Cause Unknown. It is thought that it is due to the fungus Mycosphaerella sp.; however, several other fungi colonize the affected needles but are found less frequently. They include Phaeocryptopus nudus, Phyllosticta abietina, and Rhizosphaera pini. It is unknown whether one or more of these fungi are the primary cause or simply infect necrotic needles that have died from some other factor(s). The result is a needle cast of noble fir and, to a lesser extent, grand fir Christmas trees. Cool moist weather during needle emergence and early shoot growth favors disease development.
Symptoms Needles on lower, interior branches become brown but remain firmly attached in late summer and early fall. Trees are noticeably affected and are unmarketable that season. Needles abscise and fall off by the following spring. Small, black, fruiting bodies (pycnidia, pseudothecia, or perithecia, depending on the fungus), develop on needles' undersurfaces in spring when they begin to abscise.
Cultural control
- If fungi cause the disease, then practices that increase air circulation and needle drying will be beneficial.
- Do not interplant the next rotation before the current rotation of trees has been completely harvested.
- Remove standing timber immediately adjacent to Christmas tree plantations.
- Control weeds around the bases of trees.
Chemical control Apply twice when needles emerge and new shoots are expanding, starting 4 years from expected harvest. Not needed in harvest year, especially for clearcut operations. May take 1.5 to 2 years before benefits are realized.
- Bravo Weather Stik at 5.5 pints/A. Applying chlorothalonil-based products may cause chlorotic or necrotic needle flecking. Group M5 fungicide. 12-hr reentry.
Reference Chastagner, G.A. 1997. Christmas Tree Diseases, Insects, and Disorders in the Pacific Northwest: Identification and Management. Washington State University Cooperative Extension. MISC0186.