Pest/Pathogen of the Month: June

Scientific name: Rosellinia necatrix

Common names: White root root

Rosellinia necatrix Berl. ex Prill. is the causal agent of white root rot on various plant species, including almond, apple, peach, orange, pear, grape, coffee, olive and avocado. The genus Rosellinia consists of multiple species capable of causing disease, however, R. necatrix is the most widely distributed and devastating. R. necatrix was first identified in South Africa in 1974 on apple and pear trees in the Western Cape. It is an ascomycete, saprophytic pathogen that causes rotting of the roots and collapse of host conducting vessels leading to wilting and death. This fungus is soilborne and can survive in the soil on woody debris and organic matter for long periods of time. White root rot is difficult to diagnose since foliar and root symptoms are unspecific, therefore, the disease is often mistaken for Phytophthora root rot. Some hosts do not show any foliar symptoms until the plant suddenly dies, sometimes with fruit and leaves still attached to the tree. A distinguishing symptom is the presence of white mycelial growth on the root surface, in the soil and underneath/on top of the bark at the crown of the tree. Disease control options are limited due to the pathogen’s hardy resting structures, extensive soil penetration and ability to withstand drought, acidic soils and many common fungicides.