Defending Your Orchard From the Risk of Fungicide Resistance

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Every orchard grower understands the challenge of controlling fungal diseases. Although fungicide applications are the most effective strategy for disease management, the rise of fungicide resistance poses a significant risk to efficacy.

Growers are often unaware they have a fungicide resistance issue until devastating diseases like blossom blight/brown rot, jacket rot and hull rot have caused substantial and irreversible damage to their crops. To protect tree nut health and maximize yield, growers should consider implementing proactive strategies to address fungicide resistance.

Understanding Fungicide Resistance

Fungicide resistance, as defined by the Fungicide Resistance Action Committee (FRAC), is an acquired reduction in sensitivity of fungi to specific fungicides. The key challenge is the genetic variability of fungal populations, which leads to fungicide resistance in certain fungi within the population. Without a proactive approach, those resistant fungi can rapidly reproduce, causing widespread fungicide resistance across the orchard.

Effects on Tree Nut Growers

Fungicide resistance poses a serious threat to growers’ bottom lines.

With studies indicating potential preharvest losses ranging from 10% to 23% and postharvest losses of 10% to 20%, the stakes are high for growers across the industry.1 Implementing a proactive plan at the beginning of the growing season can give tree nut growers peace of mind when diseases, such as blossom blight/ brown rot, jacket rot and hull rot, occur in their orchards.

Spotting the Signs

Fungicide resistance is generally not found until there is an issue in the crop, but staying alert during the growing season can help growers spot signs of resistance. Avoid repeated use of fungicides from the same FRAC group, talk with neighbors about their treatment outcomes for different diseases and stay informed on resistant strains to avoid ineffective treatments.

Best Management Practices

To avoid the development of fungicide resistance, consider these best management practices:

  • Rotate fungicides with different FRAC groups to reduce the selection pressure on fungi population.
  • Scout orchards daily for signs of disease development.
  • Implement integrated disease management strategies.
  • Apply the recommended label rates with adequate coverage.
  • Apply fungicides preventively — prior to disease development.
  • Select varieties with resistance to common tree nut diseases. .

Defending tree nut orchards from fungicide resistance starts with informed decisions and proactive strategies. Visit to learn more.

1Steinberg, G., and S. J. Gurr. 2020. Fungi, fungicide discovery and global food security.