Montana Field Guide

Phytophthora ramorum is an invasive fungus like pathogen that causes both Sudden Oak Death and Ramorum Blight in affected species. It is particularly lethal to some species of oaks and has caused damage to California’s oak forests. The pathogen is spread through movement of contaminated soil and vegetation. Currently it is found in California and Oregon, and containment measures are enacted to prevent the spread outside of the current distribution. The origin of this pathogen is currently unknown, but to date it is found in parts of Europe and North America.

Symptoms of Phytophthora ramorum infection vary by host species. If the pathogen infects the trunk, Bole or trunk cankers are present. In shrubs and non-woody plants the pathogen infects the foliage and shoots and causes dieback of shoots and or leaf spotting and death. In oak trees symptoms of the disease are seen only in larger host (>10 cm dbh), and manifest as cankers on the trunk. Cankers have a reddish brown to black color and weep sap. Typically they are located 1-2 m off the ground but can be found between 0 and 4 m. Infection may also cause a rapid browning of the leaves in the crown.

Definitive confirmation of this pathogen is only possible through submission of a sample to a diagnostic laboratory.

In its introduced range, Sudden Oak Death Fungus has been associated with native oak forests along the west coast of California as well as ornamental trees and shrubs in that and other areas. Affected areas range in height from 0 to 800 m above sea level (Rizzo et al. 2002). Montana has a single native oak species, the Bur Oak (Quercus macrocarpa) which is not a known host of the fungus (APHIS 2013, APHIS 2018) although the species may not yet have been exposed to the pathogen. The distribution of Bur Oak is restricted in Montana to the vicinity of the Wyoming/South Dakota border, so there is little native habitat for the fungus in the state.

Phytophthora ramorum infects over 100 host species including Oaks (Quercus spp.).

Diatoms typically reproduce by cell division (mitosis) and occasionally by meiosis—sexual reproduction in which female and male gametes combine to form a specialized zygote called an auxospore. Repeated divisions result in cells of a population becoming progressively smaller and smaller. When cells reach a critically small size, sexual reproduction is initiated, resulting in an auxospore and initial cells that are the largest attainable for the species, after which cell division and size reduction resume (Amato 2010).

Management of Sudden Oak Death is centered around containment of the causal agent. The US Department of Agriculture has quarantined host plant species in 15 counties in California and has partially quarantined a single county in Oregon (APHIS 2016). Oregon has instituted a program to slow the spread of the pathogen by using aerial detection, water sampling, and ground based surveys to identify infected plants, then removing the infected plant and other susceptible host species in the vicinity (Oregon Department of Forestry 2016).

Get more details from the California Oak Mortality Task Force.

For information and resources on plant pests and diseases see the USDA's Animal and Plant Health Inspection Service's Plant Pests and Diseases Profiles