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Watch out for sap-sucking insects

This week I answer a reader's question: Q. Our camellia is developing a black powdery substance on its leaves. This rubs off but is even discolouring the side of the house and the neighbouring rhododendron.

This week I answer a reader's question: Q. Our camellia is developing a black powdery substance on its leaves. This rubs off but is even discolouring the side of the house and the neighbouring rhododendron. The plant doesn't seem to be suffering too much, but it's really become unsightly. Any suggestions? A. Sooty Mold is very common with camellias. The two most likely causes are aphids or scale insects. All these are sap-suckers that excrete a sweet substance that coats the leaves and attracts ants that eat this honeydew. Fungi are also attracted and that's what gives the honeydew its sooty colour.

You'll need to check whether ants are crawling up the trunk of your camellia and your rhododendron. If they are, it's important to stop them. As long as the ants can get up there, your problems will continue.

Garden centres sell insect-trappping substances such as Tanglefoot. These sticky products should be smeared on a plastic or cloth band that can be fastened around the trunks of both shrubs. It should be put on so it can be removed and discarded when necessary.

All sap-sucking insects seek out shrubs with lots of tender green growth. High-nitrogen fertilizers encourage this. Balanced, slow-release fertilizers or compost is a better choice.

Checking for eggs on the underside of the leaves is also helpful in case you have scale insects, not aphids. Or you could have both.

If sunlight and air circulation are blocked, sooty mold is more likely to attack. Sometimes the camellia has very dense growth, or perhaps surrounding trees are hogging the light.

Anyway, it helps to prune off the worst infected branches. Spraying will penetrate better and so will air and sunlight. If the bush is quite small, you could pick off the most badly infected leaves.

Blasts of water can dislodge many of the aphids. For generally cleaning the bush somewhat, a tablespoon of Neem oil in a gallon of water is useful. To cut off the next generation of scale, try dormant oil sprays. These can smother scale eggs.

Anne Marrison is happy to answer garden questions. Send them to amarrison@shaw.ca. It helps if you add the name of your city.

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