MaluSim models were run on Monday May 6 for Winchester and Central Virginia (Piney River). The outputs from both locations were fairly similar, with carbohydrate deficits predicted over the next six or so days. This is mostly due to the cloud cover from a series of storms that will pass through the region. With sunnier conditions and warmer temperatures, the model predicts a carbohydrate surplus by Saturday.
Apple trees are still at late bloom to 4 mm fruitlet size in the Winchester area, and growers will probably not need to start thinning until later in the week or early next week.
In Central Virginia, fruitlets are ranging from 8 to 15 mm. Growers should consider reducing rates over the next few days as the model is predicting an aggressive response to standard rates of chemical thinners. However, as we move towards the end of the week, growers should use full rates to ensure an effective response.
It is unclear how much “natural” thinning will occur from this prolonged stretch of cloudy weather. However, work by Dr. Byers suggests that 3 days of cloudy weather with temperatures at 70F (this was a controlled study in growth chambers) can cause 50% of the fruitlets to abscise. With the prolonged stretch of cloudy weather that we are experiencing, growers should consider being less aggressive with rates until the end of the week.
Peck Winchester VA MaluSim 5_6_13