I ran the MaluSim model today (4/19 pdf). Despite the low-light conditions that are expected with this weekend’s cloudy weather and rain, the predicted cooler temperatures will allow plants to maintain a relatively high carbohydrate status. With the MaluSim model, this is depicted as a numerically positive carbohydrate balance. Based on the current weather forecasts (I checked several and also ran a MaluSim simulation using National Weather Service data) we can expect an average to below average response from chemical thinner applications that are put on tomorrow (Friday) through the early part of next week. In blocks with no or minimal frost damage, this would suggest that you can increase your rates, or add an adjuvant, such as oil to increase thinning activity. In blocks with frost damage some additional thinning may result and you may want to reduce rates.

Many varieties are approaching 10 mm size; however, the model is not showing a good “thinning window” over the next 10 days. Fruitlet growth may slow down over the next few days with the cooler temperatures, but it is unclear if we will have warmer temperatures and a better window towards the end of next week. I’ll run another MaluSim simulation again early next week.