Although frangipanis are fairly hardy, there are some pests and diseases which can affect them, predominantly fungus, scale, frangipani rust and stem rot.
Fungus, Mold & Powdery Mildew
Leaves affected by fungus or mold can be sprayed with a copper based fungicide and white oil solution. If you prefer organic solutions, try a mixture of powdered milk powder and white oil or detergent.
Keeping plants well nourished helps prevent fungal infections. Potash is particularly good for improving disease resistance in frangipanis.
Leaves affected by hemispherical scale have dark to light brown bumps that are glossy, smooth and hemispherical. Leaves may have a black sooty coating.
Scale can be treated by spraying with white oil in spring to early summer. If you prefer organic solutions, try encouraging natural predators to your garden, such as ladybugs, the scale eating caterpillar, and parasitic wasps. Many plants attract ladybugs including daisies, zinnias, and zucchini.
There is a new disease attacking frangipanis in Australia called frangipani rust. It is most noticeable in late summer and early autumn. An orange to yellow powdery substance (actually pustules) appears on the underside of leaves. They rupture and spread spores which pass the disease to other plants nearby. The upper sides of the leaves are brown and discoloured. Severe infections may cause the leaves to drop prematurely and can lead to the death of small plants, however larger trees appear to suffer no ill effects (apart from leaf drop).
To control frangipani rust try using a fungicide (such as Mancozeb) in the warmer months to slow the development of the disease. Disposing of all fallen leaves in winter and spraying the tree and the area under the tree with a fungicide may slow the reappearance of frangipani rust next season.
The good news is that recently some frangipani trees have built up a resistance to rust, so it may be on it’s way out.
Stem Rot & Black Tip Dieback
As frangipanis lose their leaves over winter, soft, withered stems may become visible. It’s a condition called ‘stem rot’ and it’s quite common in trees that have been stressed by frosts, drought, lack of sunlight or just plain old age.
The best way to keep it under control is to simply prune off any diseased growth, but when you do, it’s important to make sure you cut it right back to good, healthy tissue.
Dying tip growth is commonly referred to as black tip dieback. Some newer deciduous cultivars and evergreen frangipanis are particularly prone to the disease.
Commercial frangipani growers suggest the problem is worse in areas where fruit-spotting bug and beetle activity is high. This is because any insect attack on the tip of the plant predisposes it to a secondary dieback infection.
Affected plants typically reshoot beneath the damaged portion of stem. If plants appear unsightly or you are concerned that the rot is advancing down the stem, use sharp pruners to cut back to clean tissue. Be sure to use hot water or household disinfectant to clean pruners between cuts so as to minimise potential disease transfer.
Badly affected plants may benefit from an application of fungicide to limit the disease’s spread.