According to the apple scab model over at NEWA, yesterday’s rain was long enough to cause an infection. Ascospore maturity was at 4% in both Upper Deerfield and Pittstown. Orchards that were not covered before the rain, especially those that had scab last year are at risk of infection.
The following is excerpted from Dr. Dave Rosenberger in Scaffolds, April 14, 2014:
“Orchards that are not sprayed with fungicides before Tuesday’s rain may benefit from post-infection fungicides applied on Wednesday or Thursday. Vangard, Scala, and Syllit are the preferred post-infection fungicides for this stage of bud development, and all three of them will provide 48 to 72 hours of post-infection activity counting from the start of the wetting period. Post-infection activity may be limited to 48 hours if temperatures remain warm, but 72 hr of post-infection activity is possible with lower temperatures, such as those predicted for Tuesday evening, Wednesday, and Thursday.”
Syllit should not be applied when freezing temperatures are expected. It may not mix well with oil, and can be incompatible with other pesticides in high alkaline water. It may not work well in orchards with a history of dodine resistance.
Most orchards in southern counties are past the 1/2″ green stage so additional options would be the DMI’s Rally, Indar, and the DMI/AP premix Inspire Super. Rally at the high rate may reach back 96 hours, it is unclear what the reach back capabilities of the other products are, although they may have at least 48 hours. These products should be avoided where DMI resistance is suspected or known. All of these products will provide control of powdery mildew, while Vangard, Scala, and dodine will not. More information about early season scab control can be found in the Rosenberger and Cox article, Fungicides for Early Season Disease Control in Apples.
Regardless of what product is used, combine it with at least a half rate of a mancozeb product for improved residual control. Remember that once mancozeb is applied at either a half rate or a full rate, it must be applied at the same rate for the rest of the season. Read the label for more information.
