Mildew is said to be resistant to a fungicide after exposure to the fungicide fails to control the organism. Resistance implies that the mildew has developed the ability to detoxify the fungicide by changing either the intermediates within a metabolic pathway or by changing protein binding sites.

 

Currently grape growers (both conventional and organic) have a diverse chemical arsenal to control mildew, including several compounds with different modes of action. Compounds having a similar mode of action are said to be in the same class, which is to say that they “attack” or disrupt the same metabolic processes to affect the growth of the fungus. Repeated use of fungicides from the same class—one application after another—within a season and over seasons promotes the rapid development of resistant mildew populations. The development of resistant mildew forces growers to rely on the remaining fungicide classes that remain effective, which could lead to overuse of those fungicides, and the loss of another class due to resistance. The loss of any fungicide class also limits a grower’s ability to tailor control programs to their production systems, including cultivar, trellis type and end product. Thus, yield and quality can be compromised by the emergence of fungicide resistant mildew populations, especially on the most susceptible cultivars, such as Fiesta, Carignane, and Chardonnay. Thus, it is important to understand the different modes of action between classes to develop an effective mildew control program that minimizes the risk of resistance.

 

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