Plant Doctor Archive

Silver birch

I live in Te Horo about 60 km north of Wellington. The weather has been wet and humid over the last few weeks. I have four long lines of silver birch trees bordering a driveway — about 40 of them. They are dropping lots of leaves and the leaves are yellowed with brown spots rather like the black spot infecting my roses. Is this a blight? It's not very practical to spray as they are very tall and large, well-established trees.

Silver birches tend to be as tough as old boots. They do, however, get a leaf spot disease a lot like the symptoms you describe, but this usually occurs in autumn when they are supposed to lose their leaves, and it does them little harm. It sounds like the wet weather has caused a disease outbreak and the leaf-drop.

Perhaps the trees are not completely happy? While silver birches usually do well in either wet or dry soils, a sudden change, perhaps due to lack of drainage, could trigger an attack.

As you say, spraying is not very practical, but you could reduce the amount of fungal spores present in the area by removing dead leaves and branches from around your trees. You could also get an arborist to carefully prune your trees to let a little more airflow occur.

Advice by Dr Dan Blanchon from Unitec's Diploma in Sustainable Horticulture and Bachelor of Resource Management.

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