Rosemary Dripping "Sap"
Answered by: Conrad Richter
Question from: Cheryle Molberg
Posted on: August 14, 2003

My rosemary plant is approximately 5 years old and has always been indoors. It is approximately 24" tall and a bit leggy but continues to grow and has even given me a few blue blossoms this year. In the past few months the plant has started to throw off sticky, sap-like liquid that can cover a good part of the table on which the plant sits. Can you tell me please if I am not keeping the plant sufficiently moist or if I have a pest problem?

It sounds like you have a pest problem, an infestation of either mealybugs or scale. Scale insects are the more likely possibility because their hard brown shells blend in very well with the brown bark of rosemary and are hard to detect unless you know what to look for. Mealybugs are white, cottony insects that are much easier to see on the soft stems and on the undersides of the leaves especially. Even someone unfamiliar with these insects would notice a heavy infestation of mealybugs.

Heavy infestations of mealybugs and scale can cause the "sap" drip that you have noticed. Both of these pests exude a sticky liquid that can build up to the point of dripping. This same "sap" can turn black as fungi invade; so typically the stems and some leaves may begin to look like they are covered in soot.

Both mealybugs and scale are very difficult to eradicate, particularly in large plants. There are chemical controls such as malathion that work well with repeated application.

Organic control requires persistence: you need apply organic controls repeatedly every 5 days until the pest problem is gone. One procedure is to dab the pests with rubbing alcohol with a Q-tip followed by insecticidal soap spray. Or, you can use a solution of 5% rubbing alcohol in water and spray the plant thoroughly until the spray is dripping off the leaves, making sure that the undersides of the leaves are covered. Leave on for 30 minutes. Then spray with water only to wash off the alcohol-water mix and follow that with sprayings of insecticidal soap. Or, instead of spraying the alcohol-water mix and the soap, you can dip the whole plant in a pail filled with these solutions; dipping ensures a complete coverage of all leaf and stem surfaces.

Sometimes the infestations are so bad, and the subject plant too large, that it is nearly impossible to get the upper hand on the pests. In these cases the only practical solution is to discard the plant and start with a new one. We once had to do that with some large rosemaries and bay trees we used as display plants for shows – it was sad to see those old friends go!

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