Citrus Thrips, Scab, and Verrucosis

Dealing with Citrus Thrips, and Verrucosis (Citrus Scab)

Almost everyone in New Zealand grows a lemon tree, but unfortunately, almost everyone also deals with the frustration of ugly, bumpy skins or weird, silvery leaves. If your dream of flawless, bright yellow lemons is being ruined by rough rinds and stunted foliage, you are likely dealing with two of the most common lemon tree diseases nz gardeners face: Citrus Thrips and Citrus Verrucosis (Scab). Here is exactly how to identify and treat them.

1. Citrus Thrips: The Culprit Behind Silver Leaves and Scarred Fruit

Citrus thrips (Scirtothrips dorsalis or Kelly’s citrus thrips) are microscopic, sap-sucking insects that cause major cosmetic damage to citrus. They feed on young, tender leaves and the skin of newly formed fruit.

Damage and Symptoms –

• The Symptoms: The most obvious sign of citrus thrips nz is on the leaves.
• Leaves: Feeding punctures cause a silvery-white mottled appearance, leaf distortion, and tiny black specks of waste (frass).
• On the fruit: Larvae feed under the calyx (sepals) or where fruits touch. Thrips feeding causes a greyish-brown, scabby ring (often called a “halo”) around the stem end of the lemon, or streaks down the sides. Internal fruit quality remains fine, the rind becomes tough and ugly and market value drops.

Vulnerable Window: The high-risk period runs from petal fall until the fruit reaches about 40mm in diameter.

Monitoring: Inspect young fruit and check under sepals using a hand lens shortly after petal fall.

How to Treat Citrus Thrips

• Biological Control: Conserve beneficial predatory mites (Euseius or Amblydromalus species) which help suppress low populations.
• Organic Methods: Thrips hate humidity, so regularly hosing down the foliage during dry summer spells can disrupt them. Horticultural oils (like Neem Oil) or Pyrethrum sprays are effective organic knock-downs. You must spray in early spring when the petals are falling and the new fruit is just forming, as this is when the thrips attack.
• Chemical Methods: For severe, recurring infestations, use a specific insecticide containing spinetoram (such as Yates Success Ultra). It is highly effective against thrips. Always spray in the late evening to protect foraging bees.

2. Citrus Verrucosis (Citrus Scab): The Cause of Bumpy Lemon Skin

If your lemons look like they have warts, you are dealing with Citrus Verrucosis (also known simply as Citrus Scab). This is a fungal disease caused by Elsinoë fawcettii.

• The Symptoms: This fungus thrives in wet, humid New Zealand springs. It presents as corky, raised, scabby, and warty bumps on the skin of the fruit, leading to classic bumpy lemon skin nz. It can also cause crinkled, distorted leaves with similar raised scabs. Like thrips, the damage is purely cosmetic—the juice inside the lemon remains perfectly safe and delicious.

Citrus Verrucosis Treatment

• Cultural and Organic Methods: The fungus overwinters on infected leaves and twigs. Prune your lemon tree to open up the canopy; good airflow dries the leaves out faster, depriving the fungus of the moisture it needs to spread. Pick up and dispose of any fallen, infected leaves.
• Fungicide Treatment (Copper): The only way to stop verrucosis is through preventative spraying; once the bumps are on the fruit, they cannot be cured. Effective citrus verrucosis treatment relies on Copper Oxychloride or liquid copper sprays (which are certified organic).
• The Spray Schedule:
  1. Spray once in late winter before the flower buds open.
  2. Spray again at “petal fall” (when the flower petals drop and the tiny green lemons are exposed). This timing is absolutely critical to protect the new skin.

By timing your copper sprays for the fungus and keeping an eye out for Thrips during the spring flush, you can restore your tree to producing smooth, picture-perfect lemons.