If the plant has developed brown round spots with red and purple rings, the plant has developed Anthracnose. You need to remove the leaves which are affected and dispose of them away from the plants. Next, treat the plant with a fungicide and repeat this process as required. If you find that the margins of the leaves are fading from green to gray and then turn brown, the hydrangea plants were left dry for a long time. If the petals have turned brown at the tip, you have not applied enough water to the plant. The leaves and flowers will show a lack of water easily.