Mary asks: Some of the leaves on my plumeria tree have yellow spots on them and kind of orange bumps on the undersides. Eventually, these leaves turn totally yellow and drop off the tree. What is the problem and how might I get rid of it?
Tropical Gardener answer: It sounds like your plumeria is infected with the fungal pathogen, Coleosporium plumeriae, which causes the disease plumeria rust. The pathogen is host specific. Though it can and will spread to other plumeria, it will not infect other plant species. It was first identified on Oahu in 1991 and has since spread to all of the Hawaiian islands.
To get rid of the rust, start by inspecting your plant thoroughly and removing all the infected leaves, but not more than a third of them per week. Dispose of the leaves off your property. Do not compost them. Rust spores are easily spread. Spray the remaining leaves with a wettable sulfur mix to prevent rust attacking them. Wettable sulfur powder is available at most gardening stores and you should follow the directions on the package for mixing with water. If lots of the remaining leaves are infected, you might want to spray them with the broad spectrum bio-fungicide Serenade (also widely available). It contains a patented strain of Bacillus subtilis that controls fungal diseases and is approved for organic gardening.
Here are several recipes for low-toxic products that you can make yourself. One involves mixing water, baking soda, soap and neem oil. Mix 1 tablespoon of baking soda in a quart of water. Add 2 teaspoons of neem oil and about 1/4 teaspoon of Dr. Bronners peppermint soap to the water. Shake it to combine and spray all infected parts of the plant, especially the underside of the leaves. Of course, you can increase the recipe for larger plants. Spray every 7-10 days until you no longer see rust on the leaves.
Similar to other plant disease or pest issues, prevention is the best cure. This pathogen prefers warm moist environments. During our rainy season, conditions are ideal for rust to appear and spread, but you can take some preventive measures. Clean up all fallen leaves. The spores on infected leaves can be spread on the wind or by water.
Water in the early morning allowing time for moisture to dry out during the day. Be sure that all irrigation or watering goes into the soil not on the leaves. Damp leaves are especially vulnerable to disease.
Soft, leafy, new growth is also a prime target, especially when it results from a heavy dose of nitrogen fertilizer. Use a slow release product with a balanced N-P-K to provide good nutrition and control new growth. A thick layer of mulch or organic compost on the soil surrounding your plumeria can improve soil and plant health and serve to prevent rust spores from splashing onto the leaves.
Keep weeds down around your plumeria and allow space between plants to keep humidity levels low. Though you probably don’t currently have a resistant plumeria variety, several do exist. Lots of information on the disease and a list of resistant varieties can found on a UH free publication titled “Plumeria Rust” which you can find at https://www.ctahr.hawaii.edu/oc/freepubs/pdf/pd-61.pdf.
Email plant questions to konamg@ctahr.hawaii.edu for answers by Certified Master Gardeners. Some questions will be chosen for inclusion in this column.
Diana Duff is a plant adviser, educator and consultant living in a dryland forest north of Kailua-Kona.
Gardening Events
Tuesday: “Adding Commercial Yeast to Coffee Fermentation,” 4-5:30 p.m. at UH Cooperative Extension Service office in Kainaliu across from the Aloha Theatre with Kevin McHale of Scott Labs. Demonstration of process using commercial yeast. The preferred yeast product will be available at the event. Sponsored by Kona Coffee Farmer’s Association. Info: Visit “events” at www.konacoffeefarmers.org.
Saturday: “Work Day at Amy Greenwell Garden,” 9 a.m.-12:30 p.m. Meet at the Garden Visitor Center across from the Manago Hotel in Captain Cook. Volunteers will be able to help with garden maintenance and are invited to bring a brown bag lunch. Water and snacks provided. Info: Call Peter at 323-3318.
“Basic Grant Writing Workshop,” 9 a.m.-4 p.m. at UH-Hilo UCB331 with Jeani Navarro, who has more than 30 years of grant-writing experience. $85 registration fee. A 50 percent discount is available through the Employment &Training Fund. Info: Call 932-7830 or visit https://hilo.hawaii.edu/ccecs/
Farmer Direct Markets
Wednesday: “Sunset Farmers Market,” 2-6 p.m. in the HPM parking lot, 74-5511 Luhia St., in Kailua-Kona
Wednesday and Friday: “Hooulu Farmers Market,” 9 a.m.-2 p.m. at Sheraton Kona Resort &Spa at Keauhou Bay
Friday: “Pure Kona Market,” 9 a.m.-5 p.m. at Amy Greenwell Garden in Captain Cook
Saturday: “Keauhou Farmers Market,” 8 a.m.-noon at Keauhou Shopping Center
“Kamuela Farmer’s Market,” 7 a.m.-noon at Pukalani Stables
Sunday: “Pure Kona Green Market,” 9 a.m.-2 p.m. at Amy Greenwell Garden in Captain Cook
Tuesday-Saturday: “U-Pick greens and produce,” 10 a.m.-4 p.m. at Tropical Edibles Nursery in Captain Cook
Plant Advice Lines
Anytime: konamg@ctahr.hawaii.edu
Tuesdays, Thursdays: 9 a.m.-noon at UH-CES in Kainaliu at 322-4892
Mondays, Tuesdays, Fridays: 9 a.m.-noon at UH-CES in Hilo at 981-5199 or himga@hawaii.edu