Citrus Tree Diseases and Pests: How to Identify and Treat Them

Citrus problems fall into two groups. The diseases — citrus canker, melanose, greasy spot, sooty mold, and root rot — are mostly fungal or bacterial, and a copper fungicide is the backbone of treatment for all but canker, which has no reliable cure. The insect pests — aphids, citrus whitefly, orangedog caterpillars, citrus thrips, brown soft scale, citrus bud mite, citrus red mites, and snails — are controlled with targeted insecticides, horticultural oil, or hand-removal. Below is each problem, its telltale signs, and how to treat it.

Citrus diseases

The five diseases below are the ones citrus growers see most. With the exception of canker, a copper fungicide plus good pruning and sanitation handles the majority of them.

Citrus canker

A highly contagious bacterial infection that causes yellow, halo-like lesions or scabs on fruit, leaves, and twigs. Severe infections cause leaf loss, blemished fruit, fruit drop, and dieback. The bacterium spreads easily on air currents, insects, birds, and on people via clothing and tools. A Liquid Copper Fungicide can be used preventively, especially when canker has been detected in the area — but already-infected trees are generally destroyed quickly to slow the spread.

Melanose

A fungal infection of young fruit, primarily but not only grapefruit. It scabs the fruit rind — unsightly but not a quality problem — and is generally more severe on trees over ten years old. Because the fungus propagates in dead wood, prompt pruning is an effective control, and a copper fungicide can also be used.

Greasy spot

A fungal disease whose telltale signs are yellowish-brown blister spots on leaves, often on the undersides, which develop into oily-looking blisters. It can cause significant leaf loss, especially in winter, and can also blemish the rind of grapefruit. To control it, regularly collect and remove fallen leaves to reduce the spore source, and spray with a copper fungicide in June or July; a second application in August or September may be needed to protect late-summer growth.

Sooty mold

A fungus that blackens the leaves. It grows on the honeydew secreted by insects such as whiteflies, aphids, and mealybugs, so insect control is the most effective prevention. Spray to control the insects (covering both the tops and undersides of leaves), with a second treatment about 10 to 14 days later if the infestation is heavy. To clear mold that has already formed, spray with a copper fungicide — usually one application is enough, though a second about 14 days later may be needed in major outbreaks.

Root rot (brown rot / collar rot)

Caused by a soil-borne Phytophthora fungus. Symptoms are dark brown patches of hardened bark on the trunk, often with ooze seeping from the infected area; over time the bark dries, cracks, and dies, leaving a dark sunken canker. It can also brown and decay fruit and yellow the foliage. The fungus lives in the soil and is most prevalent in wet conditions, splashing up onto the tree from rain or irrigation. To control it: remove all fallen leaves and damaged fruit; prune off the lower branches so they sit more than two feet off the ground; and apply a fungicide when the disease is identified and again the following spring to prevent reinfection.

Citrus insect pests

The pests below feed on sap, foliage, or fruit. Many of them — aphids, whiteflies, scale — also produce honeydew that leads to sooty mold and attracts ants, so controlling the insect solves more than one problem at once.

Aphids

In small numbers aphids do little harm, but populations can explode in the growing season. They suck sap from the leaves, causing puckering, yellowing, and twisting that gives a deformed look; severe infestations bring leaf drop and twig dieback. The sticky honeydew they excrete drips onto leaves and the ground and attracts ants (the ants are a symptom, not the cause). Treat major outbreaks with a targeted insecticide directed at the undersides of leaves — usually one or two sprays achieve control — or use insecticidal soap for lighter infestations and as a preventive.

Citrus whitefly

A tiny white-winged insect about 1/12 inch long, usually feeding on leaf undersides; shake a branch and they flutter up. They lay eggs on the leaf undersides, and the near-transparent larvae attach and suck sap, causing leaves to curl and become coated in honeydew and sooty mold. Several generations emerge over a season. Full control of adults is difficult, but repeated sprays significantly reduce the juvenile population and, with it, the overall numbers.

Orangedog caterpillars

A large caterpillar, about 1.5 to 2 inches, brownish in color — the larva of the black-and-yellow swallowtail butterfly common in Florida. It eats the leaves, leaving foliage that looks partially chewed from the outer edges, and a fresh hatch can defoliate a tree in a few days. Hand-remove and destroy them where you can (when disturbed they push out two red horn-like antennae that emit a strong smell). For heavy infestations, treat with a spinosad garden insect spray or a BTK (Bacillus thuringiensis) biological insecticide — both safe around the home and garden; a second application is usually needed (about 7–10 days for BTK, 3–4 weeks for spinosad).

Citrus thrips

Tiny orange or pale-yellow insects that attack young leaves and juvenile fruit, feeding on sap. The signs are shriveled leaf buds, curled and distorted silvery-grey leaves, and fruit that is scabbed, streaked, or silvery. Damage is worst in hot, dry weather when the tree is already moisture-stressed. Treat with a spinosad garden insect spray, repeating every 14 to 21 days as needed, and keep the tree well irrigated and properly fertilized to maintain vigor.

Brown soft scale

Small, non-mobile insects that attach to wood, foliage, and sometimes fruit, most often on new tender growth, appearing as crusty or waxy bumps that are easily mistaken for part of the tree. They suck sap, turning leaves yellow and causing drop, and excrete honeydew that feeds sooty mold and attracts ants. The only mobile stage is the nymph, which emerges in spring or mid-summer. Spray with horticultural oil to suffocate the scale and eggs, and treat emerging nymphs with an insecticide in spring or early summer.

Citrus bud mite

A small, elongated mite that generally attacks lemons, especially in coastal areas. Because it peaks in summer, summer and fall lemon blooms are most at risk. It is hard to detect, but large infestations may be visible by closely examining the fruit buttons. Control it with a targeted miticide/insecticide spray.

Citrus red mites

An extremely tiny pest, only about 1/50 inch long and red or purple, that infests leaves and fruit. Intense infestations during hot, dry weather can cause leaf drop. Control it with a targeted miticide/insecticide spray.

Snails

Snails feed on citrus foliage and fruit and can be a recurring problem in damp conditions. Reduce shelter and moisture around the base of the tree, and control them with removal, copper barrier bands on the trunk, or an approved snail bait.

Which products to use

For the fungal and bacterial diseases above, a Liquid Copper Fungicide is the main preventive and treatment, paired with pruning out dead wood and improving airflow and drainage. For the insect pests, purchase insecticides, horticultural oils, and biological controls for citrus, or see the full range of disease-control products.

Frequently Asked Questions

What are the scabby spots on my citrus fruit?

Likely melanose (on grapefruit, worse on older trees) or citrus canker (raised scabs with yellow halos). Melanose is cosmetic and managed by pruning dead wood plus copper fungicide; canker has no cure and infected trees are usually removed.

Why are my citrus leaves black and sooty?

Sooty mold growing on insect honeydew. Control the insects producing it (whiteflies, aphids, scale) first, then clear existing mold with a copper fungicide.

How do I treat greasy spots?

Rake up and remove fallen leaves to cut the spore source, then spray a copper fungicide in June or July, with a second application in August or September if needed.

Can citrus canker be cured?

No. There’s no reliable cure; copper sprays help limit spread, and heavily infected trees are typically destroyed to protect nearby trees.