Pathogen
The fungal pathogen Stigmina lautii is responsible for Stigmina needle cast. It was first described from western Canada (Sutton 1973) but was not recorded in the U.S. until 1999 (Hodges 2002).
Hosts
Based on samples submitted to the UMass Plant Diagnostic Laboratory, Colorado blue spruce (Picea pungens), white spruce (P. glauca), and Norway spruce (P. abies) are the most common hosts in southern New England. Stigmina has also been identified from Oriental spruce (P. orientalis) and Serbian spruce (P. omorika), which are less common ornamentals but still widely planted in the region. Furthermore, the pathogen has also been found on Sitka spruce (P. sitchensis), Korean spruce (P. koraiensis), Schrenk’s spruce (P. schrenkiana), and Qinghai spruce (P. crassifolia) from arboretum samples. Stigmina was originally described from white spruce and black spruce (P. mariana) (Sutton 1973).
Symptoms & Signs
Symptoms of Stigmina needle cast can appear as premature shedding of one year old needles on lower canopy branches, a general thinning of the canopy, and scattered branch dieback. Infected needles often turn brown prior to shedding but may also appear from grayish brown to yellowish green. In many cases, both Rhizosphaera and Stigmina are present on symptomatic spruce, along with other pests (e.g. spruce spider mite) and fungal pathogens (e.g. Phomopsis canker). However, it is increasingly common to find Stigmina alone on spruce with symptoms of needle cast, suggesting it has become a more prominent pathogen in the northeast. Stigmina behaves very similarly to Rhizosphaera and is easily confused. One distinguishing characteristic is that Stigmina can be found sporulating on green needles, whereas Rhizosphaera primarily sporulates on symptomatic needles (e.g. those that are reddish brown to purple).
The fruiting bodies (sporodochia) appear as very small, black-colored, circular structures growing out of the stomata (pores used for gas exchange) on the needle surface (Sutton 1973). Under magnification, they appear fuzzy as the spores (conidia) grow outward from the structure. Rhizosphaera, in contrast, produces a smooth, black spore-bearing structure from the stomata and the extruding spore masses are gelatinous and clear to cream-colored.
Cool to mild temperatures and prolonged needle wetness favor disease development in the spring. The pathogen invades newly developing needles through the stomata and symptoms may develop later in the growing season or the following year. This means that current season’s needles may appear green and healthy but are infected by the fungus. This latent period between infection and symptom development is common among needle blight pathogens (Sinclair and Lyon 2005). When trees are stressed and weakened, symptoms of infection tend to develop more rapidly. Stigmina overwinters in diseased needles that have fallen to the ground or those that remain in the canopy.
Management
Prune out weak and dead branches from the lower canopy and cover dead needles beneath the tree with mulch or wood chips. This will help to reduce local inoculum that initiates new infections and increase air flow and sunlight penetration in the lower canopy. Drought stress is one of the most important predisposing stresses that facilitates needle cast development. If possible, provide supplemental irrigation during extended dry periods. Many spruces, especially blue spruce and white spruce, are adapted to cooler, higher elevation environments. While they have performed well in lower elevation locations in the past, as the climate in the northeast gets warmer and more humid, the stress of heat and short-term drought is becoming a more serious health issue. Light nitrogen fertilization may also help trees that have lost large volumes of needles to restore vigor.
Contact and locally systemic fungicides such as azoxystrobin, benzovindiflupyr, copper hydroxide, copper salts of fatty and/or rosin acids, mancozeb, metconazole and thiophanate-methyl may have some utility in suppressing the fungus. Unfortunately, little information exists regarding fungicides that have utility against Stigmina, but products that work against Rhizosphaera should produce similar results. Fungicides should be applied in the spring during new shoot and needle formation (when new shoots are 1/2" to 1" elongated and on labeled intervals) and if the infections are severe, again in autumn when temperatures cool (mid-September onward). However, keep in mind that even with good cultural and chemical control, an improvement in the tree's form may not be possible. Meaning, the best-case scenario is that the tree retains its current appearance at the time of intervention.
Prior to 1999, when Stigmina lautii was identified in North Carolina on blue spruce, it was only known from western Canada. Since that time, it has become a common and damaging needle cast pathogen on spruce in managed landscapes across the Northern Plains (Walla and Bergdahl 2016), Upper Midwest (Fullbright and O’Donnell 2011), and New England. It’s unlikely that Stigmina was previously overlooked by pathologists, given the interest and attention given to Rhizosphaera needle cast on spruce (Sinclair and Lyon 2005). Therefore, it appears the range and incidence of S. lautii has increased in recent decades although the specific factors responsible remain unknown.
References
Fulbright, DW and O’Donnell J. 2011. What’s going on with blue spruce? MSU Ext. News. https://www.canr.msu.edu/news/whats_going_on_with_blue_spruce
Hodges CS. 2002. First Report of Stigmina lautii in the United States. Plant Disease 86(6): 699. https://doi.org/10.1094/PDIS.2002.86.6.699A
Sinclair WA and Lyon HH. 2005. Diseases of Trees and Shrubs, 2nd edn. Cornell University Press, Ithaca, NY.
Sutton BC. 1973. Hyphomycetes from Manitoba and Saskatchewan, Canada. Mycological Papers 132: 1–143.
Walla JA and Bergdahl AD. 2016. Stigmina needle cast of spruce. Page 174 in Bergdahl AD and Hill A, tech. coords. Diseases of Trees in the Great Plains. USDA-RMRS-GTR-335. Fort Collins, CO, USA. 229 p. https://doi.org/10.2737/RMRS-GTR-335