Whatever resistant strains that South Carolina's peach orchards and strawberry farms have created effects me. In order to control brown rot on peaches, I have to spray at least two fungicides with different modes of action, either in a tank mix or rotated. I quit growing strawberries due to the difficulty in controlling botrytis that's resistant to almost everything available. Topsin(thiophanate methyl) used to be a super fungicide back in the 1970s. By the 1980s it was worthless on brown rot and botrytis. Quadris(azoxtstrobin) would control virtually everything in the 1990s. Within a few years there was widespread resistance. It only takes one or two hardhead farmers with a lot of acreage to cut their application rates or refuse to rotate fungicides to create a monster. Then they would blame the fungicide manufacturers instead of themselves. Fungicide efficacy trials have shown that applying some fungicides will actually increase the amount of disease compared to the unsprayed controls. Those fungicides only kill the weak, susceptible strains but allow the stronger and faster growing strains to spread like wildfire.With regard to resistance to insecticides or herbicides, any agent that is not 100% lethal to the target organism serves as a selection agent favoring an increased proportion of the resistant individuals. Sometimes, the rare genes that impart resistance may simultaneously create a competitive disadvantage compared to individuals without the resistance gene. The "cidal" agent flips that relationship around. Agronomists sometimes recommend alternating insecticides (or herbicides) with agents that utilize a different biologic mechanism.
The very same process occurs with antibiotic use for infections, only more rapidly. Whereas insects or weeds may run through a new generation once or several times a year, many bacteria create a new generation as frequently as every 20 minutes.
So every chemical agent that we use as an antibiotic, insecticide, fungicide, viricide, or herbicide will eventually become mostly useless, over a span of years--unless it is used in a manner that assures 100% lethality (e.g. tincture of iodine).
Bob