QuestionA well established (15 yrs) rosemary plant is dying off rapidly & I've discovered the branches, especially low down have a thick encrustation of a brown growth. No insects visible. When the branch has died the grwth turns grey & somewhat powdery. I have cut out as any affected branches as possible but am notsure if I can seve the shrub. I am in San Bernardino, Southern California.
AnswerI would severely prune this plant back about 50% of its size and see if the new growth is not healthy. There are few effective fungicides for use on Rosemary if it were a fungal disease. The plant are usually resistant to diseases, so it may be old age at this point.
