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Cause Several different fungi have been found by the OSU Plant Clinic associated with root and cutting rots of magnolia. Pythium and Cylindrocarpon spp. have been found most often from rotting roots followed by Phytophthora, Rhizoctonia, Fusarium and even black root rot. Fusarium, Pythium and an Alternaria were found from cutting rots. None have been formally reported from the PNW.
Symptoms Root rot symptoms above ground include yellowing of leaves, slowed shoot growth, premature leaf drop, and tree death. Phytophthora below ground symptoms include brittle, brown to red feeder roots. In time, this root rot generally progresses throughout the root system.
Cultural control
- Improve drainage where plants are grown.
- Do not overwater. Avoid areas of the field where drainage is poor or increase drainage with tiling.
- Avoid reusing pots from a previous crop for propagation. If pots must be reused then wash off all debris and soak in a sanitizing solution or treat with aerated steam for 30 min.
- Use composted hardwood or conifer bark. The suppressive effect should last about 1 year at which time potted plants may be repotted into fresh media.
- Remove and destroy infected plants.
- Collect cuttings for propagation only from healthy stock plants.
Chemical control For control of oomycetes such as Pythium and Phytophthora, focus on cultural controls. Use fungicides as preventative treatments. 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.
- 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/1,000 sq ft followed by at least 0.5 inch rain or irrigation. 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.
- Subdue MAXX is labeled as a soil spray at 1.25 to 2.5 fl oz/1,000 sq ft, irrigated in with 0.5 inch water, if rain does not fall in 24 hours. 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.
Reference Hagan, A.K. Magnolia diseases. In: R.K. Jones, and D.M. Benson, (eds.) 2001. Diseases of Woody Ornamentals and Trees in Nurseries. St. Paul, MN: APS Press.