The fungal growths on the lilac's trunk are referred to as Turkey Tails, Trametes versicolor, a common invader --    aka, a naturally-occurring decomposer -- of weakened woody plants and deadwood. Unfortunately, it may also invade nearby healthy wood.

Lilacs normally grow as multi-stem shrubs. Thus, the preferred pruning practice with lilac is to remove the oldest stems at ground level every year, while retaining 3 to 5 healthy stems.

The reason why only certain stems are attacked is explained as ". . . they were old. Then, after being stressed by drought, the trees were drenched by rain. And finally, their location on the shady north side . . . provided the ideal spot where this fungus thrives best." (To read the rest of this article, click here.)

Because it appears that all the stems are affected, consider replacing this lilac with a new one. September is a good time to plant new woodies as the fall and winter rains can ease irrigation require for a sturdy root system to develop.