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peach orchard

February 15 marked the conclusion of the measurement of chill accumulation for our area. The three chill models used to measure chilling offer mixed results and therefore, differing predictions on how crops will respond to the amount of chilling received this season. The predicted chill accumulation from the Weinberger model, which measures chill as hours of exposure at 45°F and below, is 32% higher than chill accumulation predicted by the Modified Weinberger model. The modified Weinberger measures chill accumulation as hours of exposure to temperatures between  32°F and 45°F. Additionally, Dynamic model, which measures chill as chill portions, suggests that more chilling has been accumulated than the Weinberger or Modified Weinberger models indicate. This provides a more positive outlook for chill accumulation in the peach crop. There are no means to convert between the chill hour models and the Dynamic model currently, but comparisons between the models have been made based on the extent of symptoms of lack of chilling in the fruit and bloom period duration. From these comparisons, an adjusted chill hour value was created (Table 1). According to Table 1, adjusted chill hours are 18% higher on average than the chill hours based on the Weinberger model.

Table 1. Chill Accumulation at Select Locations in Alabama as of February 15, 2020

1The 45 °F measures chill 45 °F and below.  The modified 45 model measures chill from 45 °F to 32 °F.

2The Dynamic model measures chill in terms of chill portions rather than chill hours.

3Adjusted chill hours are chill hours adjusted according to observations made of the fruit considering both the number of chill portions and chill hours received.  For example, a fruit from a tree that received 45 chill portions may have fruit similar in appearance to fruit that received 800 chill hours based on previous observations.  Values are subject to change as more observations are made and information obtained.

4Current data not available.  Most recent values reported for Sand Mountain (on 1/20/2020) were 786 chill hours, 454 modified chill hours, and 47 chill portions.  Most recent values reported for Tennessee Valley (on 1/27/2020) were 893 chill hours, 544 modified chill hours, and 52 chill portions.

LocationChill Hours (45 °F Model1)Chill Hours Modified (45 °F Model1)Chill Portion (Dynamic Model2)Adjusted Chill Hours3
Auburn Univ. Turf Unit (Auburn)61744045800
Black Belt REC (Marion Junction)78652747850
Brewton Ag Research Unit (Brewton)67844439750
Chilton County REC (Thorsby)77851450850
Durbin Farms (Clanton)75754953950
E.V. Smith Research Center (Shorter)64639035650
Gulf Coast Station REC (Fairhope)42829436650
Sand Mountain REC (Crossville)4----
Tennessee Valley REC (Belle Mina)4----
Upper Coastal Plain Substation (Winfield)1084724591050
Winfred Thomas Ag Research Unit (Hazel Green)115879865>1050
Wiregrass REC (Headland)50737742800

Chill Hour and Dynamic Models

Knowledge concerning the amount of chill accumulated in temperate fruit crops has increased over the years. It is now known that chilling is accumulated over a wider range of temperatures and that certain temperatures provide more chilling than other temperatures. Additionally, it is known that accumulated chilling can be lost if temperatures become unseasonably warm. The Dynamic model makes adjustments for these conditions and provides a better idea of the amount of chilling accumulated for our area.

Option for Management of Reduced Chilling

Low chill varieties, which includes mostly early season varieties, should not show symptoms of lack of chill according to both the Weinberger model and Dynamic model. Some moderate chill and all high chill varieties will likely display symptoms of lack of chill. Growers have used rest-breaking compounds to encourage more intense bloom and a bloom period that is more concentrated. If use of rest breaking compounds such as hydrogen cyanimide is planned, no more than a 2% concentration should be used.

In deciding whether to use rest-breaking compounds, consider the performance of the varieties from over the last 10 years. In addition, application of hydrogen cyanimide will depend on the pollen development inside of the bud. Observation of pollen development can be made in the field (Figures 1 and 2). With a sharp razor, gently cut the bud lengthwise being careful to remove only enough tissue to observe the anthers inside of the bud. If the anthers are greenish white or translucent, hydrogen cyanimide can be applied. If anthers are beginning to turn yellow, consider applying a lower rate of hydrogen cyanimide or not making an application at all in that variety. Application at this stage could result in bloom loss or advanced bloom, making blooms susceptible to late season freezes. High chill varieties with a chill hour requirement of 950 and above will benefit most from rest-breaking compounds. Application of rest-breaking compounds should be considered with caution. The effects are highly variable and depend on several factors.

This season, the chill models provide different information, which provides different interpretations of chill accumulation. There is a considerable difference between the Weinberger and the Modified Weinberger models. Both models predict a significant deficiency in chilling; however, with the Dynamic Model, less chilling deficiency is indicated. According to the Dynamic model, chill accumulation should be consistent with normal production seasons where symptoms of lack of chilling were found but production season was not severely impacted. Table 2 shows final chill accumulation over a three-year period.

Table 2. Three-Year Chill Accumulation in North, Central, and South Alabama

1The 45 °F measures chill 45 °F and below.  The modified 45 model measures chill from 45 °F to 32 °F.

2The Dynamic model measures chill in terms of chill portions rather than chill hours.

3Current chilling not available for this location in 2020.  Most recent chilling was reported on 1/27/2020.

4Data from Dynamic model not available for 2018.

LocationOld 45 °F Model1 (2018)Old 45 °F Model1 (2019)Old 45 °F Model1 (2020)Modified 45 °F Model1 (2018)Modified 45 °F Model1 (2019)Modified 45 °F Model1 (2020)Dynamic Model2 (2018)Dynamic Model2 (2019)
North Alabama (Belle Mina)3,41,3601,215>1,0001,138856>750-62
Central Alabama (Thorsby)41,1481,024730976778466-58
South Alabama (Fairhope)4706616407519348273-39
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