Alder (Alnus spp.)-Leaf Spot

Latest revision: 
March 2023

Cause Septoria alnifolia, a fungus that starts as a foliar pathogen and later causes severe stem cankers in alder nursery seedlings. Infected nursery seedlings with cankers must be culled prior to planting to prevent mortality and stem breakage.

Symptoms Small, dark yellow to pale brown, circular to irregular spots occur on leaves and stems. Spots are 1 to 20 mm in diameter with a dark brown somewhat raised margin. Large areas of the leaf may be covered when spots coalesce. Small dark fruiting bodies, pycnidia, form in the center of spots.

Cultural control None are specifically listed but the following may be helpful in a nursery or landscape.

  • Rake and destroy fallen leaves.
  • Maintain tree vigor with adequate water and fertilizer.
  • Space trees out in nursery production for good air circulation.
  • Avoid overhead irrigation that keeps plants wet for an extended period of time.

Chemical control

  • Eagle 20 EW at 6 to 12 fl oz/100 gal water. Group 3 fungicide. 24-hr reentry.
  • Myclobutanil 20 EW T&O at 6 to 12 fl oz/100 gal water plus spreading agent. May observe a PGR effect. Group 3 fungicide. 24-hr reentry.
  • Spectracide Immunox Multi-Purpose Fungicide Spray Concentrate for Gardens at 1 fl oz/gal water. Group 3 fungicide. H

Reference Constantinescu, O. 1984. Taxonomic revision of Septoria-like fungi parasitic on Betulaceae. Transactions of the British Mycological Society 83:383-398.