Tropical gardening helpline: How to treat citrus scab

  • Fruit infected with citrus scab is unsightly but still edible. (ctahr.hawaii.edu/Courtesy Photo)
  • Citrus scab pustules can spread and deform citrus fruit if they appear when the fruit is still young. (USDAidtools.org/Courtesy Photo)

Robin asks: Some of the fruit on my tangelo tree have scabby looking growths. What might be causing this? Is the fruit ruined? Should I remove them or is there a treatment?

Tropical Garden Answer: Your description sounds like the symptoms of a fungal disease called citrus scab. It is caused by the fungus Elsinoe fawcetti. This pathogen can attack young leaves and twigs as well as the fruit of many citrus species. It initially appears as small, raised, light brown, scabby looking warts on the surface of leaves, twigs or fruit.

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All parts of susceptible citrus varieties are vulnerable to the scab fungus. With four hours of continued wetness, scab pustules can develop on young shoots, leaves and immature fruit. Older leaves are not as susceptible, but scab fungus on fruit can progress and distort the fruit. Wet, rainy weather will often increase the spread and damage that citrus scab can cause. Splashing rain is often the culprit that spreads this disease. Even though scab disease is somewhat unsightly on the rind, it does not affect the quality or flavor of the fruit inside. Despite the appearance of your tangelos, you can harvest and eat them.

Once citrus scab pustules appear, they can and will likely spread to other tissue on the host plant or to nearby citrus trees. It is hard to control this fungus once it is established but a few preventive and management practices can reduce its spread. The pathogen is spread by spores. They exist wherever you see pustules including on dead and fallen leaves. Be sure to remove fallen leaves often.

Controlling weedy growth near your citrus trees as well as pruning them to thin out crowded foliage can increase air circulation and decrease the humidity in and around your plants. This will reduce their vulnerability to scab. Since overhead irrigation water can serve as a means for spores of the scab fungus to spread, consider drip irrigation for your citrus trees.

Though fungicides applied early can control the spread of citrus scab they may not be able to eradicate it once it has infected parts of your tree or trees. Know that citrus scab spores will also remain viable from season to season. At least three applications of a fungicide may be required to limit the spread of the fungus. Spray before flowering, during the early leaf flush and again as the fruit is forming for best results.

The UH CTAHR free publication at https://www.ctahr.hawaii.edu/oc/freepubs/pdf/PD-60.pdf has a list of effective fungicides for this disease. Among them are neem oil, Serenade and Sonata which are all organic. Several copper hydroxide formulas are also listed.

You can prevent this disease by planting resistant citrus varieties. A list of those can be found at the same UH free publication on citrus scab. If you decide to plant resistant citrus put them in a sunny, dry spot in your garden and consider intercropping with citrus or other plants that do not get this fungus. Plan to irrigate with drip irrigation.

Hopefully this will help you deal with citrus scab on your property and know that you can enjoy your fruit despite its unattractive appearance.

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

Gardening Events

Monday: “Hawaii Tropical Fruit Growers Monthly Meeting” from 7-9 p.m. at West Hawaii Tropical Fruit Growers office 81-6393 Mamalahoa Highway in Kealakekua. White wooden building on makai side across from the Department of Transportation yard. Park in front or on the north side. For more information contact Brian Lievens President, West Hawaii Chapter at 895-8753 or greenwizard@hawaii.rr.com.

Saturday: “Work Day at Amy Greenwell Garden” from 9 a.m. to 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. Call Peter at 323-3318 for more information.

Farmer Direct Markets

Wednesday: “Ho’oulu Farmers Market” 9 a.m. to 2 p.m. at Sheraton Kona Resort &Spa at Keauhou Bay

“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

“Kamuela Farmer’s Market” from 7 a.m. to noon at Pukalani Stables

“Waimea Town Market” from 7:30 a.m. to noon at the Parker School in central Waimea

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

Sunday: “Pure Kona Green Market” 9 a.m. to 2 p.m. at Amy Greenwell Garden in Captain Cook

“Hamakua Harvest” 9 a.m. to 2 p.m. at Highway 19 and Mamane Street in Honokaa

Plant Advice Lines

Anytime: konamg@ctahr.hawaii.edu

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