This article was written by Tom Kon and Jim Schupp (Penn State University).

Until recently, eastern apple growers did not have access to a registered and effective chemical blossom thinner. As of 2019, specific formulations of liquid lime sulfur can be used for apple crop thinning in several eastern and Midwestern states. Additionally, a series of variety-specific pollen tube growth models are available to eastern apple growers in 2019. These models are used to inform application timing of lime sulfur thinning programs. These technologies are relatively new to eastern thinning programs – this is a reference to get you started.

Introduction

Liquid lime sulfur (LS) has been registered as a blossom thinner for apple in the Pacific Northwest since 2003. Of all blossom thinners evaluated since the loss of Elgetol in 1990, the most consistent blossom thinning programs include LS (Schmidt et al., 2011). A survey by the Washington Tree Fruit Research Commission (WTFRC) found that ~60% of all apple acres in WA are being thinned with LS (Tory Schmidt, WTFRC). Growers in WA have used pollen tube growth models (PTGM) for timing LS thinning sprays since 2014. In 2018, over 1600 blocks of apples were registered to use the PTGM in WA.

Specific formulations of LS were recently labelled for blossom thinning in several eastern states as well [NovaSource™ Lime-Sulfur Solution (Tessenderlo-Kerley, Inc.); Rex Lime Sulfur Solution (Or-Cal, Inc.)]. Registration of these products extends the range of effective chemical thinning options to include bloom. Additionally, seven variety-specific pollen tube growth models are available to commercial apple growers in 2019 through the Network for Environment and Weather Applications (NEWA). These models are used to help with application timing of lime sulfur thinning programs.

How it works

Lime sulfur thins flowers by inhibiting pollen germination and pollen tube growth in the pistil, preventing fertilization of the flower (Figure 1). Unlike other pollinicides, LS has about 24 hours of kick-back after the pollen grain germinates (Yoder et al., 2009). This post-germination activity extends the time for making an effective thinning spray. LS is also a photosynthetic inhibitor (McArtney et al., 2006). A temporary reduction of apple leaf photosynthesis can also contribute to thinning by causing a brief period of carbon stress. Spray oil can be added to LS to enhance its penetration and boost its efficacy. When tank mixed with spray oil, LS is used at lower concentration.

Figure 1. Pollen tubes glow vividly (greenish-blue) when stained and viewed using fluorescence microscopy. Abundant pollen germination and growth on an untreated apple stigma and style (left). Application of liquid lime sulfur and oil can inhibit pollen germination and growth in susceptible flowers (right), resulting in reduced fruit set.

Spray Application and Rates

Application of LS as a blossom thinner has a very small target: the pistil of unfertilized flowers. Thorough coverage is essential. For well-pruned dwarf and semi-dwarf trees, a spray volume 80-100 gallons per acre via air-blast is typically used. For small trees with narrow open canopies, 50 gallons per acre may be enough, while older larger trees may require 200 gallons per acre to obtain thorough coverage. Avoid excessive spray volume to minimize leaf damage and fruit russeting. The action of LS is based on its concentration, not on the rate per acre. Do not concentrate the chemicals when spraying at lower volume.

LS is applied at 4-10% (v/v) when used alone. When mixed with oil, use LS at 1.5 to 2% (v/v). Oil options include fish oil at 2% (v/v), dormant petroleum oil at 1%, or summer oil at 1 to 1.5% v/v. Our trials in the mid-Atlantic indicate that an oil + LS combination is more effective than LS alone.

Two LS applications during bloom are suggested if possible. If weather conditions are not conducive to a 2^nd application or if less aggressive blossom thinning is desired, a solitary application would still be of benefit in multi-step thinning programs (nibble approach). In years of a protracted bloom period, a 3^rd application is possible. Until more data regarding the appropriate number of LS applications in the east is available, we suggest using no more than two LS applications during the bloom period.

Timing LS thinning sprays

Before the PTGMs were developed, timing of LS thinner was by visual estimates of the percentage of open blossoms. Typically, two sprays, one at 20-30% bloom followed by a second spray at 80-100% bloom. If you don’t have access to the models or choose not to use them, this method of timing can still be done with satisfactory results. The model just makes the timing more precise.

Inconsistent blossom thinning responses may be attributed to application timing, since pollen tubes can reach the base of the style within 48 hours (Yoder et al., 2009). Blossom thinner applications that occur at full bloom or later can increase fruit russet (Byers, 2003). To improve application timing of LS + oil, researchers at Virginia Tech and the WTFRC developed predictive models to estimate pollen tube growth rates. After over a decade of research, pollen tube growth models have been released for seven varieties (Cripps Pink, Fuji, Gala, Golden Delicious, Granny Smith, Honeycrisp, and Red Delicious). Several LS thinning trials have been conducted in the mid-Atlantic and southeast using the PTGM with positive results.

Pollen tube growth rates for each variety are estimated based on hourly temperatures. To use the PTGM, average style length must be measured in each block. Additionally, the number of open blossoms must be actively monitored. Once the number of open blossoms is equivalent to the desired crop load, the model is initiated. Detailed instructions for using the PTGM on the NEWA interface are provided here: PTGM Guidelines.

