Many gymnosperms especially those in the cypress family are affected by the fungal pathogen - Phomopsis juniperovora. Species most commonly affected include eastern red cedar, and the savin, creeping, and Rocky Mountain junipers. Some other plants that are affected to varying degrees include American and Oriental arborvitae; Douglas-fir and true firs; Chinese and common junipers; European larch; and jack pine. In addition, there are many other members of the cypress family found around the world that are affected. Besides the species P. juniperovora, there are other species of the genus Phomopsis that cause diseases on other plants.
For most people, the first symptom noticed in the spring (early summer) is the die-back of the new shoot growth (tip blight). The new growth changes from light yellow green to red brown to ash gray as it dies from this fungal disease. Phomopsis juniperovora only kills the new growth - if more than just the new growth is killed, other fungal diseases or environmental injury is involved. Infection starts on immature scale leaves or needles, whereas mature needles are resistant. This infection progresses into the underlying shoots, causing a small lesion at the junction of healthy and diseased tissues. Small stems (less than one third inch in diameter) are usually girdled by these lesions, causing tip death. Older branches (more than one third inch in diameter) are more resistant to infection, and lesions that form on this size stem usually heal. In advance stages of infection, small black spots (the fungus fruiting bodies called pycnidia) can be seen with the unaided eye or a magnifying glass. The pycnidia are embedded in the dry, ashen gray lesions on the stems and needles but often break through as they mature, releasing spores (conidia) that are spread by rain and wind. The spores can tolerate some drying and may remain viable within diseased tissue for as long as two years.
Infection occurs in the spring from conidia that overwintered on shoots or stems killed the previous year. Late season infections may occur if cultural practices (improper watering and maintaining high fertility) encourage prolonged plant growth. Prolong wet and warm conditions increase the severity of infection and symptoms on the susceptible hosts. The central part of the plant is often more affected than the outer portion, with the new growth showing almost continuous infection. Under certain conditions favorable for fungal development, entire young shrubs and trees may be affected, and all the evergreens needles and stems will die and turn brown. Severe infection for several years in a row, may result in the death of larger or older plants.
Plant only resistant juniper species, varieties and cultivars. Avoid planting in poorly drained areas. Avoid wounding or injuring when planting or cultivating. Where practical, prune out and burn or bury all blighted plant parts as the they appear, but restrict pruning and shearing to dry weather. Do not use infected branches or needles as a mulch. Sterilize pruning tools between cuts and especially between plants, using rubbing alcohol or use a twenty-five percent solution of a chlorine bleach. Keep in mind that aggressive, maintenance pruning will promote the growth of new and susceptible tissue, so wait for the dry conditions of mid June to mid July for this activity. If overhead sprinkling or in ground sprinklers must be used, water early in the day to allow for fast drying of plant tissue.
Written by James Schuster, Retired Extension Educator, Horticulture & Plant Pathology, and reviewed by Bruce Paulsrud, Plant Pathology, University of Illinois at Urbana-Champaign.