Lemon Tree Roots Are Rotting

Cause

• The Armillaria luteobubalina fungus attacks trees where spores from previously infected trees remain in the soil. Wet, sandy soils may encourage its spread. Two Phytophthora fungi, P. citrophthora and P. parasitica, thrive in most citrus orchards. P. parasitica attacks growing roots during mild weather. P. citrophthora spreads during winter weather when roots are dormant. Rain or irrigation water transports the spores to the roots. In consistently hot, wet conditions, Phytophthora fungi may infect lemon fruit as well.

Armarillia Symptoms

• Symptoms of Armarillia root rot remain concealed underground until the disease is well advanced. Newly infected roots have a noticeable mushroom odor. Decaying roots may develop a jellylike texture and dark bands, or become white and crumble. Dark, narrow filaments may spread from the fungus along the roots' surface and into the soil. In rainy fall weather, brown or honey-colored mushrooms sprout from the affected roots. The release white spores onto surrounding vegetation. As the roots decline, the lemon tree canopy experiences twig and leaf dieback that eventually engulfs large branches. Damage, which can be fatal, depends on how much root tissue the fungus destroys.

Phytophthora Symptoms

• Like those of Armarillia infection, most Phytophthora root rot symptoms develop beneath the soil. The saturated roots become slimy and black. The fungi destroy the roots’ protective outer cortex. As small feeder roots die, the tree's water and nutrient supply decreases. Advanced infection causes cracked, gummy bark and yellowing foliage. If the fungus completely encircles the trunk, the tree may topple. Having your tree professionally tested is the only way to determine whether the symptoms are Phytophthora-related, or the result of nematodes, flooding or salt accumulation.

Prevention

• Locating your lemon tree in a spot where vegetation hasn’t grown for an extended period minimizes the amount of Armarillia spores in the soil. Plant the tree so that the bud union, where it’s grafted onto rot-resistant rootstock, remains from 4 to 6 inches above the soil. This helps protect the lemon's less-resistant upper portion from Phytophthora infection. So does keeping its root crown free of standing water.

Management

• Removing damaged roots, lower branches and diseased bark may extend the fruit-bearing years of an Armarillia-infected lemon. A coating of quick-dry, water-based plastics paint on the cut roots' ends protects healthy tissue. Digging soil away from the deep roots allows air and sun to reach them, further discouraging the disease. Irrigating only as necessary is the best management method for Phytophthora infection. Applications of mefenoxam or fosetyl-al fungicides, applied according to the manufacturer's specifications, offer additional control.