Precautions

When applied properly under favorable weather conditions, LS is safe to use as a blossom thinner. LS can cause apple leaf burn and fruit russet. Avoid use of LS when high temperature is forecast to exceed 85°F within 24 hours. The potential for injury is greater under slow drying conditions (low light, high humidity). Avoid spraying under these conditions and use the lowest effective rate and spray volume if you must thin when high temperatures and poor drying conditions are imminent. Tender foliage that grew under cool, cloudy damp conditions is tender and more likely to show symptoms.

Do not tank mix other spray materials when thinning with LS or LS plus oil. Additional applications of either oil or LS immediately after LS thinning sprays is very likely to increase thinning response. Oil and Captan fungicide should never meet on your trees. To allow oil residues to weather, leave a 2 to 3 week interval between the last spray containing oil and the first captan spray.

If there is high risk of severe frost or freeze damage during the bloom period, consider delaying or avoiding application of LS as a bloom thinner.

What to expect after thinning with LS

Petal browning is common and can be observed within a day of application (Figure 2). While visually unappealing, petal browning is perfectly normal and does not indicate a problem. If applied under favorable weather conditions, minor leaf phytotoxicity (leaf curling, yellowing, marginal necrosis) may occur. If LS is applied under suboptimal weather conditions, significant damage to spur leaf tissue can occur. Excessive damage to spur leaves is undesirable. Spur leaves are important in promoting fruit growth, and injury to these tissues has been shown to have negative impacts on fruit size, fruit set, and fruit mineral content (Ferree and Palmer, 1982). Again, only apply LS under favorable weather conditions to minimize damage to spur leaf tissue.

Figure 2. Petal browning (left) and leaf damage (right) can occur after LS applications.

Since LS inhibits flower fertilization, the effects of this thinner on fruit drop can be seen shortly after petal fall. An advantage of this strategy is that a clear size difference between setting and shedding fruit can be observed in advance of making a subsequent thinning application at 8 to 12 mm fruit diameter (Figure 3). Growers can adjust rates of post-bloom thinners based on the efficacy of the blossom thinning application(s). To take advantage of this interval, the grower may not wish to apply a petal fall thinner and wait to observe the effect of blossom thinning on fruit set.

Figure 3. After thinning with LS, a clear size difference between setting and shedding fruit is observed.

Regulatory and Safety

Use a LS product that is labelled for thinning in your state. For 2019, in NC and PA, that product is NovaSource™ Lime-Sulfur Solution. Read the label of the spray oil you intend to use and make sure that it is labelled for use on apple at bloom. Do not make more than 3 applications of LS for blossom thinning per season.

NovaSource™ LS has an REI of 48 hours. Refer to the label for a list of PPE for pesticide handlers, applicators and for early re-entry. Note that LS is hazardous to eyes, so spray applicators and other handlers must wear goggles or a face shield.

Conclusion

Many eastern growers now have access to a consistent and effective blossom thinner and predictive model to aid in application timing. When compared to later application timings, blossom thinning has the greatest potential to increase fruit size and can promote return bloom in the following season. Unlike petal fall thinner applications, growers have an opportunity to assess the efficacy of LS application(s) before applying post-bloom thinner. However, these benefits are not without risk. To minimize the risk of damage to fruit and spur leaves, only apply LS under favorable weather conditions. Use common sense regarding frost risk and pollinator activity during the bloom period. We suggest small-scale evaluations of LS and/or the PTGM until experience is obtained in your orchard. Strongly biennial (Fuji, Honeycrisp, Delicious, etc.) and/or small-fruited varieties (Gala and Pink Lady) might be a good place to start. Additionally, inhibiting/reducing fruit set on young or newly planted orchards would be another natural area that LS could fit into crop load management programs.

This article was written by Tom Kon and Jim Schupp (Penn State University).

Literature cited

Byers, R.E. 2003. Flower and fruit thinning and vegetative: fruiting balance. p. 409-436. In: D.C. Ferree and I.J. Warrington (eds.), Apples: botany, production and uses. CAB International, Oxfordshire, UK.

Ferree, D.C., and J.W. Palmer. 1982. Effect of spur defoliation and ringing during bloom on fruiting, mineral level, and net photosynthesis of ‘Golden Delicious’ apple. J. Am. Soc. Hort. Sci. 107:1182-1186.

McArtney, S., J. Palmer, S. Davies, and S. Seymour. 2006. Effects of lime sulfur and fish oil on pollen tube growth, leaf photosynthesis and fruit set in apple. HortScience 41(2):357-360.

Schmidt, T.R., T.D. Auvil, I. Hanrahan, F. Castillo, and J.R. McFerson. 2011. Crop load management of tree fruits in the Pacific northwest of USA. Acta Hort. 903:759-766

Yoder, K., R. Yuan, L. Combs, R. Byers, J. McFerson, and T. Schmidt. 2009. Effects of temperature and the combination of liquid lime sulfur and fish oil on pollen germination, pollen tube growth, and fruit set in apples. HortScience 44(5):1277-1283.