Armillaria root rot

Bark cracking at the base of a tree infected with Armillaria. © NSW DPI

White mycelium at the base of the infected tree.

Shining gum (Eucalyptus nitens), killed by Armillaria species.

Armillaria luteobubalina is a native fungal pathogen distributed widely in Australian native forests and woodlands. The fungus infects more than 200 species across more than 50 plant families, and most host species are endemic to Australia.

Losses are most likely when orchards and vineyards are planted on land recently cleared of native vegetation susceptible to the fungus.

Cause

The fungus Armillaria luteobubalina.

Symptoms

Armillaria damages the root system and so above-ground symptoms will be similar to other root-infecting pathogens—plants slowly decline, leaf yellowing, reduced leaf growth and twig dieback.

Death occurs very suddenly—the leaves wilt then turn brown but stay on the tree. Often cracks in the bark are observed at the base of dead trees.

Diagnostic symptoms are the presence of cream-coloured fungal growth, sometimes fan-shaped, just beneath the bark of the crown and large roots, and a strong mushroom smell. Black, cord-like threads of the fungus often occur on the surface of the roots, forming a branched network that may extend 200–300mm into the soil. Honey-coloured mushrooms with widely separated gills can form at the base of an affected tree during wet, cold weather in early winter.

How it spreads

The fungus can survive in the soil for many years on live and decaying stumps and roots. Infection occurs by root contact with infected plants and, more commonly, by rhizomorphs present in the soil.

Rhizomorphs (root-like fungal structures) on small, infected root pieces are distributed by flowing water or on cultivation machinery from infested areas. These rhizomorphs grow through the soil, attaching to young roots and infecting the outer root tissue. Armillaria mushrooms do not have a role in the infection cycle.

Plants

Crops

• Apple
• Banana
• Citrus
• Custard apple
• Grape
• Macadamia
• Pear
• Stone fruit

Trees

• Gympie messmate (Eucalyptus cloeziana)
• Shining gum (E. nitens)

Control

• Prepare soil thoroughly before planting new orchards and vineyards. Remove as many roots and stumps as possible before planting.
• When replanting low-density orchards locate rows between the previous rows. In high-density replanting construct mounds free of root residues.
• Use resistant rootstocks when planting into high-risk areas.
• Fumigate infested soil with a recommended fumigant.

Chemical registrations and permits

Check the Australian Pesticides and Veterinary Medicines Authority database for chemicals registered or approved under permit to treat this pest on the target crop in your location. Always read the label and observe withholding periods.