It's just a symptom & Not the Cause

Black sooty mould is starting to develop on this magnolia.

Premature flower drop is common on infected plants.

Camellia’s exquisite flowers are revered for their beauty but disappoint when their beauty is diminished when their flowers drop prematurely before they open. Instead of their attractive glossy deep green leathery foliage, they are covered with a black substance that resembles soot. It is a common affliction of many plants, especially broadleaf evergreens including lily-of-the- valley shrub (Pieris japonica), holly, azaleas and rhododendrons. 

Sticky honeydew is evident on this tulip tree leaf (Liriodendron) as well as some aphids.

Aphids, and most other insects, are found on leaf undersides.

Scale insects on a lily-of-the-valley shrub also secrete honeydew.

That black stuff is mould, specifically black sooty mould. It doesn’t do any damage unless it’s severe as the soot blocks out the sunlight. Surprisingly, the mould is not the real problem, it’s a symptom of a more serious issue – sucking insect pests, which are usually aphids or scale insects.

Black sooty mould and accompanying scale insect have devastated this camellia.

Holly covered with black sooty mould.

Black sooty mold on a lily-of-the-valley shrub.

These vampire-like insects, as the name implies, feast on plants by sucking the sap out of the leaves, stems and buds. As they feed, they excrete a sticky substance called honeydew. Ants love the stuff and are often seen on plants that are infested with sucking insect pests.

Don't forget to water camellias, rhododendron and other broadleaf evergreens during the summer.

A 3-inch deep layer of an organic mulch keeps plants healthy.

Prevention: Most broadleaf evergreens, including camellias thrive in an organic rich soil, that doesn’t dry out. They benefit greatly from a 3 inch layer of organic mulch. Water in the summer from the trunk all the way past the plant’s canopy. To ensure you are watering correctly, dig down with a trowel or shovel to see how far down the water has penetrated. ​

Use hose end sprayers to spray plants.

Horticultural oil is an effective organic pest control that kills insects and helps wash off the mould.

Control: Honeydew is also a vector for the unsightly black sooty mould. It is easily washed off with soap and water, horticultural oil or neem oil. With repeated application every 7 days (until the issue is resolved), it will control the insects.  A hose end sprayer, available at garden centres and hardware stores, is an efficient way to really soak the plant inside and out. Get under the leaves, along stems and in nooks and crannies because that’s where the bugs live.