What could it be?
Have a look at this.
Here is a link that might be useful: OSU Index of Willow Diseases
Here is another picture of my diseased weeping willow tree. A close-up of the leaves. The bottom four leaves show the undersides of the leaves: http://tinyurl.com/2vdw2me
Here is a link that might be useful: leaves
bboy, thanks! "Marssonina Leaf and Twig Spot" sounds just like it. Occurs in spring, in wet climates (definitely here), and has same symptoms. All my roses (which I don't do anything with) have black spot every year, too. Apparently that's quite related. Now reading about fungicides and whether to bother or not. A systemic something or other would be nice if I could just water the tree with something and it would magically go away. :)
black spot on roses is an exterior disease.. there is no systemic cure ... that i recall /...
i would predict ... if the willow problem is closely related as you suggest ... that you are not going to find a simple systemic cure ...
do you water in the evening or at night???
when i did roses... the simplest prevention was to insure that the plants went into hot humid summer nights with dry tissue ....
which i suppose might not be possible in the PNW ..
Thank you, ken_adrian. I don't water the willow tree at all, in my climate. It's rainy about 9 out of 12 months here. That's probably the reason for the fungal disease. Fungi like it wet!
When it gets dry here is right in the middle of the growing season, making it in fact desirable - sometimes necessary - to water liberally: irrigation systems are standard on commercial installations. Anyone who want to keep a lawn (or an agricultural crop) green has to water like a fiend.
The most liberating solution to black spot on roses is to replace existing plants with those that are not bothered by it. With some care in selection this is quite possible, I have grown multiple different roses for years without spraying and see very little spotting - only one has turned out to be a stinker, it is a seedling resulting from a wild collection in China - there was no way to pre-screen it for resistance (it looked fine when purchased) as it was not a clonal cultivar with a long history of widespread cultivation to learn from.