Plumeria Rust is one of the most common, and most damaging problems faced by plumeria growers around the world. The strain of rust fungus is specific to Plumeria trees, and while it’s easy to treat, can be devastating if spotted too late.
Plumeria, or Frangipani, are grown for their fragrant flowers and bring a touch of the Hawaiian islands to your garden or indoor space. Like all house plants we can all be guilty, from time to time, of forgetting about their needs. These tropical plants need good air circulation, clear well-drained soil, and regular clean water in sunny spots ideally outdoors, but will cope with well-ventilated indoor spaces.
Those conditions are a struggle to achieve for many growers, particularly those of us with a varied plant collection, all requiring slightly different conditions. In this article, we’ll cover the main causes of plumeria rust fungus, how to treat plumeria rust, and, how to prevent it.
About Plumeria Rust Fungus
Plumeria rust fungus is, as the name suggests, a fungal infection that only affects plumeria. Botany and horticulture are obviously closely related, but with Plumeria, there was a historic anomaly, where the botanists who discovered Plumeria, seem to have failed to communicate with the horticulturalists that bred it from cultivation.
Plumeria was discovered growing wild in the late 1600s by Charles Plumier (botanists have never been very creative at naming plants), and it took until 1860 for a German horticulturist called William Hillebrand to properly develop a cultivar for commercial propagation. In 1902, just forty two years later, the first case of Plumeria rust was noted, and its spread was incredibly fast in the wild, to the point it spread to cultivated varieties within a few years. Over a century later, the fungal disease is more common in cultivation than in nature due to our preference for growing plumeria indoors.
The disease is caused by Coleosporium dominguense and Coleosporium plumeriae, which are spread through fungal spores carried by insects, wind, water, and our own hands. The fungus is killed easily by tropical summer temperatures, so is rarely fatal to plants in the wild. In our homes and gardens though, we tread a fine line between too warm, and too humid, which can accelerate the fungus reproduction. So let me share some of my own tips, as well as some common ways of preventing and treating this frustrating illness.
How to Spot Plumeria Rust Fungus
First, you need to be sure it’s plumeria rust fungus. Many foliate fungal infections in plumeria have similar visual symptoms to root rot, and drought, which can cause leaf discoloration. Fungal infections are more common in weak plants, with poor aeration, so inspecting the soil moisture levels using a moisture meter when you suspect any fungal infection should always be your first check.
Plumeria rust fungus is notable for its reddish-brown powdery pustules, which are made up of clusters of microscopic spores. Infected plants will have pustule clusters over the top of the leaf, usually with yellow-orange spots across the underside of the leaf appearing first.
Diseased leaves on severely infected plants will turn brown after a while as the fungus prevents photosynthesis entirely when it takes over. Fallen leaves should be removed stringently when you have any suspicion of rust fungus, which even in mild outbreaks can spread to nearby healthy plants to crawling insects, and promote root rot in addition to topical fungus. As new spores are produced, they will spread sporadically and can affect every part of the plant – though it is more common on leaves.
What Causes Plumeria Rust Fungus?
Plumeria rust fungus is caused by Coleosporium dominguense and Coleosporium plumeriae. These two funguses are specific to Plumeria and reproduce more easily in humid, wet climates, where the temperatures are lower.
Some strains, like Plumeria Rubra and Plumeria obtusa are particularly susceptible to the disease, and the further North you are, the more likely you are to suffer from its effects. Remember that Plumeria are tropical plants, so need to be given ideal temperatures to thrive.
Due to higher rainfall in Europe and most of North America in particular, where Plumeria are grown most commonly, you absolutely need to create ventilation through proper pruning. Poor air circulation during the growing season, and during winter, will lead to rust on old leaves. The new growth is usually less of a useful host for the fungus but should be protected by removing old and fallen leaves as soon as the fungus is apparent.
How to Treat Plumeria Rust Fungus
Treating Plumeria rust fungus is the same as treating any other fungus. We grow our plumeria in the greenhouse, which is gently heated over winter to prevent other fungal diseases. The biggest advantage of this is that our plumeria trees are grown alongside other plants that are susceptible to fungal infection, so we know that everything in that space needs pretty much the same preventative treatment all year round.
The best thing you can do to protect plumeria, and any other tropical plant – whether it’s annual or perennial, is to improve aeration, by pruning any excess growth. Plumeria flowers best on new growth, so pruning branches older than 3-4years is a great way to increase air circulation, just as you would with new trusses on tomatoes.
When you have rust fungus though, and prevention is no longer an option, you have to start thinking about exactly what is going to get rid of it. Fungicides are the obvious choice, and they are certainly an important part of your tool kit, but in addition to fungicides, regularly and carefully remove fallen leaves, affected leaves and if the fungus has spread to the stem, cut back to at least two inches below any signs of infection. Immediately place infected leaves into trash bags
Mild outbreaks need fungal treatments like neem oil as soon as possible to prevent the spread as well as remove affected growth. While neem oil and other fungicides will kill the fungal spores, they can stay productive in the soil, and on any parts of the plant you might have missed.
Make sure to clean your tools and hands thoroughly before touching any other plumeria plants nearby, as the spores spread from clothes, wind, and insects so it’s important to minimize the risk to other plants.
