Septoria leaf spot of poplar

The causative agent of this disease is the ascomycete fungus Sphaerulina musiva (with its anamorph Septoria musiva). This pathogen is recognized as a major threat to poplar cultivation, capable of causing both foliar lesions and severe stem cankers that can girdle and kill young trees.

The disease primarily affects various species and hybrids within the genus Populus. Balsam poplars and their hybrids are known to be particularly susceptible. The fungus targets leaf blades, petioles, and young green shoots, often causing significant damage in nurseries and newly established plantations where tree spacing is dense.

Symptoms initially appear as small, angular brown spots on leaves, often surrounded by a yellow halo. As the infection progresses, these lesions merge, leading to premature defoliation. On stems, the fungus creates sunken, necrotic cankers that can eventually encircle the branch or trunk, leading to dieback of the portion above the lesion.

The development of the fungus is favored by warm, humid conditions and frequent rainfall. Rain splash and wind-blown spores are the primary methods of transmission throughout the growing season. The pathogen overwinters as mycelium and fruiting bodies on fallen leaves and within cankers on infected twigs still attached to the trees.

The damage caused by Sphaerulina musiva includes reduced growth rates, branch dieback, and increased mortality among young trees. Management strategies should be centered on an integrated approach:

• Selecting and planting resistant poplar clones.
• Removing and destroying infected plant debris.
• Implementing sanitary pruning of infected branches to reduce inoculum levels.
• Applying fungicides during the spring bud-break period to protect new growth.