What can you learn about your lemon or orange, mandarin or grapefruit tree from its leaves?
Following are photos of citrus foliage mostly from the trees in my own Southern California yard, and each photo tells a small story. It’s likely that you’ll recognize some of the colors, shapes, and critters in the images. And I’m guessing that knowing why the leaves look as they do can bring comfort, satisfy curiosity, or help you care for your trees.
Zig zag tracks: leafminers
Ubiquitous on leaves of citrus trees in Southern California are the squiggly tracks of the larva of the citrus leafminer. The above photo shows a fresh track, where the larva is still in the tunnel. The photo was taken in the middle of summer, when many citrus trees are putting out new leaves and the citrus leafminers are very active.
After the larva have emerged from their tunnels, they pupate and curl the leaf around them. Eventually, they become small moths, and they leave behind distorted foliage: tracked, curled, shiny, with dead spots.
Are leafminers a significant threat to your citrus tree? What should you do about them? See my post, “Don’t spray for citrus leafminers.” Also, learn a lot more about citrus leafminers at this page from the University of California.
Cupping upward: thirsty
I greatly appreciate how citrus trees make it known that they need water: they roll their leaves like taco shells. I happen to have a handful of old citrus trees in my yard that I don’t much care about and that I rarely, if ever, irrigate. So right now, mid-summer, most of them have cupped leaves like the one in the photo above. On occasion, I water one of these trees, and I enjoy watching the leaves unroll over the next few hours.
We can use this signal of cupped leaves to notice when citrus trees need water. Conversely, we can use the absence of cupped leaves to confirm that a citrus tree does not currently need water.
Small yellow leaves: new growth
We are naturally suspicious of yellow leaves. Healthy leaves are supposed to be green, right? That’s a good rule of thumb, but it doesn’t apply to the little new leaves that emerge in pulses throughout spring and summer. Often this new growth is slightly yellow, and on some varieties of citrus it is extremely yellow. It can look sickly, but isn’t necessarily so. As the leaves mature, they green up just like the rest of the older foliage.
Deep green leaves: north side
Speaking of green leaves, have you ever noticed that the leaves on the north side of a citrus tree’s canopy are greener than the leaves on the south side? (It’s reversed in the Southern Hemisphere.) This is exaggerated in hot inland locations.
In fact, if your normal view of your citrus tree is of the north side of its canopy, then you might think the tree is healthier and more beautiful than if your normal view is of its south side.
My normal view of the above Valencia orange tree is of its north side, which is so pleasing to look at. When I see the tree from the other side, I would wonder if it didn’t need more water or fertilizer, if I didn’t know better.
Yellow areas on south side: sunburn
This photo shows some leaves on the south side of my Cara Cara navel orange tree. Unfortunately, I almost never see this tree’s greener north side. Moreover, the tree is planted beside our driveway, which adds to the intensity of the heat and sunlight that this south side of the canopy receives.
So not only is the south side less green overall, but it also has leaves with yellow areas. And if there’s been a heat wave, parts of those leaves will get so scorched that they turn light brown. It’s not a disease or an insect or a nutrient deficiency; it’s just too much sun.
Yellow mottling: sap suckers
However, citrus leaves do show yellow spots that are caused by diseases or insects or nutrient deficiencies, of course. How do you know which is which? One way is to flip the leaf over.
White fluff on bottom side: woolly whiteflies
You might find tiny insects on the underside of the leaves that are sucking the juices from the leaves and causing the mottled colors on top, such as in the photo above which shows numerous woolly whiteflies on some leaves from my Valencia orange tree.
Other sap suckers found on citrus leaves include aphids and scale.
Orange/brown bumps: scale
Scale is in love with my Bearss lime tree. The orange/brown bumps can be found all over the branches and on some leaves too. Unlike woolly whiteflies, scale seems to like living on both the bottom and the top sides of the leaves.
Note the presence of ants in the photo above. If you look closely, you’ll also notice ants in the photo of woolly whiteflies. At the end of this post, I’ll comment on the significance of the presence of these ants.
Three-leaved clusters: rootstock
It can pay dividends to notice the shapes of the leaves of your citrus tree. All of the leaves on a tree should have a similar shape. If some don’t, then you might be looking at a tree whose rootstock has been allowed to sprout.
A common type of rootstock used today to make the citrus trees that we grow in California is a type of trifoliate, meaning that it has groups of three leaves or leaflets. So look out for those clusters of three leaves, accompanied by lots of thorns on the branches.
(What to do if you find them? See my post, “Beware of rootstock suckers on citrus trees.”)
Wilted and water soaked: cold damage
If in the winter some of the leaves on the outside of a citrus tree’s canopy, especially small new leaves, look wilted, darkened, like they’re water-soaked, then you’re looking at cold damage.
Black film: sooty mold
If it’s not winter and the leaves on a citrus tree look blackened, then you might be dealing with a fungus called sooty mold, which seems to be most prolific on citrus trees near the beach, in my observations, but even my Satsuma mandarin tree in inland Ramona has some.
Sooty mold grows on the sweet excrement, called honeydew, produced by sap-sucking insects like woolly whiteflies, aphids, and scale.
Argentine ants and citrus leaves
Remember the ants that I pointed out in the photos above? They weren’t there incidentally. They were Argentine ants, and they farm those sap-sucking insects. The Argentine ants eat the honeydew that the sap suckers excrete, and then the ants herd and defend the sap suckers, their food source.
If you find sap-sucking insects on your citrus leaves, I guarantee you’ll also find ants.
If you get rid of the ants, then the population of sap-sucking insects will be vastly reduced by predators, parasites, and pathogens. Without the ants to defend them, the sap suckers are vulnerable.
Mark Hoddle, entomologist at U.C. Riverside, reports getting over 90-percent reductions of sap-sucking insect populations in citrus groves through merely reducing the Argentine ant population. Getting rid of the ants allows the natural enemies of the sap suckers to do their work. Hoddle finds it most effective to kill Argentine ants through some form of bait and poison. One example of this that is available to home gardeners for purchase is the KM AntPro bait station.
It’s a wild world out there. There are lots of environmental stresses, and there are bugs and microbes that feed on our citrus leaves. But don’t automatically worry at the sight of imperfect leaves. Do automatically try to learn their stories.
A list of all my Yard Posts is HERE