Phytophthora root rot

Phytophthora root rot is the most destructive and important disease of avocado. It can be extremely serious, killing most trees in an orchard. Nursery plants and young replants are particularly sensitive to root rot and often die soon after infection.

Phytophthora cinnamomi also causes heart rot and green fruit rot in pineapples.

Cause

Soil-borne oomycete Phytophthora cinnamomi.

Symptoms

Phytophthora species can invade the roots and crowns of woody trees but foliar symptoms may not become evident for months or even years.

Leaves are pale green, wilted and fall readily. Shoots die back from the tips so that eventually the tree is reduced to a bare framework of dying branches. Death of the tree may take from a few months to several years. Declining trees commonly set large crops of small fruit (avocado). Lack of foliage and dieback of small branches exposes fruit and major limbs to sunburn.

Feeder roots are black, decayed and few in number. As infected roots lose the ability to exclude salts, leaf margins in affected trees develop brown, necrotic symptoms typical of salt burn.

Under severe waterlogging rapid decline of trees may occur. The leaves wilt and die, leaving a canopy of brown, dead leaves. A weeping stem canker may occur on the lower trunk.

When the pathogen is present, the soil environment plays a very important role in the development of symptoms. Symptoms do not appear unless there is an upset in the balance between water requirements of the leaves and the capacity of roots to absorb water.

How it spreads

Zoospores (motile asexual spores) are important for the rapid spread of the pathogen. This occurs when free water is present in the soil or on aerial plant surfaces. In the soil they are attracted to root tips of plants by chemical stimulus as well as root-generated electric fields.

Phytophthora cinnamomi is mostly soil-borne and can survive in infected roots under adverse conditions for several years. Infection is usually confined to the roots and lower trunk.

Crops affected

Avocado, macadamia, pineapple and stone fruit.

Control

• Use an integrated approach that relies on pathogen-free nursery trees, cultural and biological controls, resistant rootstocks and chemicals.
• Plant on well-drained soils or improve drainage using mounds.
• Irrigate carefully, avoiding both over- and under-irrigation.
• Increase the organic matter content of the soil using ground covers and mulch. Keep mulch away from tree trunks.
• Apply gypsum (or lime if pH needs correcting) under the canopy of the trees to suppress the formation of spores. High pH favours development of the disease.
• Provide adequate nutrition.
• Use recommended systemic chemicals in addition to these cultural practices.

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.