Hello

Are there any blackened areas on the twigs and branches? If so it is likely black canker or willow scab.

When did spots begin to appear?

Willow scab occurs earlier in the season. The undersides of infected leaves are often covered in velvety, olive-brown spore masses. Cankers are common on smaller willow twigs and shoots. Black canker occurs later in the season. Cankers are commonly observed on larger twigs and shoots and are often covered in distinctive pinkish spore masses.

If there is no twig or branch dieback it is likely tar spot or anthracnose on the leaves.

Fungicide sprays must be done early in the spring as buds begin to open. Thorough coverage is important, and repeat applications may be necessary in wet springs until leaves are full sized. Rotate fungicide types used so resistance to one doesn’t build up. Always read and follow all label directions and precautions, please:

http://extension.msstate.edu/publications/the-plant-doctor-how-spray-fungicides-protect-landscapes-gardens-and-turf-residential

Here are common diseases and their treatment:

From the client’s pictures it looks like tar spot.

Tarspot (fungus – Rhytisma salicinum): Spots are very thick, jet black, discrete and about one-fourth inch in diameter. It looks like a drop of tar on the leaf. Rake up an burn dead leaves, as the fungus overwinters on them. Spray early in spring, as buds begin to open,  with a fungicide.( from Texas A&M Agrilife Extension, https://plantdiseasehandbook.tamu.edu/landscaping/trees/willow/ )

Managing Black canker -

https://www.montana.edu/extension/pubs/a-guide-to-pests-problems-and-identification-of-ornamental-shrubs-and-trees-in-montana/diseases/black-cancer-of-willow.html

Managing Willow scab-

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

Twig and leaf samples can be submitted to MSU Plant Diagnostic lab for diagnosis under a microscope to positively ID the disease. The client should submit still live leaves with some spots and still attached to the twigs. There is a modest fee for this service.

Instructions, fee schedule, sample submission form, and mailing information is on the lab website

https://www.canr.msu.edu/pestid/