Galls form on stems and portions of the stem distal to the gall eventually die.
The cause of this malady is unknown, but it is suspected to be a gall-forming fungus. No control recommendation, other than pruning out galled stems, can be made at this time. Insect Problems Plant bugs. The fourlined plant bug , Poecilocapsus lineatus, and tarnished plant bug, Lygus lineolaris , lay eggs in soft stems. These hatch about the middle of May and the young bugs suck the sap from the tender leaves of forsythia and many other plants.