Pathogen
DiseasesA microorganism such as a fungus, bacterium, virus, or phytoplasma that causes disease in plants.
A pathogen is any microorganism capable of causing disease in a host plant. Tree pathogens include fungi (the most common group), bacteria, viruses, phytoplasmas, and oomycetes (water molds). Each type of pathogen has different modes of infection, spread, and management, making accurate identification essential for effective treatment.
Disease development requires the convergence of three factors, known as the disease triangle: a susceptible host, a virulent pathogen, and favorable environmental conditions. Understanding this triangle is key to disease management because disrupting any one element can prevent or reduce disease. For example, selecting resistant tree species removes the susceptible host, applying fungicide eliminates the pathogen, and improving air circulation changes the environment.
Pathogens spread through various mechanisms including wind-dispersed spores, rain splash, insect vectors, contaminated soil, and infected planting material. Some pathogens like oak wilt fungus can even spread through connected root systems between adjacent trees. Proper sanitation practices, including disinfecting pruning tools between trees and avoiding pruning during high-risk periods, are important measures for limiting pathogen spread in managed landscapes.