Complex Forest Tree Diseases – Diagnostics Beyond Koch’s Postulates

Abstract

Purpose of Review: Forest tree diseases are a major contributor to forest degradation and loss of productivity. They are often quite complex in their causation (etiology), especially in the case of forest syndromes, i.e. diseases with multiple causes and concurrent symptoms. Traditionally, to prove pathogenicity of a microbial agent, and thus correctly diagnose the etiology of a disease, plant pathologists must satisfy all of the so-called Koch’s postulates, as mandated by their deontological code. This review examines whether this approach is still current. Recent Findings: Koch’s postulates state that a pathogen is a microorganism that, after being isolated in pure culture, can reproduce the disease when it is inoculated into a healthy plant. Over the decades, plant pathologists as well as medical scientists have discovered that these postulates are not always applicable in their entirety and that, furthermore, novel approaches based on molecular biology can be very helpful in uncovering relationships between microbes and diseases that are not easily proven using Koch’s postulates. Summary: I conclude that Koch’s postulates are not a viable approach for many forest tree diseases and propose a set of new guidelines, based on the preponderance of the evidence principle, to integrate this proven approach and bring it into the twenty-first century.