Tom's Blog

Monday, September 24, 2018

2018 Survey for Bur Oak Blight at Pleasant Valley Conservancy Update 9-30-2018

Update Sept 30, 2018

The report below was written on Sept 23. Since then, lots more trees are showing symptoms of bur oak blight.

Many are large trees, including some icons.

I am hoping that the unusually humid weather this year is responsible, and that no long-term damage will befall our bur oaks.

We'll see what next year brings!


I first reported bur oak blight in a small number of trees at PVC in 2017.

Infected leaves exhibit purple-brown lesions along the veins on the under side of the leaves. These lesions gradually expand and in many cases the leaf turns completely brown. Black fruiting structures of the causal agent, the fungus Tubakia iowensis, appear and are diagnostic for the disease.

Although not fatal, we felt it desirable to continue monitoring this affliction. The trees that were infected last year leafed out normally, but symptoms started to appear in late August. Yesterday, about a month later, the symptoms were more extensive, although some leaves were still not infected.

We also noticed blight on some trees that had apparently been fine last summer, including the classic open-grown bur in Unit 10 that I have often used in photographs. Also, our oldest bur oak, the patriarch of the savanna, does not seem to be infected as much this year as last.

But most of our bur oaks, both small and large, remain healthy.

Bur oak blight infection is apparently associated with humid weather cycles and the past two years have been unusually rainy. Since long-term weather patterns generally alternate between wet and dry years, we can hope for some drier years (although nothing like the 2012 drought!).

Since the symptoms do not appear until late summer/early fall, the leaves on infected trees have most of the growing season to function normally, photosynthesizing and translocating nutrients to the roots. Even now, most leaves are only partially affected.

Bur oaks regularly lose there leaves earlier in the fall (usually mid October) than do most other oak species, so it won’t be long before the leaves will be dropping. Thus, there is a narrow window, mostly the month of September, when bur oaks can be monitored for blight.

The scientific paper describing this disease in detail can be downloaded from this link.


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