L.D. Combs

Apple Pollen Tube Growth Rates Are Regulated by Parentage and Environment

A greater understanding of apple (Malus ·domestica) pollen tube growth rates can improve crop load management in commercial orchards. Specifically, applications of caustic bloom-thinning chemicals need to occur when enough, but not too many, flowers have been fertilized to achieve crop load densities that balance yields with marketable fruit sizes. In this study, the pollen tube growth rates of five crabapple (Malus sp.) cultivars were measured in the styles of three maternal cultivars at 12, 18, 24, and 30 8C after 24 hours in a growth chamber. Pollen tube growth rates were greatest for ‘Selkirk’ and ‘Thunderchild’ at 12 8C, and greatest for ‘Indian Summer’, ‘Selkirk’, and ‘Thunderchild’ at 24 8C. Pollen tube growth increased with increasing temperatures until 24 8C. There were minimal pollen tube growth rate increases between 24 and 30 8C. Overall, ‘Snowdrift’ had the slowest pollen tube growth rate of the five evaluated crabapple genotypes. At 24 and 30 8C, ‘Indian Summer’ and ‘Thunderchild’ pollen tubes reached the base of the style most frequently, and ‘Snowdrift’ pollen tubes the least frequently. Pollen tube growth rate was also influenced by the maternal cultivar, with Golden Delicious having relatively faster pollen tube growth than Fuji at 24 and 30 8C. Interactions among paternal and maternal genotypes as well as temperature after pollination reveal complex biological and environmental relationships that can be used to develop more precise crop load management strategies for apple orchards. Apple pollination occurs when pollen is transferred, often by an insect vector, from the anthers of one blossom to the stigma of another. After pollen grains are deposited, they are rehydrated by stigmatic secretions, and pollen tube growth begins (Dennis, 2000; Jackson, 2003). Genetically compatible pollen tubes grow through the stigma and style and toward the ovaries. At the end of the style, the pollen tube enters the ovary through the micropyle, where two sperm cells are released into the ovule. Fertilization occurs when one nucleus fuses with the egg cell, and the second nucleus fuses with the embryo sac, initiating seed formation (Dennis, 2000). Investigations into apple pollen biology have been underway for over a century (Adams, 1916) and have recently been reviewed by Ram ırez and Davenport (2013). In commercial apple orchards, cross-pollination between two distinct genotypes results in greater fertilization rates and thus a greater fruit set, even in orchards containing self-fertile cultivars (Dennis, 2003). Various Malus species, colloquially referred to as crabapples, are preferred pollinizers in commercial orchards due to their long bloom periods, disease resistance, and abundant pollen production (Fitzgerald, 2005). In particular, the crabapple Malus baccata ‘Manchurian’ has been widely planted in commercial orchards as a pollinizer. However, it has fallen into disfavor in recent years due to its high susceptibility to canker diseases, which could limit apple exports into Asian markets (Hansen, 2014). Other popular crabapple pollinizers include ‘Snowdrift’, ‘Thunderchild’, ‘Indian Summer’, ‘Mt. Blanc’, ‘Wickson Crab’, and ‘Chestnut Crab’. The pollen tube growth rate of ‘Snowdrift’ has been used to develop a temperature-driven pollen tube growth model that can predict the time between pollination and fertilization (Peck et al., 2016; Yoder et al., 2013). Themodel is now being used by commercial apple growers to precisely calculate when to apply bloom-thinning chemicals (Lehnert, 2014). The majority of bloom-thinning chemicals are caustic and prevent pollination or fertilization by destroying flower parts and inhibiting pollen germination and pollen tube growth. Liquid lime sulfur (calcium polysulfide) is the most studied and commercially used bloom-thinning chemical (McArtney et al., 2006). Although Snowdrift pollen tube growth can differ by as much as 3-fold among maternal cultivars, published reports on the pollen tube growth rates of other crabapple species and cultivars are limited (Embree and Foster, 1999; McArtney et al., 2006; Stott, 1972; Williams, 1965). Successful application of blossom-thinning chemicals is highly dependent on understanding the rate of pollen tube growth among different pollen donor genotypes (Jackson, 2003). Analyzing the pollen Received for publication 13 May 2016. Accepted for publication 29 Aug. 2016. This project was funded by Virginia Agricultural Experiment Station and Virginia Tech’s Department of Horticulture. We thank Abby Kowalski, Ashley Thompson, Taylor Mackintosh, and David Carbaugh for their assistance in the field and lab, Mizuho Nita for guidance with statistics, and Roger Harris and Tony Wolf for review of the manuscript. Corresponding author. E-mail: gmp32@cornell.edu. 548 J. AMER. SOC. HORT. SCI. 141(6):548–554. 