloem may occur."

I narrow my searches to ".edu" sites only so I can be sure they are based on scientific research; even here many of the sources had some differences in species susceptibility so perhaps there just needs to be more focused experimentation on this curious disease.

Wishing you good luck with your new plantings!