Types

• Two fungal diseases cause black mildew growth on holly: sooty mold and downy mildew. Sooty mold is a non-parasitic fungal infection that grows on honeydew deposits on plant foliage. Sap-sucking insects that feed on holly foliage excrete honeydew during their feeding process. While the disease is non-parasitic, it can cause problems with foliage growth and plant vigor. Downy mildew is a parasitic fungal disease, caused by the Plasmopara fungi, that affects the foliage of the plant.
  
  

Symptoms/Identification

• Sooty mold infections produce black fungal growth or mold that has a spot-like appearance on foliage. Sooty mold differs from other leaf spot infections because the fungus is easily removed when rubbing the leaf. Heavy infections block sunlight, which causes poor foliage growth, plant wilt and reduced vigor. Symptoms of downy mildew infections consist of stunted leaf growth, black mildew growth, leaf yellowing and premature defoliation. Fungal buildup has a fuzzy or feather-like appearance, which distinguishes it from other fungal infections.
  
  

Favorable Conditions

• Sooty mold requires sap-sucking insect infestations for sooty mold to grow. Common insect pests that produce sooty mold consist of scale insects, aphids, whiteflies, mealybugs, leafhoppers and psyllids. Downy mildew fungi favor temperatures between 50 and 75 degrees Fahrenheit, humid weather conditions and water on leaf surfaces for infection. The fungus can spread through water splashing from infected to non-infected leaf surfaces.
  
  

Management/Control

• Control honeydew-producing insects to prevent and control sooty mold growth on holly. Chemical insecticides can help reduce insect populations to control sooty mold buildup, while a strong water spray can dislodge the mold growth on the foliage. However, use insecticides sparingly because they can kill beneficial insects that eat honeydew-producing insects, thus increasing infection rates. Avoid overhead irrigation and free moisture on leaf surfaces to prevent downy mildew infections. Water during the morning hours and provide good air circulation with proper spacing and pruning to prevent infections. Chemical fungicides are effective for controlling downy mildew infections.