May 3, 2016
Time for Lakso carbohydrate model in Pennsylvania

More Pennsylvania orchards entered full bloom over the weekend. In some cases, there was as much as a week’s difference in full bloom with some cultivars.

For example in Bedford, Pennsylvania early blooming cultivars like McIntosh and Ginger Gold recorded full bloom on April 22, while later cultivars like Honeycrisp and Golden Delicious were not in full bloom until April 28. At Rock Springs we had early blooming cultivars such as Sunrise in full bloom on April 24 but did not declare full bloom on Gala until April 29. This provided a unique opportunity to look at the difference the full bloom dates made upon the model results, which was nothing. I ran several sites that had different full bloom dates and although there were differences in the 4 Day Average Balance (4DAB) initially prior to full bloom, these disappeared as the sites approached full bloom and are nonexistent now.

Photo: Penn State
Photo: Penn State

Interpreting the Carbohydrate model

There seems to be some confusion on the interpretation of the Carbohydrate model. The 4 Day Average Balance is based on the weather forecast including the current weather forecast and the next 3 days. (Today, plus the 3 day predicted outlook). If you are planning on spraying today (5/3) then the model will look at the predicted high, low and sunlight for 5/3 to determine the daily balance and then also include weather forecasted for 5/4, 5/5 and 5/6 adding the estimated daily balance for those day and average the sum for the four days.

Illustration:

PennCarbModel

To illustrate how the value is derived look at the sample in the table above. Assume that you want to spray April 30, that day is predicted to have a –17.22 4 day average balance (4 DAB). The next three days are predicted to have daily balances of -64.56, -57.62, and -7.38. The average of Lines 1, 2, 3 and 4 = -36.69, which is the 4 DAB for April 30th. Using the same table, if the predicted value of May 1 is correct then the 4 DAB is the average of May 1 through May 4 and equal to -31.53. Finally for May 2nd, the 4 DAB of May 2 through May 5 is -17.18. The 4 DAB is an estimation of how the tree might respond to thinner applications on that date.

Some of the sites are recording small surpluses in carbohydrate balance for the third day out, which is May 6. The decision rules for using the output of the carbohydrate model to adjust chemical thinning rates recommend increasing chemical thinner rates by 30% anytime the 4 DAB is greater than 0. Please note that 4 DAB values beyond today (the bold faced line) are not the average of the next four days but rather the difference between the estimated daily production and the estimated daily demand. The weather has been cloudy and cool, at least in central Pennsylvania. Normally carbohydrate deficits are greater (more negative) when it is cloudy and warm and lower (more towards positive) when it is cool and sunny making it harder to thin. Be mindful of the body of work that Ross Byers did at Winchester where he showed that cloudy weather was a prime factor in inducing fruit drop and proceed cautiously.

In talking with Rich Marini we were thinking the first wave of fruit drop will probably not be evident until the fruit reach 8-9 mm diameter. This may not give much of a window for thinning sprays aimed at the 10 to 12 mm diameter window.

Caution for using the model following freeze injury

As in the past the tables show the carbohydrate production, demand and balance for the last several days and the estimated values for the next few days. The future estimates are only as accurate as the weather prediction and will change if those predicted values do not occur. I asked Alan Lakso about the potential impact of the frosts and any damage to flowers may have on the results, and here is his response:

“The model assumes a healthy tree with good water relations, nutrition and normal function, so it does not try to account for stresses like frost. Such stresses will change the carbon supply if the leaves are hurt and would affect the demand if the number of fruit are reduced. It still will give an idea if there are major carbon deficits in addition to any frost problems. Please do not depend on the model under these conditions.”

If weather forecasts differ from what is predicted, be sure to re-run the model. Chemical thinning recommendations are at Penn State Extension Crop Load Management.

During the next several weeks the carbohydrate model data will regularly be posted at the Penn State Fruit Research and Extension Center Seasonal Monitoring Page, and you will receive a Fruit Times update that links you to the models.

For more information, visit http://extension.psu.edu/plants/tree-fruit.

Robert Crassweller, Penn State University

Source: Penn State University Extension





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