Magnolia-Bacterial Blight

Cause Pseudomonas syringae pv. syringae, a bacterium favored by cool, wet weather in spring. It is found on almost all plants and causes a wide variety of diseases. Although the bacteria survive on the outside of the plant they must get inside and multiply in the space between plant cells (apoplast) to cause disease. These pathogenic bacteria inject several proteins and small-molecule toxins to get past host immune mechanisms. Once inside, the bacteria induce a watery, nutrient rich environment between the plant cells where they can multiply and continue colonization of the plant tissues. Bacteria also produce a protein that acts as an ice nucleus, increasing frost wounds that bacteria easily colonize and expand. Twig dieback has also been associated with old pruning wounds.

Symptoms New shoots wilt, become necrotic, and die back in spring. Dead, necrotic leaves may remain attached to the shoot after the branches die back. Large, dark, irregular spots also may be on leaves and have been reported on flowers. Leaf spots are small, dark brown, water soaked, and often surrounded by a yellow halo. Spots enlarge and may be limited by leaf veins becoming angular in shape.

Cultural control

  • Keep plant foliage as dry as possible.
  • Minimize wounds to limbs and new shoots.
  • Prune out and destroy infected shoots and branches during the late dormant season.
  • Space plantings to provide good air circulation.
  • Container-grown plants may need a different pH depending on the cultivar or species grown. Magnolia soulangiana and M. stellata like a low 5.5 pH, while M. acuminata and M. salicifolia can withstand higher pH ranges.

Chemical control In British Columbia, apply copper-based bactericides at least once in fall and twice in spring near budbreak. Bacteria resistant to copper products and antibiotics have been found in Pacific Northwest nurseries.

  • Badge X2 at 1.5 to 2 lb/A. Group M1 fungicide. 48-hr reentry. O
  • Copper-Count-N at 1 quart/100 gal water. Group M1 fungicide. 48-hr reentry.
  • CuPRO 5000 at 1.5 to 5 lb/A but only up to 2 lb/A when new growth is present. Group M1 fungicide. 48-hr reentry.
  • Junction at 1.5 to 3.5 lb/A. Group M1 + M3 fungicide. 48-hr reentry.
  • Monterey Liqui-Cop at 3 Tbsp/gal water. H
  • Nu-Cop 50 DF at 1 lb/100 gal water. Group M1 fungicide. 48-hr reentry.
  • Phyton 27 at 1.5 to 3.5 oz/10 gal water. Group M1 fungicide. 48-hr reentry.

Reference Canfield, M.L., Baca, S., and Moore, L.W. 1986. Isolation of Pseudomonas syringae from 40 cultivars of diseased woody plants with tip dieback in Pacific Northwest nurseries. Plant Disease 70:647-650.