The pollen tube growth model (PTGM) was developed at Virginia Tech. It is based on apple pollen tubes growth rates that were empirically derived under controlled temperature conditions. Model validation has been conducted in Washington, Virginia, and New York orchards.
The PTGM begins when the desired number of king bloom flowers are in full bloom (that is, when the petals no longer cover the reproductive organs thus allowing for cross-pollination). The desired number of open king bloom flowers is equal to the desired crop load and is determined by counting the number of open king bloom flowers per tree or by visual assessment of full bloom density in the orchard. Average style length is measured at this time and is used as a variable in the model.
Hourly temperatures recorded in or near the orchard are used with the pollen tube growth rate equations to calculate cumulative pollen tube growth. Chemical bloom thinning applications are made when the pollen tube lengths are equivalent to average style length. The supposition is that fertilization has occurred at this point.
Assuming that pollen tubes must grow the entire average style length on flowers that reached full bloom after an application of a bloom thinner, the model is reset after the bloom thinning application is made. Additional bloom thinning applications occur before pollen tubes grow to the end of the style to prevent additional fertilization. Applications cease at the end of bloom. Typically, two chemical thinning applications are necessary each year. Occasionally, a third application is necessary.
Additional information about the Pollen Tube Growth Model and bloom thinning in the Eastern U.S. can be found in this Fruit Quarterly article and this Penn State Extension publication