Solutions to problems gardeners may find with plumeria trees

Plumeria, Hawaiian name melia, is the most commonly used flower for lei in Hawaii, although it isn’t native to Hawaii.

It’s native to Central and South America. At this time of year, it’s blooming all over Maui, filling the air with its wonderful fragrance. This is also the time of year when gardeners begin to notice some of the problems found on plumeria.

Plumeria performs best with a soil pH of 6.0 to 6.5. They should be grown in an area of full sun and good drainage. While they can withstand some light salt spray, they shouldn’t be planted in areas where they will be exposed to wind or medium to heavy salt spray.

They do well in dry areas below 2,000 feet, but to encourage healthy growth and good flower set, plan on irrigating them during the driest part of the year. They should be fertilized every three to four months with a complete fertilizer. General recommendations call for 10-30-10, but to be more precise, plan on getting a soil test.

The easiest time of year to prune plumeria is in the winter after the leaf drop, although heavy pruning can cause a significant reduction in spring flowers. Pruning plumeria can cause a tree to “bleed.” The tree will form a callus where it is cut, so it’s best to refrain from using a product to seal wounds. The sap from plumeria can irritate skin and eyes.

One problem we are seeing quite a bit of this year is plumeria rust. Plumeria rust symptoms and signs include orange powdery pustules on the underside of the leaves that are easily rubbed off. You may or may not see the pustules on the top surface of the leaves, but you will most likely see yellow spots.

Eventually the tree may experience 100 percent leaf drop. This fungal disease does not affect the flowers. The disease is spread through wind-borne spores during wet or humid conditions. The pathogen can survive on the infected leaves as well as debris from the plant. Commonly found Plumeria rubra, a deciduous type, and P. obtusa, the evergreen type, are susceptible to this disease. The lesser-known species, such as Plumeria caracasana, are highly resistant. While there are some fungicides registered to treat this disease, the most important control is sanitation, which includes removing and destroying falling leaves. Do not compost them.

Spiraling whitefly is another common problem of plumeria. In fact, spiraling whiteflies attack a variety of plants, but it’s believed that plumeria is the favorite host of spiraling whiteflies. Incidence of this pest is highest during warm, dry weather.

Spiraling whiteflies can cause premature leaf drop as well as spread viral diseases. One sign of spiraling whiteflies is sooty mold, a black mold that grows on the honeydew produced by the whiteflies.

Honeydew is a sticky substance produced when whiteflies feed on leaves.

The mold then grows on the honeydew. While the sooty mold does not infect the leaves, it can interfere with photosynthesis. Controls for whiteflies include washing the undersides of the leaves with sharp streams of water, foliar applications of insecticidal soaps and oils, removal of heavily infested leaves, thinning the canopy, and managing weeds in the area. Ants must be managed as well because they feed on honeydew. They will protect the whiteflies from natural enemies and will carry them from one plant to another. There are a number of natural enemies of whiteflies that are found on Maui, including a tiny parasitic wasp, and some species of ladybeetles. Avoid spraying pesticides, especially broad-spectrum pesticides, because they can kill these natural enemies.

A long-horned beetle, plumeria stem borer, is a pest that damages the insides of plumeria stems.

Symptoms include shriveled stems and a small hole, dripping black material. The only recommended treatment for the borer is to remove the infected stems. Pesticides are ineffective against this pest.

Plants under stress, such as drought stress, are more susceptible to this pest than plants receiving proper maintenance.

* Lorraine Brooks is an urban horticulture extension agent and the Maui Master Gardener coordinator with the University of Hawaii’s Cooperative Extension Service, part of the College of Tropical Agriculture and Human Resources. Her email is Gardening In Maui is published when there’s a fifth Sunday of the month.