Tropical gardening helpline: Taking care of plumeria trees

  • The orange pustules on the underside of plumeria leaves is a symptom of the fungal disease plumeria rust. (plumeria101.com/Courtesy Photo)
  • Plumeria rust causes light colored spots on the surface of the leaves. (gardenguyhawaii.com/Courtesy Photo)
  • Plumeria leaves get light colored spots on the surface and have orange spots on the underside when infected by the plumeria rust fungus. (hiveminer.com/Courtesy Photo)

Larry asks: The leaves on my plumeria trees do not look healthy. Some are turning yellow, others have brown spots and many are falling off the tree. I also see orange spots on the underside of the leaves. What’s up and what can I do?

Tropical Garden Answer: Plumeria trees are somewhat drought tolerant and will suffer if their roots are constantly wet or if the area around them is moist. Plumeria trees with “wet feet” are especially vulnerable to root rots or other fungal diseases.

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The orange spots on the underside of the leaves is indicative of the presence of the plumeria rust pathogen Coleosporium plumeriae. This is a fungal disease and is hard to get rid of once it starts. The best cure is usually prevention but using a few different treatments will likely help.

The symptoms of the disease are light-colored spots on the upper surface of the leaves and orange pustules of the rust spores on the undersides of the leaves. In severe cases, the leaves will yellow and may turn brown and drop off.

Rust spores are airborne and can spread from one plumeria to another on the wind, from water or rain backsplash. Once the spores come in contact with moist leaves, they stick and begin to grow and multiply.

To start dealing with this disease you can remove undergrowth around your trees and thin out some branches on the tree to improve air circulation and reduce moisture in and around the tree. If you only have a few infested leaves, remove them. Do not remove more than one-third of the tree’s leaves, however, or it will go into additional stress.

Remove any fallen leaves from around the tree as well. The disease can be spread from infected leaves on the ground. Plumeria rust only affects the plant’s leaves, not the stems or flowers. It is also specific to plumeria plants and will not spread to other species.

This disease can be prevented and/or controlled by planting disease resistant varieties, removing weeds and dead leaves and by applying fungicides when necessary. Research has shown that both Plumeria stenopetala and Plumeria caracasana are resistant to rust.

Several homemade fungicides can be effective against rust. An easy one is made by mixing 1 tablespoon each of baking soda, light vegetable oil and mild liquid soap in a gallon of water. Using neem oil and safer soap can increase the insecticidal properties of this formula if you want a multipurpose product. Spray the mixture in early morning or late afternoon so that it contacts the rust on the undersides of the leaves.

Fungicides like this can be used to spray all parts of the plants as well as the soil around them. Several organic commercial fungicides are also available and should be used according to the directions on the product. Both Serenade and Dr. Earth Final Stop Disease and Fungicide Control are usually effective and generally available. Fungicides should be applied about twice a month until the pathogen is no longer spreading and your plant looks better. To avoid pathogen resistance, it is best to use different types of fungicides over the season.

For more information and photos of rust fungus and infected leaves go to the UH CTAHR free publication at https://www.ctahr.hawaii.edu/oc/freepubs/pdf/pd-61.pdf.

Diana Duff is a plant adviser, educator and consultant living in a dryland forest north of Kailua-Kona.

Gardening Events

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Farmer Direct Markets

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“Sunset Farmers Market” 2-6 p.m. in the HPM parking lot at 74-5511 Luhia St. in Kailua-Kona (across from Target)

Saturday: “Keauhou Farmers Market” 8 a.m. to noon at Keauhou Shopping Center

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“Waimea Homestead Farmers Market” from 7 a.m. to noon at the Waimea Middle and Elementary School Playground

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Plant Advice Lines

Anytime: konamg@ctahr.hawaii.edu

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