I think some of the problem is sooty mold which is a build up of fungi on the leaf surface. It is often stimulated by aphids (plant feeding insects) and the sugar that they secrete onto the leaf.
"Sooty molds don’t infect plants but grow on surfaces where honeydew deposits accumulate. Honeydew is a sweet, sticky liquid that plant-sucking insects excrete as they ingest large quantities of sap from a plant. Because the insect can’t completely utilize all the nutrients in this large volume of fluid, it assimilates what it needs and excretes the rest as “honeydew.” Wherever honeydew lands—e.g., leaves, twigs, fruit, yard furniture, concrete, sidewalks, or statuary—sooty molds can become established.
Although sooty molds don’t infect plants, they can indirectly damage the plant by coating the leaves to the point that it reduces or inhibits sunlight penetration. Without adequate sunlight, the plant’s ability to carry on photosynthesis is reduced, which can stunt plant growth. Coated leaves also might prematurely age (senesce) and die, causing premature leaf drop."
http://ipm.ucanr.edu/PMG/PESTNOTES/pn...
Please check to see if you have aphids on the leaves- I didnt see them but it can be hard with a photo.
It is also promoted by wet conditions.
Sooty Mold
Aphids on walnut leaf
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