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.
Soil-borne oomycete Phytophthora cinnamomi.
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 does it spread?|
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.
Avocado, macadamia, pineapple and stone fruit.
Chemical registrations and permits
Check the Australian Pesticides and Veterinary Medicines Authority chemical database and permit database for chemicals registered or approved under permit to treat this disease on the target crop in your state or location. Always read the label and observe withholding periods.