[From A Year of Roses, by Stephen Scanniello, p. 90:] one of the reasons why a rose's petals could turn prematurely brown... Botrytis is especially prevalent when flowers are frequently wet, the air is warm, and the humidity is high. At first the petals will merely appear to be bruised, but they will soon turn to mush. Other times, the flowers simply fail to open, or result in a shattered mess of brown petals. This is very common with very densely petaled roses... [to prevent botrytis] deadhead any infected flowers immediately to prevent the fungus from spreading.