dropping heads

I have a large garden which has a great deal of multiple headed sunflowers. I have been noticing heads dropping off of some of my plants as if somethings chewed off the bloom several inches from the blossom on the stem.. In addition a few of entire plants are suddenly wilted and droop baldly until I remove them and dispose of their remains. My garden is a mountain desert garden so I have to water by hand daily and I thought that perhaps I have some sort of fungal issue....not sure....please help?....Thanks David