Swiss needle cast
Swiss needle cast (SNC), Nothophaeocryptopus gaeumannii, is not an introduced disease but a native foliar pathogen. Its name comes because of it’s initial identification by Swiss researchers who were examining it on imported Douglas-fir growing there.
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Swiss Needle Cast (SNC) can be found worldwide almost everywhere Douglas-fir grows. It is the only host for this fungus. SNC was rarely noticed in the past, usually overshadowed by another foliar disease of Douglas-fir, Rhabdocline needle cast (Rhabdocline pseudotsugae). However, in the 1990’s Oregon began reporting large areas of Douglas-fir plantations defoliated by SNC and that trend has continued with over 250,000 ha being mapped there to date and an additional 100,000 ha mapped in Washington state. SNC ascospores infect newly flushing needles via the stomata. The fungus develops internally but causes no reaction or necrosis of tissue that would lead to visible symptoms like banding or foliar discolouration. The only visible sign is the production of black fruiting structures that develop blocking stomates a few months after infection. This blocking leads to lowered CO2 uptake and hinders metabolic processes like photosynthesis. Once the leaf becomes a net liability to the tree, it is dropped usually as 2 to 3 year-old foliage. This premature foliar drop leads to growth loss and, occasionally, mortality.
The occurrence of SNC is correlated to weather. During flush and early leaf development mild moist weather that promotes leaf wetness increases the chance of successful infection. Mild winter temperatures allow continued fungal development in the leaf that increases stomatal blocking and disease severity. Planting Douglas-fir into wetter coastal areas where it historically was not present encouraged the development of SNC whereas more inland areas remained unaffected. Since B.C. does not share the same coastal geography as Oregon, our increased incidence is likely due to changing weather patterns that have increased the amount of spring moisture during the infection window in some years. The area most severely affected by SNC is on the north side of the Fraser River between Stave and Harrison Lakes, but the disease can be found, at varying levels, on regenerating Douglas-fir throughout the coast. There is evidence that moving seedlots from high elevation or dry site sources to the coast can increase risk. Foliar diseases disproportionately affect genetically maladapted seed sources.
Monitoring plots in Oregon average 23% growth reductions in epidemic areas. Severely infected stands may suffer up to 60% reductions in growth. The value of this growth loss has been estimated at US$128 million annually. Mortality is rare but has been documented in stands with repeated annual infections especially on younger trees.
Since there is no resistance to SNC, choice of seedlots for reforestation is of limited use. There is no genetic resistance to SNC. Every tree can potentially be infected so the risk is tied directly to the weather. There is evidence of family tolerance to the disease, but such families are still being assessed for tree improvement purposes. Fertilization (N, Ca, P or blends) does not seem to compensate for needle loss. Infected trees respond to thinning more slowly (thin early and from below) but it does not help needle retention. There is no evidence that mixed species stands reduce infection, but loss of basal area may be compensated for by other species. Fungicides are effective, and used extensively in the Christmas tree industry, but would need to be applied 1 to 3 times during the infection window and annually if weather conditions are suitable for the disease.
SNC has come to the fore in the last decade primarily because of its appearance in young stands prior to free growing. An extensive network of monitoring plots was established starting in 2017 in the CWH dm, xm and vm subzones of the coast to monitor stand infection and weather attributes and determine an impact estimate for the disease.
Ritokova, G., D. Shaw and D. Mainwaring. 2020. Silvicultural decision guide for Swiss needle cast in coastal Oregon and Washington. Swiss Needle Cast Co-operative, Corvallis, Or. Download at: http://sncc.forestry.oregonstate.edu/sites/default/files/SilvGuide_July2020.pdf