16 April 2014: Low temperatures early this morning (Wed) varied by region, but ranged from 28 to as low as 23 across the Hudson Valley. More freezing temperatures are predicted for early Thursday morning. Green tissue in apple buds that have been frozen is especially susceptible to uptake of spray materials for a day or two after being frozen because ice crystals that form in the green tissue create microscopic injuries in plant cells that serve as entry points for anything that is applied to the injured leaves. The injuries created by freeze damage will heal as leaves grow, but applications of pesticides should be delayed as much as possible during the day or two immediately following sub-freezing temperatures.
Products such as oil, copper, and captan can be especially toxic if absorbed into leaves, but I suspect that many other fungicides can also exacerbate freeze-injury if they are applied immediately after leaves thaw. Of course, the worst-case scenario is to spray trees BEFORE buds thaw because the melting ice crystals will create mini-vacuums that will actively pull the the spray material into the leaf tissue. (Remember, ice occupies more volume that the equivalent amount of water!)
Even in the absence of any spray-exacerbated injury, freeze-damaged cluster leaves are likely to show crinkling and other malformations as they expand. However, the severity of that injury will be much greater if damaged leaves are sprayed too soon after a freeze. At the recent ISHS Symposium on Physiological Principles and Fruit Production that was held in Geneva, several speakers noted that cluster leaves provide much of the energy required for flowering and fruit set. Therefore, minimizing injury to cluster leaves is important for maximizing productivity.
Most areas of the lower Hudson Valley (Ulster/Dutchess Counties and south) had at least a light Mill’s apple scab infection period on Tuesday, and in some areas the temperature and duration of wetting was sufficient to generate a moderate infection period. Obviously, apple scab can also damage cluster leaves, so orchards that were unprotected during Tuesdays infection period may need a post-infection spray no later than about noon in Friday. However, given the freeze-damage that will be present in cluster leaves, I would avoid applying captan, oil, or copper in any sprays applied Thursday or Friday.
