Citrus foot rot, when roots slowly die away unnoticed…

Burst bark on a citrus tree due to phytophthora

For citrus trees, root rot leads to trunk rot. It’s sometimes detected on fruits, too. The disease results from an infection by Phytophthora, a rot fungus.

Root rot is definitely a gardener’s worst nightmare – especially when it starts infecting citrus trees. A young sapling, planted with love and care, simply doesn’t seem to want to grow. Worse still, after a few years, it dies off abruptly. No insects, no visible damage… it just wilted away. In cases like these, the culprit is, most often, Phytophthora, a soil inhabitant that infects trees, wood plants, and even vegetables. For instance, tomato blight often doesn’t stop until the entire crop is devastated.

What causes citrus root rot?

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.

How does this root rot spread?

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.

Symptoms of citrus root rot

On citrus trees, it’s (sad to say) very easy to identify citrus root rot:

  • Citrus fruits infected by this particular type of mildew turn brown, fall off of the tree before they’re even ripe. They release a foul but very distinctive smell.
  • The bark, at the root crown, rots and exudes a gumSymptoms of root or foot rot on citrus: gummosismy sap. It bursts open in places and brown wounds form.
  • Foliage turns yellow, dries out in stages, and leaves that remain on the stems start forming strange angles with the branch.

Usually, it takes years for the tree to slowly wane, and then it suddenly dies. On a young tree, the disease spreads particularly fast.

Preventive treatment

  • Never get the trunk wet. Water with drip irrigation, or use a tool/protection to keep the root crown from getting and staying wet. For instance, a technique called “double ridge” watering will help keep the base of the trunk dry.
  • Disinfect your tools even as you switch trees during your work. Either run them across a hot flame (careful!) or dip them in pure chlorine (and dry or let it evaporate).
  • When you buy young saplings, chose those that have a graft joint at least 10 inches high (25 cm). The rootstock should be tall enough.

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).

Curative treatment

  • Sick fruit like this browning lemon should be eliminatedEliminate sick fruits and any leaves and branches that fall to the ground.
  • Treat apparent wounds (even the smallest ones) by scraping them until you reach healthy wood. Then, cover them with fungus-killing pruning paste, such as that used against canker.
  • Water around the tree with a copper-containing mix, or (if you’re not exclusively organic) Aliette, a common fungicide that also treats against Verticillium and fire blight. If leaves are also hit, spray them, too (but don’t overdo it).
  • Complement this curative treatment with a lather of bordeaux mixture on the trunk.

Good to know

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!


Credits for images shared to Nature & Garden (all edits by Gaspard Lorthiois):
Bark bursting by JIRCAS Library under © CC BY 2.0
Sticky gummy sap by Scot Nelson under Public Domain
Telling brown color by Gerald Holmes, Strawberry Center, Cal Poly via Bugwood.org