Common Citrus Problems


Are your citrus tree leaves covered in black stuff? Do the stems and leaves have white fluffy spots? Are there hard oval bumps on the stems and branches? Or tiny black or white specks on the leaves? Or a white cloud that emerges from the leaves when you touch them?

What are they?

All of these are common with citrus. The black stuff is black sooty mold. White fluffy spots are mealybugs. Hard bumps are scale insects, while tiny black or white specks are aphids. The white cloud is whitefly.

Where do they come from?

Aphids, scale, and mealybugs are “farmed” by ants. These bugs suck sugary sap from the plants and exude sugary droplets – called “honeydew” – from their rear ends. Ants collect the honeydew to feed their young. Ants move the bugs from plant to plant to ensure an ongoing supply of honeydew.

Black sooty mold forms in the honeydew that inevitably drips onto leaves. Whiteflies are ubiquitous and from time to time, when conditions are just right, their populations explode.

What to do about them?

None of these pests will kill your citrus but they can weaken the trees and leave them vulnerable to other pests.

Controlling ants is key to controlling aphids, scale, mealybugs, and black sooty mold. Before you get out the big guns, however, start with the least toxic and mildest approach.

Look for a trail of ants heading up and down the trunk. Once you find them, apply sticky stuff like Tanglefoot® around the trunk of the tree. When ants get caught in the Tanglefoot®, it interrupts their “highway.” Don’t just slather it onto the trunk. Instead, make a band of duct tape or a similar material around the trunk, then smear Tanglefoot® onto the duct tape. Wear disposable gloves as this is messy! Replace the band every month or two so it doesn’t lose its effectiveness. Also, if left on too long, duct tape can constrict the growth of the tree.

At the same time, set out boric acid-based ant traps like the Terro® brand or the KM Ant Pro® bait station. Ants are attracted to these boric acid-based baits, which they take back to the nest. Boric acid destroys the nest.

This one-two punch should go a long way towards controlling ants. Once the ants are gone, natural mealybug and scale predators usually keep their populations under control. If they are still present after a month or so, spray the leaves and branches with insecticidal soap (NEVER DISHSOAP) or light horticultural oil (NOT NEEM). Always follow label instructions.

Next, use a sharp blast of water to clean the leaves of black sooty mold, and to kill aphids and whiteflies.

Normally, whiteflies are kept under control by natural predators, but from time to time, their populations explode. When that happens, spray leaves – especially the undersides – once a week for a month or two. I use a small hose attachment called a Bug Blaster ™, which creates a high-pressure blast in 360 degrees. The pressure of the water and the fall to the ground kill the soft-bodied aphids and the whiteflies, especially their eggs which are attached to the undersides of leaves.

Enjoy your citrus and their bloom, which should smell like heaven about now!

Garden expert, designer, educator, and author Nan Sterman specializes in low water, sustainable, and edible landscapes. Nan hosts, co-writes, and co-produces opens in a new windowA Growing Passion, a show that explores the power of plants in our world. Nan is also author of California Gardener’s Guide vII, Waterwise Plants for the Southwest, and Hot Colors, Dry Garden. More information is at opens in a new