Disease of the Month - Aspen/Poplar Tree Leaf Spots / Splotches / Blights

  Hort Snacks - August 2018
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 Leaf & Shoot Blight of Aspen (Venturia macularis); Septoria Leaf Spot (Mycosphaerella populicola); Marsonina Leaf Spot of Aspen (Marssonina populi)

Crops Affected: various aspens and poplars, depending on pathogen species

Disease Cycle:

  • Fungal pathogens
  • Spores are generally produced in the leaf litter, or on twig lesions (depending on pathogen), spreading to infect new leaves
  • Continuous wet weather in spring, and warm/moist conditions in summer, can result in the production of spores and rapid spread
    • Secondary disease development is more common in some diseases (e.g. Marssonina), regardless of conditions, although rate of spread is linked to presence of conditions
  • Angular black spots form on host foliage, with spots growing and merging to cover the entire leaf/shoot, resulting in wilting and necrosis of the entire structure
    • Affected parts continue to elongate, producing a drooping “shepherd’s crook” symptom
  • Can kill a significant number of terminal shoots, in severe situations
  • Initially starts as small, circular to angular, white, grayish, tan, brown, or purplish spots with a dark border
    • Spots may coalesce to form blotches
    • Dark specks (pycnidia – fruiting structures) are scattered in older lesions
  • Small, brownish spots appear on infected leaves from as early as the end of leaf expansion, through July and August
    • Spots enlarge, turn brownish black, and often have a yellow margin and a white center
    • Spots may coalesce to form vein-limited, necrotic blotches
  • Infected leaves may fall prematurely, particularly when infection occurs at/on/near the leaf petiole
  • Stands may appear thin, due to loss of leaves
Venturia leaf blight on aspen
Septoria leaf spot on poplar
Marssonina blight on aspen leaves
Photo by: tidcf.nrcan.gc.ca
Photo by: tidcf.nrcan.gc.ca
Photo by: William Jacobi, Colorado State University, Bugwood.org

Conditions Favouring Development:
  • Generally favoured by cool, wet weather in spring
  • Warm, moist conditions in summer can result in further spread
  • Remove and destroy leaves that drop, and/or twig/branches or other infected material, to reduce inoculum levels and reduce spread
  • Plant resistant species or clones, if available
  • Ensure that there is adequate spacing between plants, to improve air circulation and reduce humidity
  • Fungicides (applied in spring) are largely impractical in the vast majority of situations
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For more information about the content of this document, contact Robert Spencer.
This information published to the web on July 30, 2018.