When it comes to rooting out root rot in your avocado tree, it pays to do a little detective work to find out which pathogen is plaguing your tree. While many of the root rot types affecting avocados initially reveal themselves through drooping, yellowing leaves, other aspects determine whether the tree can be saved. Saving your prized avocado from root rot may be challenging, but in many cases it is possible. These subtropical fruits generally grow best in Mediterranean climates where the temperature doesn't dip lower than about 25 degrees Fahrenheit, although Mexican varieties are slightly higher.
Examine the tree to determine which type of root rot your avocado true suffers from. The fortunately rare rosellinia root rot (Dematophora necatrix) is generally considered fatal to avocados. It appears as white patches along the base of the tree and surface root system and by purple cankers near the soil line. The Armillaria mellea pathogen, known as oak root or amarillo root rot, often shows itself as large mushrooms at the base of the tree. The most common type, avocado root rot (Phytophthora cinnamomi), often produces brownish tips on the leaves, dead branches near the top of the tree and blackening or decaying feeder roots.
Scatter 25 pounds of gypsum under the branches of the avocado tree and gently work the amendment into the top 6 to 10 inches of soil around the root system. Because it is rich in calcium, gypsum helps keep new root rot pathogen spores from forming.
Lay a 4 to 6-inch layer of wood chip mulch around the base of the tree, starting several inches from the trunk and extending about 5 feet beyond the tree's outer branches. Mulch promotes beneficial organisms which can help kill the pathogens associated with root rot.
Reduce the amount of water your normally give the avocado tree. Root rot may indicate that the tree it is receiving too much water through drip irrigation or other watering methods.
Test your water supply for salinity, sodium, boron or chloride, and treat your water supply if possible. An excess amount of any of these elements may hasten the advance of root rot.
Dig shallow tunnels to channel excess water away from the tree. Drainage problems can hasten the advance of the root rot pathogens. Depending on the slope and soil conditions on your property, tunnels may be as simple as earth-packed, spade-dug trenches, or as sophisticated as deeper tunnels into which PVC pipes and gravel have been set to encourage better drainage.
Remove soil from the crown of the root system if you suspect armillaria root rot. Air-drying the roots for several days may help halt the advancement of the pathogens which cause this type of root rot.
Set a shade cloth over the root system to protect the exposed roots from the sun's rays while you are drying out the tree's roots.
Replace the soil around the tree's root system with clean topsoil.
Fertilize the tree with a balanced fertilizer, such as 20-20-20, four times each year, starting in late winter. Moderate amounts of fertilizer, especially those which contain nitrogen, promote tree health, which in turn allows the tree tree to fight off pathogens such as root rot.
Treat avocado root rot with phosphite fungicides in late spring and late summer, following package directions. According to the University of California's IPM program, choosing a phosphite fungicide that you apply by injecting the trunk or by watering into the root system is more effective than the kind that can only be applied as a foliar or trunk spray. Follow package directions closely.
Things You Will Need
- Wood chips
- PVC piping and gravel (as needed)
- Shade cloth (as needed)
- Top soil
- 20-20-20 or similar fertilizer
- Phosphite fungicidal treatment
- Get your new avocado trees off to the best start by planting on a raised berm or otherwise well-drained area, and by purchasing disease-resistant stock. Avoid planting new trees in soil that previously contained diseased plants.
- If you have uninfected avocado trees and are attempting to save one or more infected trees, direct watering tunnels or drip irrigation systems from the infected trees away from uninfected trees. This measure lowers the risk of your healthy trees coming in contact with the pathogens.
- Ask any expert before expending too much time and energy into saving an avocado tree infected with root rot -- especially if you have nearby healthy avocado trees. The pathogens associated with root rot can linger in the soil for years or be spread by tools, water and soil erosion.
- Destroy your tree if it is infected with rosellinia root rot, advises the University of California's Integrated Pest Management program. Trees do not recover from this type of root rot, which can infect nearby trees.
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