Rains and warmer temperatures have promoted rapid growth over the past week.  Now that the root zone has expanded and N uptake has increased exponentially, expect very rapid plant growth in areas which have adequate moisture as we move into flowering.  For those who have not applied an early shot of a plant growth regulator (PGR), you should consider doing so very soon.  Several points should be considered when attempting to regulate growth in 2015.

First, it appears TN has more acres planted to ‘new’ varieties (i.e. varieties which less than three years of data has been collected) than ever before.  Although we have some suggestions from industry trials as to how these varieties will respond to plant growth regulators, much of this dataset is limited to the 2013 and 2014 seasons.  As you recall, temperatures were moderate and rainfall was plentiful during those seasons.  Although 2015 appears to be similar to the past two years, we do not have data on most of these varieties from a more ‘normal’ year.  The table below represents our best guess as to how these varieties will respond.  Classification is based on determinacy (more determinate varieties are typically more responsive and indeterminate varieties are commonly less responsive), growth habit (more vigorous varieties will require more and are therefore considered less responsive) and direct response to the regulator (variety sensitivity is largely influenced by the aforementioned factors, but specific response can vary independently based on plant response to the regulator).

The best approach to managing growth in a new variety is to use multiple, low-rate applications based on plant growth measurements. Several different measurements can be used (and multiple should be considered) but the most emphasis should be placed on current growth.  One of the best ways to gauge current growth is to measure the internode length between the fourth and fifth nodes.

If the distance between fourth and fifth internode is:

• Less than 2”: Growth not adequate. Application is not warranted.
• Between 2” to 3”: Growth is adequate; a low application may be warranted on ‘strong’ ground, but applications in this range are not universally warranted.
• Exceeds 3”: Growth is excessive. The potential for rank growth is high and an application should be strongly considered.

  

Additional thoughts:

• The main goal is to use PGRs to govern vegetative growth until the reproductive sinks begin to naturally govern vegetative growth. Given fruit retention is adequate, the transition to reproductively governed vegetative growth should occur during the effective flowering period.
• PGRs do not shrink the plant; instead, they ultimately reduce internode elongation (among other things). Although yield increases due to PGR applications are not consistent, PGRs consistently increase the penetration of insecticides into the canopy and increase harvest efficiency.
• If applied during a stress (particularly drought stress) or immediately prior to the onset of stress, yields can be negatively affected.
• Several PGRs are currently available, but results of a regional Beltwide study noted no significant differences in effectiveness of formulations of mepiquat chloride, mepiquat chloride + kinetin, mepiquat chloride + cyclanilide, and mepiquat pentaborate when used at equivalent rates (Dodds et al., 2010).