Leaves turn brown and the plant withers and dies. Spreads slowly to other plants. Daisies are affected first. No insects are present. The garden is in full sun and watered regularly and fertilized. Could this be a fungus in the soil?
It is impossible to say from the information that is given exactly what is killing your annuals. Fungi that kill plants do not usually kill all plants. Is this area lower than others and does water sometimes stand there? Too much water will kill plants. Too much fertilizer can kill plants. It is possible that digital photos will provide us with more information. vw