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Some dogs tend to like the texture of certain things such as leaves or a certain object. I have seen some dogs that love to eat the darnest things. Unfortunately this is a condition called Pica, (Simple term for when a animal eats a non food item). It is always recommend to speak with your vet to have a blood test done they can do a panel to make sure they are not missing anything that is causing the behavior.
If all turns out well, and he continues the habit you will have to break him of it because the leaves can cause a blockage of his digestive track. The last thing you want is a expensive surgery to clear the blockage.
You can help break him of the habit by firmly telling him no when he gets around the leaves. Spraying the leaves with a mixture of bitter apple and water (With a hose is easiest) will deter the behavior as most dogs despise the taste of bitter apple. Other things you can use is a product called Forbid or Tabasco sauce works well to.