How to Prevent Plumeria Rust Fungus
Spores produced all year round can occur if the conditions are right, but are most effective during the spring flush of growth that your plumeria tree puts on as soon as the days start to lengthen. While new leaves are less susceptible to the illness, they are not immune, so if you live in a cold climate, it’s really important to take precautions against splashing rain in the early growing season.
Wet leaves are more likely to develop fungal infections, and the yellow-orange spots on the underside of leaves will quickly begin to spread if they are subjected to cold and wet conditions.
To prevent this from happening, the obvious solution is to keep your frangipani undercover either indoors or in a greenhouse where you can completely control the watering and make sure the leaves never get any water directly to them at all. For plants grown outdoors, the best way to counteract the rain is to ensure clear space beneath your plumeria and removing tall weeds in spring, and regularly weeding around the base of the pant all through the year.
Failing all that perhaps consider something drastic, and invest in frangipani that is better suited to your environment. Plumeria Caracasana is a hybrid plumeria, developed for greater disease resistance, and higher cold tolerance. The result is an astonishingly disease-resistant plant, with a far stronger fragrance in colder climates.
Best Fungicides for Plumeria Rust Fungus
Fungicides can be preventative as well as useful for treating ongoing infections, but you need to get the right one. I’ll make no attempt to hide my personal preference for neem oil, which is an incredible organic fungicide, as well as an insecticide, but it does require careful use as it can be toxic to beneficial insects and pollinators if used outdoors.
I’ve dug out a few products from my own garage below, which we use for a variety of treatments. all are brilliant options for plumeria rust fungus treatment, but they each have limitations that should be adhered to:
Personally, I haven’t found this to be particularly effective as a cure for Plumeria Rust, but it is an effective preventative treatment if applied as a foliar spray in warm weather. The citric acid gives reasonable protection against rust and other funguses, so it’s a useful addition to your cabinet of cures.
The biggest benefit of Grower’s Aly fungicide is that because it’s a general fungicide it’s gentle to your plants using natural ingredients that won’t harm wildlife, humans, or pets, and if you need to use it on crops, it’s safe to eat too.
Until recently, I was against copper fungicides as they seemed like an excessive way to control a simple problem, but copper is actually an incredibly effective way to stop funguses in their tracks – plumeria rust included.
Because its active ingredient is copper it also helps (in a very small way) to keep slugs away from leaves while they recover from illness or infection. Even a small amount of sprayed copper during a fungal infection will dissuade the pests from eating your plants.
Bonide’s Copper Fungicide is also none harmful to insects, but avoid spraying directly onto them as they might not die but they certainly won’t thank you.
Trifecta’s pesticide and fungal treatment is a really great fungicide, which is a little odd, as it’s marketed primarily as an insecticide. I find it completely useless as an insecticide if I’m completely honest, but it’s useful for discouraging insects and pests, rather than killing them so is a much better choice for a truly organic garden.
Trifecta’s blend is made up of thyme oil, peppermint rosemary, clove, garlic, corn, and geraniol oil, as well as citric acid. The smell is effective at discouraging pests, but it’s the speed at which it dries out plumeria fungus which is most impressive.
As well as being an effective plant fungicide it’s a= also a useful treatment for fleas, mites, and ticks on livestock, which causes the animals no harm. For most plumeria growers, it’s unlikely to be a consideration, but it’s a useful tip for any agricultural growers!
Before the springtime flush of growth, you can sprinkle it onto the soil, but during the growing season, this powdered fungicide can be mixed in water to use as a direct topical spray for infected foliage.
Neem oil is an essential store cupboard supply for any gardener. It’s a completely organic topical treatment for pests and diseases, which dries out areas and acts and a surfactant, starving funguses and insects or oxygen while raising pH to a point they can’t survive.
Because of its natural properties, it should only ever be used indoors and away from the risk of beneficial insects that can be killed by any wet neem oil left on the plant. On dry it is completely harmless but remains as a deterrent which covers the leaves with a distasteful substance to put of insects.
Plumeria Rust FAQs
How do you get rid of rust on Plumeria?
The only way to get rid of rust on plumeria is to remove the affected leaves and dispose of them. Once all infected materials have been removed, spray the entire plant with an organic fungicide, and make sure the soil is not too wet. Move the plant to a bright sunny position with good ventilation while it recovers, and consider leaving it there long term.
What causes rust on plumeria leaves?
Plumeria leaves develop rust due to over-humidifying, and poor ventilation. The fungus is caused by two strains of fungus specific to plumeria, called Coleosporium dominguense and Coleosporium plumeriae. To prevent it, you can use sulfur-based fungicides as a soil additive that improve your plant’s resilience.
What kills rust on frangipani?
Neem, sulfur, and copper sprays are the most effective fungicides for direct application on plumeria fungus. They work quickly and effectively to kill funguses before they develop, but should be used in conjunction with a program of disease removal (cutting off any infected growth, as even with fungicides there is a chance it can spread to other leaves before they are treated.
Final Thoughts on Plumeria Rust Fungus
If you are an avid plumeria collector, it’s more than likely you will encounter plumeria rust in your lifetime. It is an unavoidable part of plumeria care but isn’t fatal as long as you find it in time.
If left untreated it can kill the entire plant, but by removing affected leaves and treating them with appropriate fungicides you can keep your plumeria plants happy and healthy for years to come.
As always, we recommend prevention rather than cure, so make sure you understand how to prune plumeria, as well as the right light conditions and watering requirements, which will work together to stop plumeria rust fungus in its tracks.