Severe powdery mildew infections can abort blossoms. Photo: K. Peter, Penn State
Many areas are in the tight cluster stage of bud development on apples. Now is the time to begin powdery mildew control. Powdery mildew is considered a “dry weather” disease. The fungus does not like prolonged leaf wetness (i.e., apple scab conditions); high humidity and temperatures ranging from 55 - 70°F are enough for the spores to germinate. Be mindful of the dry weather days occurring from tight cluster until the shoots harden off (approximately second cover spray), which is the most susceptible time for infection since the powdery mildew fungus likes young tissue. As tissue begins to grow, the fungus will colonize young, green tissue as it emerges. Symptoms of primary infection include “flag shoots,” which may have stunted growth or die back. On very susceptible varieties, severe blossom infections can occur and fail to produce fruit.
Rain is in the forecast this week and an apple scab infection period is predicted for April 24 – 25, 2018. There is the challenge of controlling for powdery mildew and scab combined with practicing fungicide resistance management during these mixed days of dry and wet conditions. During the current period of green tip through tight cluster bud development, when scab spores are on the lower side, consider using fungicides from FRAC Groups 3 or 9, such as Rally, Indar, Inspire Super, Procure/Trionic, Scala, or Vangard. Syllit, which is FRAC Group M7 can also be included; however, include sulfur for powdery mildew control since Syllit is ineffective for controlling powdery mildew. Alternative management options include sulfur or lime sulfur only (both will manage scab and powdery mildew). Be sure to rotate FRAC Groups and tank mix with a broad spectrum fungicide (mancozeb, ferbam, metiram, ziram) for fungicide resistance management.
There are many products available with the FRAC Group 7 mode action. Several of these products are excellent for both scab and powdery mildew control. Growers are highly encouraged to wait to use the FRAC Group 7 fungicides during pink through petal fall; these fungicides are best saved for peak apple scab pressure, which is from pink through petal fall. Even if dry conditions persist during late pink through petal fall, the FRAC Group 7 products will control for powdery mildew control during this time period.