No. 6 — April 29, 2019

Rainfall and Fungicide Efficacy

The amount of rainfall that occurs reduces the amount of fungicide residue remaining of the grape leaf surface. Simulated rainfall events of 0.1, 0.5, 1, and 2 inches removed 25, 30, 65, and 75 percent of one-day old Ziram fungicide residues (Annemiek Schilder, 2010). Even though Ziram residues were removed by rainfall, Phomopsis control remained above 90 percent for all rainfall events. Similarly, this was demonstrated for Captec and Abound. From this research some guidelines were established for when a fungicide should be reapplied in regards to rainfall amount.

  • If 2 inches or more of rainfall on a recently applied protectant fungicide, then the fungicide should be reapplied.
  • If the protectant fungicide is greater than or equal to 7 days old and 1 inch of rainfall, then the protectant fungicide should be reapplied.
  • All fungicides, which includes both protectant and systemic fungicides, must dry completely on the grapevine prior to a rainfall event.

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