Modern fungicides generally provide better disease control with lower potential negative impact on the environment and nontarget organisms, including farm workers, than contact fungicides (chlorothalonil, copper, mancozeb, sulfur). Superior disease control is partly due to their ability to move into plant tissue, which they can safely due without damaging plants because of their targeted activity against pathogens. Often the movement is just translaminar within the leaf where they were deposited when applied, but very important as this means they are moving to the underside of leaves where many pathogens primarily infect and develop best, plus inside they are protected from weathering, in particular from rain.
The disadvantage of targeted activity is it often is due to single site mode of action, and some pathogens have proven adept at evolving such that the fungicide is no longer active against them, most commonly by a simple change to the molecular site where the fungicide binds. Research on fungicide resistance is essential for developing sound fungicide recommendations for growers. Growers applying a fungicide ineffective because of resistance likely will not have an observable impact on control when the other fungicides applied to the crop for the disease have not been impacted by resistance (multiple fungicides legally have to be used because of label restrictions on number of consecutive and total applications for each fungicide that can be applied).
The cucurbit powdery and downy mildew pathogens are two that have proven adept at developing resistance. Seedling bioassays have been used to detect fungicide resistance in pathogen populations, in particular in commercial crops. Leaf disk bioassays have been used to examine resistance in pathogen isolates (individuals). This research has documented multi-fungicide resistance to unrelated chemistry in the cucurbit powdery mildew pathogen.
Cucurbit powdery mildew:
Cucurbit downy mildew:
See also: