SNC Definitions


is the major group (Division/Phylum) of fungi to which the Swiss needle cast (SNC) pathogen belongs. This fungal group produces sexual spores (ascospores) in a sac called an ascus (plural: asci), and this is the defining characteristic of this large and diverse group. Many ascomycetes also (or only) produce asexual spores, called conidia, but the fungus that causes SNC does not have this type of spore stage.




Phaeocryptopus gaeumannii is an ascomycete, but the basidiomycetes are the other major group of fungi. Many (not all) basidiomycetes produce spores in showy fruiting bodies, such as conks on woody plants or fleshy mushrooms on soil, wood or tree roots.




(Pseudotsuga menziesii) is the only tree species affected by SNC disease (i.e. the only host of Phaeocryptopus gaeumannii). Symptomatic trees have chlorotic (yellow) needles and decreased needle retention, resulting in sparse crowns and reduced growth. Growth losses in the area of epidemic range from 20-55%. Estimated annual growth impacts exceed $200 million per year.


Pseudotsuga menziesii

Integrated pest management

(IPM) is an integrated, common sense approach to crop or forest management to minimize losses from insect pests and disease. The general components of an IPM strategy are to: 1. set action thresholds, 2. monitor, identify and quantify pests, 3. exercise prevention, and 4. control. According to the Environmental Protection Agency, Integrated pest management (IPM) “is an effective and environmentally sensitive approach to pest management that relies on a combination of common-sense practices. IPM programs use current, comprehensive information on the life cycles of pests and their interaction with the environment. This information, in combination with available pest control methods, is used to manage pest damage by the most economical means, and with the least possible hazard to people, property, and the environment. IPM is not a single pest control method but, rather, a series of pest management evaluations, decisions and controls”. Although some of the focus of IPM strategies is on reduced pesticide or fungicide use, particularly when these solutions are not economically viable, the feature common to all IPM strategies is that thresholds for action are set, which allow managers to make decisions on how to best manage their resource in the face of disease and/or insect pressure using a variety of tools.

Mechanism of infection

The fruiting bodies (pseudothecia) cause a physical blockage (occlusion) of needle stomates, and this inhibits the uptake of carbon dioxide from the atmosphere for photosynthesis. When a threshold proportion of stomates are blocked (30-50%), the carbon cost of respiration exceeds the carbon gained through photosynthesis, and the needle becomes “carbon starved”. In these cases, it is beneficial for the tree to cast the needle. Researchers have also postulated that the reduction of carbon intake may also lead to the accumulation of damaging photo-oxidative chemicals within the needle (greater amounts of excess absorbed light). This helps to explain why more severe defoliation is observed in the upper crowns of diseased trees. Trees that have fewer than three years of needles are likely to experience moderate to significant growth loss compared to trees with longer needle retention.


Phaeocryptopus gaumannii: Mechanism of Infection


are symbiotic (intimate) associations between fungi and the feeder roots of a vascular plant. The symbiosis is mutually beneficial and is necessary for the fungus to complete its life cycle. Plants supply carbon from photosynthesis to the fungi; in turn, fungi absorb minerals and nutrients from the soil and transfer them to roots. In a mycorrhizal association, the fungus colonizes the host plants' roots, either intracellularly (within cells) as in arbuscular mycorrhizal fungi, or extracellularly (between cells) as in ectomycorrhizal fungi. Ectomycorrhizal fungus (EMF) diversity is an important attribute of many temperate forest ecosystems. For instance, ectomycorrhizae help to stabilize below-ground processes after disturbance. Ectomycorrhiza communities are particularly influential with respect to nutrient availability and tree nutrition, so may be influential in the predisposition of Douglas-fir to SNC. Seedlings associated with a high diversity of ectomycorrhizae may be better adapted to disturbance as compared to seedlings with less diversity, and ectomycorrhiza diversity also seems to increase a tree’s competitive abilities. Douglas-fir has about 2000 EMF symbionts throughout its range, and will not grow in soil without ectomycorrhizal fungi. Although preliminary fertilization trials have not yet found evidence of nutritional amelioration of SNC, it is still plausible that imbalanced nutrition may contribute to the susceptibility of Douglas-fir to SNC.


ectomycorrhizae, EM


is a physical blockage, and has many additional definitions. With regards to Swiss needle cast disease, occlusion is used to describe the physical blockage of stomates by Phaeocrypotopus gaeumannii’s fungal fruiting bodies (pseudothecia).


