Gum oozing from split bark at the root crown, foliage yellowing and drying in stages, leaves hanging at strange angles, half-grown fruit dropping with a foul smell: these are the late signals of citrus foot rot, the work of Phytophthora. Keep water off the trunk and out of waterlogged soil, and your grove stays clear.
Root rot is definitely a gardener’s worst nightmare – especially when it starts infecting citrus trees.
A special micro-organism called Phytophthora is responsible for this disease. When the soil is too wet, it infects roots and they start rotting.
Some experts call it “citrus mildew“, but confusing since another citrus disease caused by Oidium also has the same name.
A big problem with this disease is that symptoms only appear very late:
In cases like these, the culprit is Phytophthora, a soil inhabitant. It infects trees, wood plants, and even vegetables. For instance, it also causes tomato blight that doesn’t stop until the entire crop is devastated.
It’s spread by water, either through contact with the tree’s bark (during heavy rains, for instance), or through fruits (those that are nearer the ground), as rain drops spatter back up. The peel then starts showing signs of damage. There are a great many different types of phytophthora. Some are triggered by warmth, whereas others are more potent when the weather is cool. Most of them appear in spring and in fall.
Soil that stays waterlogged is what lets the spores swim to citrus roots.
Runoff and irrigation water can carry Phytophthora propagules downslope, so citrus planted below an infected tree faces increased risk of gummosis, though the order in which nearby trees develop symptoms varies.
Infested soil also travels on boots, spade blades and wheelbarrow wheels, so always work your healthy citrus first and the suspect tree last.
Irrigation water drawn from a pond, ditch or open tank can carry Phytophthora, and delivering it at the root crown is the worst possible combination.
The same soil-borne infection shows up on three very different parts of the tree, and each one calls for a different response. Use this table to work out which part of your citrus is talking to you.
| Part affected | What you see | What to do |
|---|---|---|
| Fruit hanging near the ground | Turns brown, drops before ripening, gives off a foul but distinctive smell | Pick off and remove sick fruits, plus fallen leaves and branches |
| Bark at the root crown | Rots and exudes gum, splits open, brown wounds form | Scrape wounds back to healthy wood, then seal with fungus-killing pruning paste |
| Foliage | Yellows and dries out in stages; remaining leaves sit at strange angles to the branch | Spray the leaves as well as watering the soil, without overdoing it |
| The whole young tree | Never really gets going, then dies off abruptly after a few years with no insect damage | Plant saplings whose graft joint sits at least 10 inches (25 cm) above the soil |
Whichever part shows first, the underlying cause is the same: water sitting where it should not.
On citrus trees, it’s (sad to say) very easy to identify citrus root rot:
my sap
. It bursts open in places and brown wounds form.Usually, it takes years for the tree to slowly wane, and then it suddenly dies. On a young tree, the disease spreads particularly fast.
Double-ridge watering: citrus trees hate getting water at the base of the trunk (root crown). This is because water often brings on disease. The root crown is a vulnerable spot because the bark is much thinner there. A “double ridge” is a means to protect the trunk from getting touched by water even when there’s a lot of it, like during heavy rains. An inside ridge keep water from reaching the trunk, while an outside ridge channels water to roots all around the tree: it’s set 2-3 feet out beyond the drip line (50-80cm).
Just like animals (and, of course, us humans), citrus trees can and do fall victim to diseases because of fungus, viruses, or lack of nutrients (vitamins, trace minerals…). Such issues have consequences that are often much more critical than those caused by pests and parasites.
In previous times, even until recently, what is called Phytophthora was thought to be a fungus. Nowadays it’s been classified as an Oomycete. This is a life form that looks like a fungus, but is actually genetically related to algae. Even though this might seem to be a detail, this new understanding is already opening inroads into new forms of treatment. Let’s hope they’re made available soon!
