Symptoms

• If the leaves and branches on a lemon tree are speckled with charcoal-colored marks, this is symptomatic of a fungus called sooty mold. A type of mold that affects both indoor and outdoor lemon trees, sooty mold may look like an ash tray was emptied over the plant.
  
  

Causes

• Pests such as aphids, mealy bugs, whiteflies and scale are the major culprits behind sooty mold. These insects secrete a sticky substance called honeydew, which attract the airborne spores of sooty mold. To determine if the bug causing the sooty mold on the lemon tree is a whitefly, shake the leaves of the lemon tree. If white-colored bugs fly out from under the leaves, the culprit is the whitefly. The other culprits can be seen on the top of the leaves. Aphids can be various colors, including yellow, green, black, brown or red, and are round and tiny in shape. Female mealybugs are covered in a cotton-like wax, and male mealybugs resemble gnats. Whitish and covered in a waxy substance, the insects known as scale are tiny enough that a magnifying glass may be needed to see them.
  
  

Treatment

• The only way to eradicate the problem of sooty mold is to kill off the bugs secreting honeydew. One method is to wipe the leaves with a wash cloth and 70 percent isopropyl alcohol, and then rinsing the leaves off afterward. This will remove the mold, but requires additional pesticide treatment, such as a vegetable and fruit pesticide, to kill the insects once the mold is removed. The second method is to remove the mold and kill the bugs simultaneously by spraying the tree with horticultural oil. Once the oil has been sprayed on the affected area, wait 30 minutes and then rinse the tree with a garden hose. Horticultural oil is non-toxic and can be purchased at a garden center.
  
  

Prevention

• To prevent the mold from spreading to the other plants in the vicinity, quarantine the lemon tree and inspect the other plants for similar signs of sooty mold. Whitefly traps, which are made of bright yellow tape and attract the flies, also can be hung from the branches to prevent future sooty mold attacks.
  
  

Warnings

• Citrus trees being treated for sooty mold should never be sprayed with pesticide or horticultural oil in the middle of the day. Spray in the early morning, late afternoon or at night time. If the weather is cloudy or overcast, the tree can be sprayed at any time of day. Additionally, fungicide will not treat sooty mold, so it is not advised for use.