An organism that causes disease. Many plant diseases are caused by fungi or fungus-like organisms (water-molds), but several other types of organisms can also cause plant diseases (bacteria, viruses, parasitic plants, etc.). Disease symptoms are the impact of the pathogen on its hosts (chlorotic foliage, growth reductions, contorted form, etc.). Signs of disease are the visible parts of the pathogen (fruiting bodies, hyphae, etc.).

Phaeocryptopus gaeumannii

is the ascomycete fungus that causes SNC disease. This fungus is native throughout the range of its host, Douglas-fir, and until the mid-1980s was considered an unimportant and minor pathogen. Annual aerial surveys conducted by the Oregon Department of Forestry (ODF) estimate that more than 300,000 acres have been impacted by the fungus each year since 2006, a consistent increase since aerial surveys began in 1996. Disease is most damaging close to the coast, and severe disease has been associated with several climatic and topographic variables, including abundant spring leaf wetness from precipitation and fog, mild temperatures in the winter and spring, and low-elevation valleys. Much of the current research is focused on assessing the impacts of soil and foliage nutrition on Swiss needle cast disease development and severity.


(singular: Pseudothecium): The fruiting bodies of Phaeocryptopus gauemannii. Fruiting bodies mature in needle stomates, and physically block needle absorption of carbon dioxide, resulting in carbon starvation. Sexual fungal spores of P. gauemannii, called ascospores, are contained in sacs (asci) within pseudothecia, and spores are discharged from these sacs in spring and early-summer when the fruiting bodies are mature, environmental conditions are favorable and new Douglas-fir foliage is expanding.



Signs of disease

Signs of disease are the visible parts of the pathogen (e.g. fruiting bodies, hyphae, etc.).

SNC Disease Cycle

Infection of needles takes place in late-spring and early-summer, when ascospores are discharged from mature fruiting bodies (pseudothecia) on infected needles. Spore release coincides with the emergence and elongation of new foliage from buds, and newly formed needles are highly susceptible to infection (little or no infection of older needles occurs). Ascospores are wind and water-splash dispersed, and cause infection when they land on susceptible foliage, germinate, and enter needles through needle stomates. The threadlike body of the fungus (hyphae) continues to grow in the intercellular (between cells) space within the needle over many months, and fruiting bodies begin to develop in stomatal cavities (the epistomatal chamber). Under “ideal” conditions, immature pseudothecia may be visible on needles within 6 months of infection. Infection by one spore may result in the development of many pseudothecia on a single needle, because the fungus is able to expand within leaves in the months and years following infection. The lifecycle is completed when pseudothecia mature, and release ascospores in the spring. Because infection can increase on infected needles over several years, relatively more fungal biomass and fruiting bodies are present within/on older needles compared to younger needles. As a higher proportion of stomates on a needle become blocked by the fungal fruiting bodies, there is increase likelihood that the needle will be shed.


(Plural: Stomates/Stomata): The structures, usually most concentrated on the undersides of leaves, that allow for uptake of carbon dioxide for photosynthesis, and the release of oxygen and water vapor. Phaeocryptopus gaeumannii enters needles through needle stomates, and produces fruiting bodies in stomatal cavities.


Stoma, Stomata, Stomates

Swiss needle cast

(SNC): the name of the foliage disease of Douglas-fir caused by the fungal pathogen Phaeocryptopus gaeumannii. It is known as a “cast” disease, because it causes the needles to be prematurely shed, or cast, from the tree, resulting in sparse tree crowns and reduced growth. Although it is called “Swiss” needle cast, the fungus that causes this disease is native to the western United States throughout the range of Douglas-fir, and is an exotic pathogen in Europe and New Zealand. However, the disease was first discovered in Douglas-fir plantations in Switzerland in the mid-1920s, and the name has persisted.


Symptoms of disease are the impact of the pathogen on its hosts (e.g. chlorotic foliage, growth reductions, contorted form, etc.).