Garden Word of the Day
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Avocado Root Rot
If you have a large, lovely, and productive avocado tree, you will want to be sure to prevent avocado root rot. This disease is commonly caused by overwatering and it is usually fatal to the tree. [By the way, removing large trees is expensive and dangerous.]
Avocado root rot is perhaps the most common disease of California’s avocado orchards, but nearly all fruit and nut trees are susceptible, as are azaleas, blueberries, boxwood, camellias, cinnamon trees, conifers, cycads, ferns, lilies, mosses, pineapples, and roses. Avocado root rot also threatens California’s rare endemic Ione manzanita (Arctostaphylos myrtifolia).
What causes avocado root rot?
Large trees and shrubs use a lot of water. Adding too much water creates habitat for tiny water molds, called oomycetes. Oomycetes are responsible for Phytophthora [Fie-TOF-ther-uh] root and crown rot diseases. This is one of those. The oomycete responsible for avocado root rot is called Phytophthora cinnamomi. The Global Invasive Species Database includes Phytophthora cinnamomi in its list of "100 of the World's Worst Invasive Alien Species". These water molds were originally found only in tropical and subtropical countries. They are now found around the world.
Water molds fall between fungi and algae. They fly through the air and exist in waterways and the soil. Normally, they are not a problem. Until environmental conditions are suitable, the oomycetes that cause avocado root rot mostly remain dormant in the soil. Add too much water and spores germinate, producing mycelia or hyphae (tiny vegetative threads) and sporangia (reproductive parts). The sporangia release spores that are small enough to enter a root through the root tip. Once they enter the root, they start absorbing nutrients and carbohydrates, breaking down the root structure and preventing the plant from absorbing water and nutrients.
Symptoms of avocado root rot
The most common symptom of avocado root rot, or Phytophthora dieback, is wilting. This is a problem because many people respond to plant wilting by adding more water, which is what caused the problem in the first place. Other symptoms include brown edges on leaves, chlorosis (yellowing), collar rot, darkened patches of bark and root tissue, gumming, leaf curl, reduced fruit size, stem cankers, and young shoot dieback. Flagging may also be seen. Flagging refers to the way dead leaves are retained by the plant, rather than allowed to fall.
Preventing avocado root rot
Proper tree planting depth is the best thing you can do to keep your trees healthy. Other preventive measures include:
Commercial orchards infected with avocado root rot may inject or spray affected trees with fungicides, or resort to soil solarization. These are extreme measures that should be avoided by the home gardener unless absolutely necessary. One thing you can do is cover the area with composted mulch, which suppresses the oomycetes that cause the disease and prevent healthy trees from becoming infected. Or, you can get a load of free arborist chips which will, eventually, compost themselves, preventing disease and improving soil structure.
When we bought our place back in 2012, there was a lovely old apricot tree in the backyard. She wasn’t performing very well, however. Upon closer inspection, the tree seemed a little “loose” in the ground. When I gave it a shove, the whole thing fell over! The root system was practically gone. Luckily, the tree had been kept at a manageable size and no one was hurt. I decided to use the trunk to create a stumpery and plant a new apricot tree. We later learned that the sprinkler system was pointed directly at the tree, creating a recipe for disaster.
While you may not be able to eliminate the oomycetes that cause avocado root rot from your property, you can control the conditions that allow them to cause harm.
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