Hello,

There are three diseases common to willow that might be causing these symptoms on your tree. These three diseases are leaf spot, black canker and willow scab.

Willow scab and black canker, both caused by fungal pathogens, can sometimes infect a tree at the same time. If this happens, the combination of the two diseases is called Willow blight. Willow scab occurs earlier in the season, March/April, and black canker occurs in May/June. 

With willow scab you should see olive and brown masses of spores on the undersides of the leaves. Black canker develops as a pinkish mass of spores. With both fungal infections, the leaves eventually turn brown and fall off the tree. Treatment of both of these diseases is difficult. A fungicide spray could help but larger trees such as yours are impractical to spray. 

There are also a couple of leaf spot diseases common to willow. Pseudocercospora spot is one and Marssonina spot is the other. 
Pseudocercospora spot causes brown lesions with purple centers and Marssonina spot causes brown or black lesions, sometimes with white centers. 

 Several insects are attracted to willows. From what I can see in the pictures, there does not seem to be insect damage. You might examine the tree closely and search for signs of insects or fungal spores.

In looking at the pictures that you have sent, I cannot make a definitive identification of the problem, though it does appear to be a fungal issue. The most accurate option for disease identification is to send a sample of the infected tree branch and stems to an independent lab. The C. Wayne Ellett Plant and Pest Diagnostic Clinic (PPDC), run by OSU, is such a lab. I have listed a link below for their website. The website explains how to send the lab a sample to them if you wish to do so. The PPDC can make recommendations for treatment according to the diagnosis.

For now, pruning away the diseased limbs and disposing of that plant material would provide some benefit to the tree. Keeping the tree well watered would also help, willows thrive on water. The tree should receive at least an inch of water a week. The willow is also competing for moisture with the hosta that's planted at the base of the tree. Moving the hosta off to the side and away from the tree trunk, or to another area, might help the 
willow receive more water. 

It would also be a good idea to create a tree ring around the tree, cover the area with a 1-2" inch layer of good quality mulch, and keep it weed-free. Trees and other plants that are stressed from adverse environmental conditions such as drought are often more prone to disease and insect infestation. 

I have listed several links below for articles about willow diseases and insects. Please take a look at this information and write back if you should have more questions.`

Thanks for asking Ask Extension.

Betsy B.- MGV

https://www.uaex.uada.edu/yard-garden/plant-health-clinic/disease-notes/posts/willow-common-diseases.aspx

https://www.purduelandscapereport.org/article/black-canker-of-willow/

https://hyg.ipm.illinois.edu/article.php?id=734

https://hortsense.cahnrs.wsu.edu/fact-sheet/willow-marssonina-leaf-twig-spot/

https://bygl.osu.edu/node/1061

https://ppdc.osu.edu/