PLANT GROWTH REGULATORS:
WHAT IS NEW AND WHAT WORKS

By
Dr. Earl J. Seeley

Unlike my oral presentation to the 2002 Utah State Horticultural Association, in this "written presentation" I will attempt to outline which uses of Plant Growth Regulators (PGR’s) have been shown to give consistent, economic returns and define what is required to maximize the results.

Promalin: This material has been available and used widely for over 20 years, and is now available in several ‘generic’ formulations. Tests of three of these ‘generic’ materials have shown that they are as effective in affecting fruit size and type as the original material. The key factors in obtaining the desired effects appear to be the material, application factors and the temperature during and immediately following application.

Type Enhancement: For many years this material has been used to enhance the length to diameter ratio (type) of apples. For the first 10 to 15 years of use, the results were somewhat variable, but we finely figured out that the temperature during the 3-5 days following the application plays a significant role in determining the degree of effect obtained. The material acts by enhancing the rate of cell division in the fruit during the period when the accelerated growth rate at the base of the sepals causes the ‘calyx to close’. The cell division process is highly temperature dependant (will roughly double for each 10 degree rise in temperature above 600 F.). When the application of Promalin is followed by 5 days when the temperature reaches the high 70’s or 3 days when the high reaches the mid 80’s we see maximum effects. On the other hand, if the temperatures remain in the low to mid 60’s little if any effect is obtained.

After many years of experience, the following rules (suggestions) have been found to maximize the economic returns from the use of this material:

For Size Enhancement: Promalin and Provide have been used to enhance the size of fruit, particularly Gala. There are several different ways in which it has been used, but generally a number of "low rate" applications have been used at 10 to 14 day intervals following petal fall. Temperature is important in obtaining consistent results form these materials when used for size enhancement. The following rules (suggestions) should enable the grower to obtain consistent results.

The size enhancing value of this material is dependant upon its ability to stimulate cell division during the early stages of growth of the ovary wall (fruit). A slight acceleration early may be very significant. In most cases the most noticeable effect will be on the ‘fruit type’ with an extended calyx. This can consistently increase fruit weight by 10% - 15%. Under favorable temperatures the size increase can be 20% as some increase in diameter will also occur. This is particularly important where significant premiums are obtained for larger fruit sizes as in Gala.

There is significant disagreement as to the effect of these early season Promalin applications on return bloom. Based on my experience, if the grower uses these materials when significant economic returns can be expected, (i.e. when daily high temperatures are expected to be 700 or above) there will be little or no adverse effect on return bloom. If, however, these materials are used during years when the post petal fall daily high temperatures are 600 to 650 or lower for 3 weeks or more, an adverse effect on return bloom may be experienced.

During the post bloom period in the 2000 growing season we experienced average daily temperatures that ranged from 50 to 70 below normal. The following year (2001) most blocks had much less bloom than we expected, given the degree of thinning that was obtained by the bloom and post bloom chemical thinning programs in the spring of 2000. This effect has been observed several times in Washington State during the last 20 years (particularly following the 1984, 1985, 1992, and 1996 seasons). This lack of return bloom probably occurs because photosynthetic rates are much lower when the temperature is in the 500 to 700 range than when temperatures are 700 to 850. During these "cool seasons" use of promalin is much more likely to result in a decrease in return bloom.

Use of Promalin for Fruit Finish Enhancement: In some varieties and environmental conditions the value of the crop can be drastically reduced by formation of russet on the fruit surface. This is particularly a problem with Golden Delicious in the Eastern U.S. and in the West when we have a wet early spring. Extended periods when the fruit surface is wet contribute to the formation of russet. The fruits are particularly sensitive to this adverse reaction to surface wetting during the period when the calyx is closing and the ‘stem basin’ is forming. During these periods susceptible varieties should not be sprayed when the drying time will exceed 30 – 40 minutes. Other measures should be taken that will tend to reduce the duration of surface wetting of the fruits.

Significant control of russet can be obtained by the use of Provide at 10 to 12 ounces per acre in 100 gallons of spray solution at petal fall and petal fall plus 10 to 12 days. In some cases a 3rd or 4th application may be necessary if the wet weather persists for an extended period. Equal russet control can be obtained by use of Promalin at the same timing and rates as are used in Gala for fruit size enhancement. When these materials are used for this purpose it generally means that temperatures are cooler than would be desired for the size and type enhancement that can be obtained from these materials but the reduction in lenticel and surface russet can be dramatic. Timing is very important as the material must be applied before the russet conditions begin. A delay of only a few hours can result in unacceptable russet levels.

Use of ‘Retain’ for color enhancement, harvest delay, and ‘stop-drop’ activity: Over the past 10 years we have done a great deal of work with this material with results ranging from spectacular to dismally disappointing. In the last 6 years we have found that it can be very useful and the results are much more predictable when we use the material at 50% of the regular recommended rate. For reasons unknown, when the full rate is used there will be times when fruit will undergo rapid softening as the fruit approaches the ‘normal’ harvest date.

As the material is expensive, delays color formation and is often used to delay harvest timing into better coloring weather, this unexpected result can be costly. Consistent results have been obtained with Red Delicious and Gala with the use of ½ rates, without the risk of the "premature fruit softening".

An additional ‘side effect’ of this material when used at full rates is that following CA storage the fruit flavors are somewhat surprised for the first several days that the fruit is out of CA conditions. This problem is particularly noticeable if fruit is removed from CA after 30 to 90 days, and is much less when the CA period is 120 days or longer. Generally, this effect can be minimized by: 1. Allowing at least 5 days out of CA before consumption of the fruit, or 2. This has not been a problem when ½ rates were used in the field.

The effect of this material on the amount of Red color developed by Delicious and Gala are usually dramatic, and the treatments will return several times the treatment costs if treatment dates are properly timed. Given the costs and potential returns of these treatments I would strongly suggest that you consult and work closely with your packer and/or crop advisor.

Use of Gibberellin in Sweet and Tart Cherry Production: Pro-Gibb, when applied to the foliage of a young tree, 2 to 4 weeks after ‘petal-fall’, at a concentration of 25 ppm will inhibit formation of flower buds for the following season. This will result in significantly more "vegetative growth" during these early years and will allow the grower to bring his trees into production earlier. If wind, excess heat, or marginally dry soil conditions should occur, the treated tree is less likely to set terminal buds, stop growing and form flower buds than an untreated tree. In addition, it is apparent that use of this material at this time, when combined with other sound cultural practices can contribute significantly to the economic life of a planting. This is probably because the faster vegetative growth, stronger root development and other interacting factors, delay the onset of the "death or loss of function" of the inner layers of the Xylem. Since the effect is seen the year following the application it is important that the practice be properly timed.

We are investigating the effect of GA materials at the pit hardening and ‘straw color’ stage on the dry weight of individual fruits. As the use of the tart cherries has shifted from the 5 + 1 institutional pack, to the dried fruit product, our knowledge of ways to enhance the % dry weight has lagged. If tart cherries behave in the same manner as sweet cherries we may be able to decrease the ratio of pitted cherries to dried product significantly, which would further enhance our competitive position over other growing areas for this product.

We have instigated research into the use of Apogee for shoot growth control, color and return bloom enhancement, and will be testing it further in commercial fireblight control. Because of the timing of application, the materials anti-GA mode of action, and other factors it will take significant testing before this material should be widely used. I would suggest that you began to get familiar with this material and its uses, because I think that it will be a major tool in the near future.