2016. tube growth rates of additional pollen donors could contribute to the selection of improved pollinizers for apple orchards. The objectives of this study were to determine how pollen tube growth rates were affected by paternal (pollen source) and maternal cultivars, temperatures after pollination, and the interaction among these factors. We did this by comparing the pollen tube growth rates of five crabapple cultivars on three maternal culinary apple cultivars at four temperatures. Materials and Methods Experiments were conducted at Virginia Tech’s Alson H. Smith Jr. Agricultural Research and Extension Center (AREC) in Winchester, VA. In 2013 and 2014, pollen was collected from mature ‘Evereste’, ‘Indian Summer’, ‘Selkirk’, ‘Snowdrift’, and ‘Thunderchild’ trees located at the AREC. Only healthy blossoms in the late pink flower stage (when petals still covered the sexual organs and before dehiscence of anthers) were used (Chapman and Catlin, 1976). Pollen was collected by removing anthers from the blossoms using a fine-toothed comb, allowing them to air-dry, and then pressing the anthers through a 200-mesh sieve (74 mm between wires). Pollen was stored in microcentrifuge tubes at –12 C until use. Pollen collected in 2013 and 2014 was combined for each crabapple cultivar. Pollen viability was scored in two samples for each crabapple cultivar after 24 h at 21 C on an agarose (10 g L), sucrose (100 g L), and boric acid (10 mg L) medium by calculating the percent germination (Williams and Maier, 1977). Mature ‘Autumn Rose Fuji’/‘M.9’, ‘Golden Delicious’/ ‘M.27’, and ‘Cripps Pink’/‘M.9’ trees grown in 19-L root bags (Lacebark, Stillwater, OK) were the maternal cultivars selected for this study. Whole trees (including the bagged root system) were removed from the orchard in Jan. 2015, placed in 19-L buckets, and kept in cold storage to accumulate sufficient winter chill hours to complete endodormancy. ‘Cripps Pink’ and ‘Fuji’ trees were kept at 4.4 C for 400 h, and ‘Golden Delicious’ trees were kept at 4.4 C for 700 h; after the chilling hours had been met, trees were moved into a greenhouse (mean temperature 24 C) to force bloom. Maternal flowers used in the experiment were selected during the late pink flower stage. During each temperature trial, pollen from the five paternal cultivars was hand-pollinated onto 35 flowers from each maternal cultivar; seven unique pollinated flowers for each paternal cultivar treatment. An additional seven emasculated, nonpollinated flowers were left on the tree to serve as nonpollinated controls. All side blooms and extra flower clusters were removed from the tree to prevent cross-contamination. Flowers were tagged to identify the pollen source, and a sterilized, size 6 artist’s brush was used to apply pollen directly to the stigma. Pollen was applied in a uniform way until pollen grains were visible on the stigma. Immediately after pollination, trees were placed into a temperature-controlled growth chamber (Intellus Control System; Percival Scientific, Perry, IA) for 24 h under a 12-h photoperiod. For each maternal cultivar, the experiment was repeated at four ambient air temperatures: 12, 18, 24, and 30 C for the duration of the experiment. After 24 h in the growth chamber, flowers were removed from the tree in the order they were pollinated and placed into glass vials filled with a 5% aqueous sodium sulfite (Amresco, Solon, OH) solution. Storage vials containing the flowers were placed in boiling water for 20 min to soften the tissue. The pistillate portions were then excised from each flower and rinsed with distilled water. The styles were individually detached from the ovary using forceps and scalpel. The styles were stained with a water-soluble fluorescent solution containing 0.01% aniline blue dye (MP Biomedicals, Solon, OH) in 0.067 M K2HPO4 for pollen tube visualization, then pressed between two microscope slides (Embree and Foster, 1999). Slides were viewed at 10· and 40· with a light microscope (Eclipse Ci; Nikon, Table 1. Percent crabapple pollen germination, Spring 2015 first bloom and petal fall dates, and flower color of crabapple trees used for paternal cultivar pollen collection. Cultivars were all planted in 1999 on M.9 rootstocks. Paternal cultivar Pollen germination (%) 2015 First bloom 2015 Petal fall Flower color Evereste 65 22 Apr. 4 May White Indian Summer 75 17 Apr. 28 Apr. Pink Selkirk 85 15 Apr. 22 Apr. Pink Snowdrift 65 22 Apr. 4 May White Thunderchild 90 17 Apr. 24 Apr. Pink Fig. 1. ‘Thunderchild’ pollen tube growth in a ‘Fuji’ apple 24 h after pollination at a constant temperature of 24 C, visualized with water-soluble fluorescent solution containing 0.01% aniline blue dye in 0.067 M K2HPO4 and magnified 10· using a light microscope and a fluorescent light source. J. AMER. SOC. HORT. SCI. 141(6):548–554. 2016. 549 Tokyo, Japan) and fluorescent light source (Intensilight C-HGFI; Nikon). Histological examination for each individual style was conducted as described in Yoder et al. (2009) and Yoder et al. (2013), and included pollen germination/tube growth on the stigmatic surface (0% t

Managing apple crop load and diseases with bloom thinning applications in an organically managed ‘Honeycrisp’/‘MM.111’orchard.

