Fungicides are pesticides that kill or prevent the growth of fungi and their spores. They can be used to control fungi that damage plants, including rusts, mildews and blights. They might also be used to control mold and mildew in other settings. A fungicide is a specific type of pesticide that controls fungal disease by specifically inhibiting or killing the fungus causing the disease. Not all diseases caused by fungi can be adequately controlled by fungicides. These include the vascular diseases Fusarium and Verticillium wilt Diseases caused by other types of organisms, disorders caused by abiotic factors, and insect damage are not controlled by fungicides. Thus it is essential to first determine the cause of symptoms before applying a fungicide.
Many fungicides have targeted activity that imparts high efficacy against specific pathogens, which means low potential for toxicity to humans and other organisms, but also results in a high risk of pathogens developing resistance to the fungicide. A resistant pathogen is less sensitive to the action of the fungicide, which results in the fungicide being less effective or even ineffective. Fungicides that are designed to target specific enzymes or proteins made by fungi do not damage plant tissue, thus they can penetrate and move inside leaves enabling curative properties and increasing the amount of plant tissue protected to more than just where fungicide was deposit when applied. Since the mode of action of these fungicides is so specific, small genetic changes in fungi can overcome the effectiveness of these fungicides and pathogen populations can become resistant to future applications. Disease management strategies that rely heavily upon curative application of fungicides often lead to more resistance problems due to (a) the large size of the pathogen population when the application is made from which resistant individuals are being selected and (b) the difficultly in eradicating a pathogen entirely from inside the plant. Fungicide resistance is covered in more detail in a separate section.
Growers often use disease forecasting systems or action thresholds, when these are available, to ensure fungicides are applied when needed and to avoid the expense and possible environmental impact of unnecessary applications. Forecasting systems have been developed for a number of diseases based on an understanding of the environmental conditions favorable for their development. Typically these are based on temperature and relative humidity or leaf wetness in the area where the crop is grown. Threshold-based fungicide programs involve routinely scouting the crop for symptoms, then applying fungicides when the amount of symptoms reaches a critical level beyond which the disease cannot be controlled adequately. An example of a critical level is one disease spot per five leaves examined. Knowledge of the disease cycle of the pathogen is important when developing and using forecasting systems and thresholds.
