Crop management, fungicide chemistry, disease pressure, and application patterns profoundly affect the emergence of pathogen strains resistant to fungicides. Resistance results from prolonged and repeated use of the same or closely related fungicides. The onset of resistance can be delayed by minimizing selection pressure on the pathogen; it is best to begin this process early in the life of the fungicide or fungicide group. Some crop management and cultural practices can lower disease and, therefore, selection pressure 1) avoid planting in high-pressure areas such as ravines or draws where relative humidity may be excessive, 2) properly manage nitrogen fertilization and plant vigor, and 3) practice sound and effective orchard sanitation and maintenance. All of these methods should reduce the reproductive rate of fungal pathogens and therefore reduce selection pressure. The following fungicide use patterns may delay onset of resistance: 1) apply fungicides only when absolutely necessary, 2) use in a protective, rather than eradicative, 3) rotate fungicide chemistries, and 3) use mixtures of chemically unrelated fungicides 4) use formulations that contain two or more fungicide active ingredients against the target organism.
Fungicide classes differ in their potential for resistance. The Fungicide Resistance Action Committee (FRAC) has developed numeric codes for that various fungicide classes in order to better design resistance management programs. The FRAC class code (or codes in the case of premix fungicide formulations) are normally found at the upper right on the first page of the fungicide label. Compounds possessing low inherent resistance risk include sulfurs, soaps, carbonates, oils, and biologicals. Fungicide classes having high resistance risk include the strobilurin (QoI; FRAC code 11) and benzimidazole (FRAC code 1) classes. Fungicide classes having moderate resistance risk include the DMI (FRAC code 3), quinolone (FRAC code 13), anilinopyrimidine (FRAC code 9), SDHI (FRAC code 7) groups. Mix or rotate all moderate and high risk with fungicides having different modes of action. For example, alternating the QoI fungicides pyraclostrobin (FRAC code 11) and trifloxystrobin (FRAC code 11) is virtually the same practice as using either compound continually. Starting resistance management strategies early in the life of the class and compound is important. When using resistance-prone fungicides for powdery mildew control, alternate with oils, soaps, sulfurs, or calcium polysulfide (Do not use cyprodinil for powdery mildew control). When using these chemicals for apple scab control, alternate them with ziram, captan, or mancozeb. When using premix fungicides for resistance management, it is imperative that both fungicide components are active against the causal organism.