Although demand for organic produce continues to increase in the midAtlantic, few apple (Malus 3domestica Borkh.) growers in the region have adopted organic management practices due to the considerable disease, insect, and weed pressure, as well as the lack of effective crop loadmanagement tools. In this study, lime sulfur (LS) and Regalia (R) were applied in different sequences (i.e., LS/LS, LS/R, R/R, and R/LS), each in a mixture with JMS Stylet-Oil, to chemically thin apple flowers in an organically managed ‘Honeycrisp’/‘MM.111’ orchard. There was also a nontreated control, a ‘‘grower standard’’ control (LS at 11 mm fruitlet diameter), and a hand-thinned control. The treatments were evaluated for their ability to reduce crop load, as well as to control powdery mildew [Podosphaera leucotricha (Ellis & Everh.) E. S. Salmon], cedar apple rust (Gymnosporangium juniperi-virginiana Schwein.), and quince rust (Gymnosporangium clavipesCooke & Peck). All treatments reduced crop load compared with the nontreated control, and after the first application of LS or R, the number of fertilized king blooms was reduced and fertilization was prevented in all side blooms. All bloom thinning treatments had more fruit peel russet than the control and russet was more severe when LS was one of the applications. Bloom thinning applications of LS and R did not reduce powdery mildew leaf infection compared with the nontreated control. Cedar apple rust incidence was reduced by all bloom thinning treatments, though some lesions were detected in all treatments. There were minimal quince rust infections in any of the treatments, including the nontreated control. These results suggest that when LS and/or Regalia are mixed with JMS Stylet-Oil and applied as bloom thinners, they can reduce crop load, and, as a secondary benefit, they can also decrease cedar apple rust incidence from infections that occur during bloom. There is a need to conduct research specifically for organic apple (Malus ·domestica) production in the mid-Atlantic region (including the states of Delaware, Maryland, New Jersey, North Carolina, Pennsylvania, Virginia, and West Virginia), so that local growers can supply the increasing demand for organic products. In 2015, organic product sales exceeded

Ethylene Inhibitors Delay Fruit Drop, Maturity, and Increase Fruit Size of 'Arlet' Apples

43 billion in the United States, of which produce accounted for 13% of the total sales (Organic Trade Association, 2016). However, under the current U.S. Department of Agriculture-National Organic Program (USDA-NOP) regulations, growing apples organically in the eastern United States can be challenging due to the lack of effective biological, cultural, or chemical controls for the immense insect, disease, and weed pressure experienced in this region (Cromwell et al., 2011; Peck et al., 2010; Williams et al., 2016). Additionally, crop load management options are limited in organic production because the USDA-NOP prohibits the use of the synthetically derived plant growth regulators that are used for this purpose in conventional systems (U.S. Department of Agriculture, 2016). Managing crop load in apple orchards is essential for improving fruit size and color, preventing broken limbs due to the excess weight of fruit, and ensuring adequate return bloom to minimize biennial bearing (Byers, 2003; Dennis, 2000). Thinning, or reducing the number of apples per tree and within each fruiting cluster, can also decrease disease pressure by allowing for more rapid drying conditions within fruit clusters and allowing for more complete spray coverage of the developing fruitlets. Thinning during bloommay increase these benefits and decrease the likelihood of biennial bearing cycles by eliminating excess fruit before flower bud initiation occurs (Batjer and Hoffman, 1951; Greene, 2002). In the experiment described in this paper, we evaluated bloom thinning materials applied according to output from a ‘Honeycrisp’-specific pollen tube growth model for their ability to reduce crop load and disease incidence. The pollen tube growth model uses hourly temperature data to predict the amount of time between pollination and fertilization (Peck et al., 2016; Yoder et al., 2013). This information is then used to allow a preset percentage of flowers to set fruit, while fertilization in later blooming flowers is prevented by treatment (most commonly liquid LS) with a chemical that kills the pollen tubes before they fertilize additional eggs. Liquid LS tank mixed with oil (either fish or a petroleum-based product) is a recommended chemical combination for apple bloom thinning under organic management in the eastern United States (Peck and Merwin, 2010). However, before OR-CAL, Inc. (Junction City, OR) changed the label on Rex Lime Sulfur Solution in 2015, LS was only legally approved for bloom thinning and for use with oil inWashington State (Lehnert, 2014). Additionally, LS is caustic and often causes fruit russeting, therefore