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Ask Mr. Smarty Plants is a free service provided by the staff and volunteers at the Lady Bird Johnson Wildflower Center.

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Monday - February 25, 2013

From: Plum, TX
Region: Southwest
Topic: Non-Natives, Diseases and Disorders
Title: Black Sooty Mold on Bay Tree
Answered by: Anne Van Nest

QUESTION:

I have a large bay tree and all the leaves are covered with a black mold-like substance on the top part of the leaf. Under each leaf are some black/brown spots. I have washed the leaves with soap and water using a wash cloth. This cleans the leaves some but this stuff is stubborn. I really do not want to hand wash 1000+ leaves. I have tried fungus sprays, mold spray, and high pressure washing with just water. Nothing seems to help. All the new growth is getting the same gunk. Help!

ANSWER:

Sorry, bay laurel trees (Laurus nobilis) are a little out of our line since they are native to the Mediterranean. The Lady Bird Johnson Wildflower Center, home of Mr. Smarty Plants, is committed to the growth, protection and propagation of plants native not only to North America but to the area in which they are being grown. In Texas, this is Texas A&M. Below though are links to several websites which we believe have the information you are looking for. Look at the Texas A&M AgriLIFE Extension web page on bay laurels.

Your plant symptoms though are similar to what sooty mold does when it attacks some native plants. The sooty mold grows on the honeydew that is excreted by small soft-bodied aphids or scales that suck sap from plants. In your case there is a laurel aphid that attacks bay laurel trees and excretes a sticky, sugary substance that drips down onto the tops of lower leaves. Soon thereafter, a black, mold grows on the sugar. The sooty mold blocks the light to the leaf and weakens the plant.

Look for small pale green insects (aphids) on the new growth or small round bumps (scale) to find the ultimate culprit of the problem.  Once you have determined the honeydew cause (and solved this problem), you will be able to get your mold under control. For more information look at this ehow article on bay laurel tree problems.

 

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