We do not see any disease problem but there are a few cultural care tips that might help. Peace Lily plants do best indoors in bright, indirect light, w/regular water, but do not allow the plant to sit in water as this may lead to root rot. Since peace lilies are low light plants, do not place where there is direct sunlight, but do provide bright, indirect light. Black or brown leaf tips sometimes develop when humidity is low or irregular watering practices; place your plants on a pebble tray with water just up to the bottom of the pot to raise ambient humidity. Peace lilies are susceptible to salts in tap water which sometimes causes blackening of leaves. Try using distilled water (steam iron water). Some other things to check on: how often do you fertilize? Make sure you feed with a slow-release or organic fertilizer low in salts that is formulated for container houseplants. Again, make sure the pot has drainage holes, water in the sink and allow the salts to leach out and do not allow it to sit in a saucer or pot with water. Peace lilies are dramatic wilters when they get too dry. They bounce back but it weakens the plant.