For both Winchester and Central Virginia, the Malusim model is predicting a mild to no carbohydrate stress over the next week. Although temperatures at the time of application, are less important then the carbohydrate running average (2+4 Running Average), expect chemical thinners applied Sunday (10 May) or Monday (11 May) to cause more thinning than applications made late last week or for the rest of this week. Expect a “typical” response from chemical thinners applied Tuesday (12 May) through the rest of this coming week.

The National Weather Service is predicting sunny weather later this week, as well as daytime temperatures in the 70s. This should result in ideal conditions for plant growth, and thus less carbohydrate stress in the apple trees.

One of the weaknesses of weather-based models, is the need to use forecast data to make decisions about future events. When I ran the MaluSim models for both Winchester and Central Virginia last Thursday, the forecast was for hot, cloudy weather. Although it was in the mid-80s in most of Virginia over the weekend, there was also a lot more sunlight than predicted. Temperatures are generally a lot more reliable than cloud cover predictions, but both seemed to be less severe than originally forecasted.

Cloudy weather predicted for later in the week is causing wild swings in the MaluSim model output. My expectation is that actual carbohydrate levels will be more moderate and that running average values will remain in the 20 to -20 g CHO/day range.

Full model output:

Peck Winchester MaluSim 5_11_15.pptx

Peck Central VA MaluSim 5_11_15.pptx

Due to warm days (>80F) and intermittent cloud cover, the MaluSim carbohydrate model is showing a strong carbohydrate deficit for both Winchester and Central Virginia over the next four to five days. Expect an aggressive to very aggressive response to chemical thinners applied today through early next week. Reduce rates, and/or do not include oil or other surfactants if overthinning is a concern.

Pay close attention to the weather forecasts. If actual temperatures are closer to 90F and/or there are prolonged periods of cloud cover, then overthinning will likely occur.

The greatest amount of thinning occurs when fruitlets are on average between 8-12 mm in diameter. Thinning when fruitlets are slightly smaller or larger will result in less thinning, which may be desirable if there is a severe carbohydrate deficit.

Cooler temperatures forecasted for the middle to end of next week should result in less carbohydrate stress and reduced chances of overthinning. However, fruitlets that are greater than 15 mm in diameter are more difficult to thin with NAA or 6-BA.

During these warm days, expect the average fruitlet size to increase by at least 0.5 mm per day.

Full MaluSim model output:

Peck Winchester MaluSim 5_7_15.pptx

Peck Central VA MaluSim 5_7_15.pptx

On Monday, I ran the MaluSim model for Winchester and a season recap for the Central Virginia region.

For the Winchester area, the carbohydrate status has been in the -20 to 20 g CHO/day range since May 15. This means that thinning sprays made between last Thursday and today are predicted to have an average response. For the next couple of days, the model is showing a significant carbohydrate deficit. This is being caused by the cloudy weather that is predicted, as neither day nor night temperatures are forecast to be very high. Cloud cover remains the hardest variable to forecast. If the next two days are sunnier than forecast, or if the rain showers occur at night, then I would not expect the actual carbohydrate balance to go into such a low deficit. However, if the weather forecasts that I used in the model are accurate, then growers should avoid thinning on Wednesday. Either way, the best predictor of the potency of chemical thinners is the 2 days before, day of, and 3 days after (2+4-day) running average (the black line). As you can see, the running average remains between 0 and 20 g CHO/day, meaning that chemical thinners will have an average response.

For the Central Virginia region, I’ve run a recap of the season using weather station data through Monday, May 19. Growers should compare their thinning results to the carbohydrate status at the time they made their application to see how well the model predicted the potency of thinners this year. From my experience, using the MaluSim model as a forensic tool can really help explain why chemical thinners work so differently from one year to the next. As I repeatedly have said, the most difficult part of using the MaluSim model to make real time decisions is having accurate weather data to use as inputs. As I mention above, cloud cover can really impact the carbohydrate status in the tree, and thus the responsiveness of thinners, but it is the hardest variable to forecast. Additionally, weather can be very local. We experienced this issue with the major differences between Tyro and Piney River weather forecasts. We also saw a lot of variable temperature data, with several days between May 8 and 14 being 5 or more degrees Fahrenheit warmer than forecast.

Even with these issues, I hope that the model is providing useful information that helps you make sound management decisions. If you have any feedback, please don’t hesitate to contact me.

I will continue to run the Winchester model until fruit size is too big to thin with NAA and 6-BA.

Download the full reports:

Central VA MaluSim 5_19_14

Winchester VA MaluSim 5_19_14