Cause Phytophthora cinnamomi and P. lateralis, soilborne fungus-like microorganisms that have been reported from Oregon. Other species have been reported from several other states. The disease is favored by wet or waterlogged conditions during warm weather. The fungus-like microorganism can produce swimming spores that are attracted to roots and can move from one infected tree to another in saturated soil or between flooded pots in a nursery. Inoculum also may be found in containers previously used to grow other crops. Many cultivars are susceptible; however, some may show similar symptoms simply from waterlogging and not necessarily from infection.
Symptoms Plants are stunted with blue-gray foliage. This symptom intensifies as heat and water stress begin in early summer. Foliage eventually turns brown as the plant dies. Single branches may die first, which makes the disease look as if it is on only one side of the plant. Root and root crown tissues are extensively decayed or necrotic. Sometimes, necrotic root crown tissue extends several inches above the soil line. Some cultivars show an "off" color in the winter months but "green up" when growth resumes in spring; however, plants affected by root rot do not "green up" in spring.
- Improve drainage where junipers are grown.
- Isolate new shipments from main growing area and observe them for signs of disease.
- Use only new pots to propagate plants. If pots must be reused then wash off all debris and soak in a sanitizing solution or treat with aerated steam for 30 min.
- Remove and destroy infected plants.
- Some junipers, such as J. horizontalis, prefer a pot pH of around 5.5.
Chemical control Focus on cultural controls. Use fungicides as preventative treatments. Tank-mix and/or alternate products from different groups with different modes of action to prevent the buildup of resistant fungi. The Group 4 and P7 fungicides used to manage Phytophthora do not kill this organism. They can only prevent establishment of the organism before it gets into the plant. They can also prevent continued growth if the organism is already inside the plant thereby delaying symptoms that might have developed. Once chemical activity has subsided with time, the organism can resume growth within infected plants.
- Aliette WDG at 2.5 to 5 lb/100 gal water. Foliar spray only. Do not use with adjuvants. Group P7 fungicide. 12-hr reentry.
- Alude at 5 to 10 fl oz/100 gal water applied as a soil drench at a rate of 1 gal solution/sq yd. Follow application with irrigation. Can also be used as a foliar spray at 26 to 54 fl oz /100 gal water at 14- to 21-day intervals. Group P7 fungicide. 4-hr reentry.
- Areca at 2.5 to 5 lb/100 gal water as a foliar application. Group P7 fungicide. 12-hr reentry.
- Banrot 40 WP at 6 to 12 oz/100 gal water. Group 1 + 14 fungicide. 12-hr reentry.
- Empress at 1 to 3 fl oz/100 gal water can be used for cuttings or seedlings. Group 11 fungicide. 12-hr reentry.
- Fosphite at 1 to 2 quarts/100 gal water. Do not use copper products within 20 days of treatment and do not use spray adjuvants. Group P7 fungicide. 4-hr reentry.
- Mefenoxam 2 AQ at 0.98 to 1.96 fl oz/100 gal water as a soil drench or at 1.23 to 2.45 fl oz/1000 sq ft followed by a least 0.5 inch rain or irrigation. Not a curative treatment. Group 4 fungicide. No restrictions on reentry when used as a soil drench or media incorporation.
- MetaStar 2E at 1 to 4 fl oz/100 gal water as a drench. Group 4 fungicide. No restrictions on reentry when used as a soil drench or media incorporation.
- Organocide Plant Doctor at 16 fl oz in 16 fl oz water plus 1 oz Pentra-Bark as a basal trunk spray. Also labeled for injection, see label for details. Can be used in landscape sites. Group P7 fungicide. H
- Phospho-Jet at 1 to 2 qt/100 gal water as a foliar spray or soil drench. Group P7 fungicide. 4-hr reentry.
- Subdue MAXX is labeled as a soil spray at 1.25 to 2.5 fl oz/1000 sq ft, irrigated in with 0.5 inch water within 24 hours of application. Not a curative treatment. Group 4 fungicide. 48-hr reentry.
- Terrazole 35 WP at 3.5 to 10 oz/100 gal water. Group 14 fungicide. 12-hr reentry.
- Truban 30 WP at 3 to 10 oz/100 gal water. Group 14 fungicide. 12-hr reentry.
Reference Peterson, E. K., Rupp, F., Eberhart, J., and Parke, J. L. 2020. Root rot of Juniperus and Microbiota by Phytophthora lateralis in Oregon horticultural nurseries. Plant Disease 104:1500-1506.