What kind of plant is it and where are you (state, region would be helpful)? If it is not an annual, how old is it? Has it been really hot where you are like out here this summer. If so, it could be related to the heat and the amount water and fertilizer you put on the plants. The hotter it is the more dangerous it is to fertilize as it becomes stressful to the plant when it is really hot. It is impossible to accurately guess what is causing the problem without knowing what plant of plant family we are looking at.