Identification

• Japanese magnolia often develop a black fungal growth known as sooty mold. This fungal coating often feels like velvet and rubs easily from the plant, but sooty mold fungi soon returns to the infected tree. Sooty mold does not invade or attack plant tissues and is considered a cosmetic disease. However, some Japanese magnolias can experience reduced vigor when very heavy coverings of sooty mold are present because severe sooty mold fungi can block sunlight from the tree.
  
  

Causes

• The primary cause of sooty mold is insect infestations. Japanese magnolias are susceptible to infestations of magnolia scales, which feed on the leaves of the tree. Scale insects extract plant sap from host trees with their piercing mouths. After feeding on large amounts of plant sap, magnolia scales excrete honeydew, which is a sticky substance. This substance falls onto the leaves, stems and branches of infested trees and sooty mold fungi sticks to the honeydew.
  
  

Prevention

• Preventing insect infestations is the key to preventing sooty mold on your Japanese magnolias. Scale insects gravitate toward plants under stress from drought or the environment. Keeping your Japanese magnolia healthy and free from disease is important in preventing scale insect infestations. Avoid applications of nitrogen-rich fertilizers, as this may promote growth and encourage scale insects to attack the tree.
  
  

Control

• Sooty mold is a sign of heavy insect infestation in Japanese magnolia. Spraying your tree with a forceful stream of water may help reduce scale insects and other damaging pests. Spraying your tree with water twice each week can keep insect numbers low. Horticultural oils suffocate insects and kill them. Oil products also help loosen sooty mold fungi, making it easier to remove. However, if your tree has very heavy infestations of scale insects, horticultural oils may not be effective. Applying insecticidal soaps may also help eliminate insects on your Japanese magnolia. Some trees do not tolerate soap treatments, so test these products on a small area of your tree before applying it all over. If your tree has no adverse effects within seven days, it is safe to apply insecticidal soaps all over the tree. Your local garden center also has a wide range of insecticidal sprays that may help eliminate insects on your tree. Always follow the directions on the label for best results. Repeat applications as necessary for continued insect control.