reducing the value of the fruit (Peck et al., 2010). Other negative impacts of LS result from its phytotoxicity, which disrupts photosynthesis, and thus potentially yield, tree growth, and return bloom (Holb et al., 2003; McArtney et al., 2006). Additionally, LS can be a very potent bloom thinner and may over thin the crop in some years. For these reasons, additional bloom thinning product options are needed for use in the eastern United States. In prior tests, the biofungicide, Regalia (Marrone Bio Innovations, Inc., Davis, CA), made from the extracts of Giant Knotweed [Reynoutria sachalinensis (F. Schmidt) Nakai syn. Polygonum sachalinense (F. Schmidt)], was found to function as an apple bloom thinner (Peck et al., 2016; Yoder et al., 2013). Our interest in the product began after observing damaged petals in an apple disease control field experiment where Regalia was applied during bloom. We hypothesized that the Received for publication 3 Oct. 2016. Accepted for publication 11 Jan. 2017. Funding support was provided by a U.S. Department of Agriculture-Specialty Crop Block Grant administered by the Virginia Department of Agriculture and Consumer Services, Virginia Agricultural Experiment Station, and Virginia Tech’s Department of Horticulture. We thank Sierra Athey, Matt Borden, David Carbaugh, Allen Cochran II, Scott Kilmer, Abby Kowalski, Taylor Mackintosh, Jared Repass, William Royston, Jr., Ashley Thompson, and Jim Warren for their assistance with this experiment. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the authors and does not imply its approval to the exclusion of other products or vendors that also may be suitable. Corresponding author. E-mail: gmp32@cornell. edu. HORTSCIENCE VOL. 52(3) MARCH 2017 377 product was caustic and potentially damaging to other floral organs. The disease control efficacy of Regalia is reported to be from a unique mechanism of induced plant resistance that upregulates chalcone synthase and chalcone isomerase in the phenylpropanoid pathway and induces the production and accumulation of phytoalexins (Su et al., 2012). In a previous formulation, namedMilsana (Marrone Bio Innovations, Inc.), R. sachalinensis extracts were shown to decrease powdery mildew infection on cucumbers, tomatoes, begonias, and wheat (Daayf et al., 1995; Dik and van der Staay, 1995; Herger et al., 1988; Herger and Klingauf, 1990; KonstantinidouDoltsinis et al., 2006; Konstantinidou-Doltsinis and Schmitt, 1998). Regalia is currently labeled for use as a fungicide on many horticultural and agronomic crops, but its mode of action as a bloom thinner is unknown. Among the many fungal diseases that require grower intervention in the mid-Atlantic, cedar apple rust (G. juniperi-virginiana) and quince rust (G. clavipes) are formidable barriers to organic apple production because they are not effectively controlled by organically approved fungicides such as sulfur and copper (Yoder et al., 2006). An experimental formulation of R. sachalinensis (MOI-106020) that was tested before the commercial release of Regalia was found to provide cedar apple rust control (Halbrendt et al., 2010). Similarly, prior tests in Virginia indicated that control of rusts by a combination of Regalia and JMS Stylet-Oil (JMS Flower Farms, Inc., Vero Beach, FL) provides effective control of cedar apple rust and after-infection control of quince rust (Peck et al., 2016; Yoder et al., 2015). Thus, we saw an opportunity to target two of the key barriers for organic apple production in the mid-Atlantic, namely reliable crop load and disease management. The objective of this project was to test the disease control achieved while using different sequences of LS and/or Regalia , each in combination with JMS Stylet-Oil, for bloom thinning. We hypothesized that these bloom thinningmaterials would aid disease control in an organically managed apple orchard. Materials and Methods The study was conducted in an 8-year-old ‘Honeycrisp’/‘MM.111’ apple orchard located at Virginia Tech’s Alson H. Smith, Jr. Agricultural Research and Extension Center in Winchester, VA (39 06# N, 78 17#W). Trees were spaced 3 m between trees and 7 m between rows. In 2015, the year of this experiment, only USDA-NOP approved products were applied to the orchard (Table 1). Before 2015, all trees had been managed conventionally. A row of border trees on the west side of the block, as well as four trees on each end of each row with treatment trees were not sprayed for the entire season. The experiment was conducted on the remaining four rows. Treatments were applied at the dilute rate to the point of runoff with a single nozzle handgun at 1723 kPa. Maintenance sprays were applied with an airblast sprayer (Swanson Model DA-400; Durand-Wayland, LaGrange, GA) calibrated to deliver 935 L·ha. Weeds were managed by using Suppress Herbicide EC (Westbridge Agricultural Products, Vista, CA) at 28.3 L of product in 467.2 L of water/ha on 23May and 4 July. Full bloom occurred on 25 Ap