Fungi that cause needle blight (dieback) and cast (premature drop) are weak pathogens. Even when environmental, plant, and fungal characteristics coincide to favor needle disease, usually only the foliage produced during a single season is damaged or drops prematurely, so trees rarely suffer severe stress or are killed.
Identification
Most needle fungi infect only one or several related conifers and can be identified by their host and characteristic spore-forming structures on dying and dead needles. For example, Douglas-fir needle cast, Rhabdocline spp., affects only Douglas-fir, and mostly those in the North Coast. Cercospora needle blight, Cercospora sequoiae, affects coast redwood after warm, wet conditions.
Elytroderma disease caused by Elytroderma deformans affects only pines, commonly those grown alongside lakes and streams. Unlike other needle cast fungi that develop in foliage, Elytroderma infection becomes systemic and persists in branch tissue, and it can seriously damage pines.
All conifers are susceptible to brown felt blight, or "snow mold." Needles buried under snow die in dark brown to gray masses matted together with fungal mycelium. Brown felt blight of pines is caused by Neopeckia (=Herpotrichia) coulteri. In other conifers, Herpotrichia juniperi (=H. nigra) causes brown felt blight.
Life cycle
These fungi often have life cycles that are timed with the growth and development of the host and may require 1, 2, or more years to complete. Sporulation, spread, and infection by these fungi are frequently restricted to a specific season, such as spring, summer, or fall.
Successful infection depends on favorable conditions. For example, in some needle casts, infection is frequently restricted to the newly developing needles, and sporulation is timed to coincide in spring with needle development.
Solutions
To minimize needle blights, choose a planting site with good air circulation and avoid planting conifers too close together. Provide appropriate cultural care to encourage vigorous plant growth. When irrigating, use drip irrigation instead of overhead sprinkling and avoid irrigating in the late afternoon or evening. Generally, no other management is practical or necessary.
