1. As Predicted Previously Weather in the Past 15 Days Favors Apple Bitter Rot; 2. Apply Fungicides According to the Rain Amounts and Frequency

Apple bitter rot is favored by rain events following the warm to hot weather periods like we had in the last 15 days. We also had a recent rain event above 2 inches (Wednesday, 22 June) when fungicide residues from a recent cover spray are completely washed off by rain. A cover spray needs to be re-applied as soon as possible after 2-inches or above rain events to assure protection of fruit from infections by bitter rot and other diseases. When rain events are more frequent like in the last 30 days (Fig. 1) fungicide cover spray applications must follow the rain events and be more frequent when severe rain events occur. One fungicide cover spray is worth 14 days or 2 inches of rain, whichever comes first, but if by any chance no rain falls during the 14 days counting from the last cover spray, you can extend the spray interval to 21 days, provided that no rain falls during the 7 additional days. If rain event does occurs any time during these additional 7 days, apply a fungicide cover before that rain event. Cover spray fungicides must be selected and delivered according to the rain amount and rain frequency, not a fixed calendar spray schedule. For example, apply the single site action fungicides that are very effective against bitter rot in mix with contact fungicide, that broadens the specter of efficacy, just before severe rain events following hot weather periods (for e.g. today’s rain after the past weekend with upper to mid 80F temperatures, Fig. 1). Those would be options in bold below:

  • Aprovia (30 PHI) + Captan 80 WDG 3 lb (or Ziram 6 lb)
  • Aprovia (30 PHI) + Captan 80 WDG 3 lb (or Ziram 6 lb)
  • Flint Extra 2.9 fl oz or Pristine 14.5 oz or Luna Sensation 5.8 fl oz + Captan 80 WDG 3 lb (or Ziram 6 lb)
  • Omega 500 (28 PHI) + Captan 80 WDG 3 lb (or Ziram 6 lb)
  • Flint Extra 2.9 fl oz or Pristine 14.5 oz or Luna Sensation 5.8 fl oz  + Captan 80 WDG 3 lb (or Ziram 6 lb)
  • Omega 500 (28 PHI) + Captan 80 WDG 3 lb (or Ziram 6 lb)
  • Captan 80 WDG 3 lb (or Ziram 6 lb)
  • Merivon 5.5. fl oz or Pristine 14.5 oz + Captan 80 WDG 3 lb
  • Captan 80 WDG 3 lb
  • Merivon 5.5. fl oz or Pristine 14.5 oz + Captan 80 WDG 3 lb

The order of cover sprays can change as long as you rotate fungicides of different classes from cover to cover (group 7 or SDHIs like Aprovia, with group 29 like Omega, with group 11 fungicides like Merivon, Luna Sensation, Pristine, and Flint Extra) and pay attention to use the fungicides with longer pre-harvest interval (PHI) period early in the summer i.e. far from harvest (e.g. 28 and 30 days for Omega and Aprovia) so you do not risk getting too close to harvest. Reserve your zero days PHI fungicides like Merivon and/or Pristine + Captan for the last cover sprays, i.e. just before harvest. Finally, any small rain amount(s) and likely the dew or fog events will favor bitter rot infections on apple fruit without any fungicide residues on them or with very low fungicide residues remaining after a previous severe rain event (2 inches and above).

Limit the application of group 11 fungicides only to 4 times per season in total (four underlined fungicides above) to avoid development of fungicide resistance in Colletotrichum species of fungi that cause apple bitter rot. This does not mean you can apply 4 applications of each of these materials: Flint Extra, Sovran, Luna Sensation, Pristine or Merivon. It is quite the opposite. Since they all belong to the same group 11 fungicides, which means that they have the same mode of action in the fungi, you can only use 4 applications in total of any of the fungicides in this group 11, per growing season. You can use either one group 11 active ingredient or choose different group 11 active ingredient just as long as their total number of applications does not exceed 4 in total per growing season (whichever is cheaper for your budget). It is best to save Merivon or Pristine for your last, i.e. third and fourth application of group 11 fungicides, just before harvest due to the zero days PHIs of these materials. This is especially needed if you will do multiple picks. Captan can be replaced with ziram and/or ferbam, however, do not apply Ziram within 14 days of harvest and ferbam is not recommended for late season cover sprays on fresh market fruit due to the potential for dark residue spots on fruit at harvest. Do not apply Ferbam within 7 days of harvest. Flint Extra, Aprovia, Omega, Inspire Super, Topsin M, Merivon, Flint, Pristine, Luna Sensation, Ziram and Ferbam will also be effective for control of sooty blotch and flyspeck.

Figure 1. Weather conditions in Winchester VA showing sharp high temperatures in the last 15 days overlapping with severe rain events which favor bitter rot infections (Courtesy of RIMpro BV, Keizersgracht 482, NL-1017 EG Amsterdam, The Netherlands).