The "burnt" tips on the leaves are caused by soluble salts that are either in your tap water or in the fertilizer you use, if you use one. Sometimes, even if you don't fertilize, you might notice a white buildup on the soil's surface in your pots. Those are minerals in your tap water that can be harmful when they build up. If you see the white residue, you can scrape it off. It's also a good idea to take your plants outside or to the bathtub and flush them with water for about five minutes. That can help wash the excess minerals down through the soil profile and out the drainage holes in the bottom of the pot. The yellow spots on your corn plant are probably caused by a bacterial or fungal disease. Such diseases are basically untreatable in houseplants, but you can remove the stricken leaves and hope that it won't spread to the entire plant.