Keep green tissue protected from apple scab. Photo: K. Peter, Penn State
The summer days late last week pushed the apple trees in Pennsylvania to finally reach green tip. Coinciding with green tip, the first mature apple scab ascospores have started to disperse. We detected a few mature ascospores from overwintering leaves on April 9; however, the numbers are still very low. The ascospores do not all mature and release at once. The ascospores will gradually disperse, peaking from late pink until petal fall (see figure 1). Consequently, save your best apple scab control products for your sprays from pink until petal fall when the disease pressure can be at its greatest.
Although it is on the chilly side with the temperatures, the amount of leaf wetness hours occurring is enough to trigger an apple scab infection period right now. April 15 – 16 (and maybe April 17) will be an apple scab infection period. If you had applied your dormant copper in the last week, your trees are protected during this infection period. However, this system has dumped at least 2 inches of rain on the region, and there is a good chance whatever had been sprayed has been washed off. With the cooler temperatures this week, another copper spray could be applied if your trees have not reached ½ inch green tip. If you are well past ½ inch green tip, consider using mancozeb only, or Syllit at 1.5 pt/A plus mancozeb 3 lb/A.
To review apple scab, especially the relationship between average temperature and required leaf wetness hours, be sure to check out our Orchard IPM - Scouting for Apple Scab fact sheet.
Figure 1. The release of overwintering scab ascospores peaks from late pink until petal fall.