Looks like you've got a bunch of things going on there. There's fungus eating some leaves. There's fungus eating the fungus that's eating the leaves (saprophytes). The plant looks a bit starved for nitrogen. That general yellowing could come from lots of stuff but that fact that it's happening in the older leaves but the younger (top) leaves look okay makes me think at least part of the problem is nitrogen. That's because nitrogen can move around in a plant and it will be shuffled away from older leaves to younger. That helps weaken a plant and decrease its ability to fight off diseases. It also looks as though the nitrogen problem may be masking, to a some extent, micronutrient defficiensies as well (these too weaken plants' resistance to infection). Check out the the yellow leaf with green viens. It's tough to pin point a micro problem when lack of nitrogen sort of "washes out" signs so I would say give the plant some triple fifteen and some common micros. Most common are iron, magnesium, zinc, and mangenese (and maybe copper). Also, are you over-head watering? If so, switch to drip irrigation or hand watering to keep water off the leaves. You may also want to water early in the day rather than in the late afternoon or evening. And, it's hard to tell what that is behind the plant but it looks like it might be a window or shower door. If the plant is in a pot or something, move it to a place where it can get good air circulation. If you can't move the plant, tear down that part of the house. (J/K ;P) The right fertilizers and water can go a long way to help the plant fight disease. Also, it's tough to tell if it's mildew or what, but I see what might be some whitefly on the underside of one leaf. If so, just pick it off and toss it. Check the rest of the plant for the same.