Richard P. Marini

Repeated applications of NAA control preharvest drop of ‘Delicious’ apples

‘Delicious’ apple (Malus domestica Borkh.) trees were treated with NAA, at concentrations of 0-20 mg I“1,7 d before harvest, or on five occasions during the final 60 d before harvest. Although the response varied with the experiment, repeated NAA applications delayed fruit abscission more than single applications, but two or three applications were as effective as five. Five applications of NAA at concentrations of 5-20 mg T1 had little influence on flesh firmness, soluble solids concentration, or starch index at harvest or after four months of storage.

Seasonal correlations of specific leaf weight to net photosynthesis and dark respiration of apple leaves.

Specific leaf weight (SLW), net photosynthesis (Pn), and dark respiration (Rd) of apple leaves were monitored for an entire growing season. Leaves were sampled from the canopy interior and periphery to provide a range of SLW. Leaf Pnwas linearly correlated with SLW until mid-August, when Pnbegan to decline. During September the relationship between SLW and Pnwas a quadratic. Leaf Rdand SLW were linearly correlated throughout the season. Leaf Pnand Rdwere significantly correlated through most of the season, but the relationship was not always linear. Specific leaf weight appears to be a reliable index of the previous light environment of a leaf, but use to estimate Pnis probably limited to the first half of the season, because of increased variation after mid-August.

Effect of Aphis spiraecola and A. pomi (Homoptera: Aphididae) on the growth of young apple trees

Abstract One-year-old ‘Redchief Delicious’ apple trees grown in pots in an unheated greenhouse with screened ends were inoculated during the summer with either spirea aphid, Aphis spiraecola Patch or apple aphid, A. pomi DeGeer. Photosynthesis and greenness (the extent of chlorophyll colour) of apple leaves were reduced by increasing accumulated aphid-days for both species. Accumulation of fresh and dry weights in leaves, lateral shoots, rootstock and roots during the first growing season were affected by both species. Differences in dry weight were still present when trees were harvested at the ten-leaf stage the following spring. The percentages of non-structural carbohydrates (NSC) in shoots, roots and leaves were reduced by both aphids; amounts of NSC were reduced in all tree partitions. At the ten-leaf stage of the second season, the percentage and amount of NSC were reduced in all tree partitions. Tree response to the two aphid species was generally similar.

Crop load management.

Problems with over production are related to a decrease in the number of leaves per fruit, especially in heavy crop years. On average, it takes 8-10 functional leaves to adequately fill one nut. Large fruited cultivars may require even more leaves per nut. Much of the stress on the leaves to fill nuts can be relieved by fruit thinning of prolific cultivars. Fruit thinning may decrease total yield per tree for the current year, but this can be offset by an increase in marketable yield. The benefits of nut thinning include increased nut quality in terms of higher kernel percentage and kernel grade, as well as more stable yield and cash flow for the grower from year to year.

Fruitlet size and position within the cluster influence apple fruitlet susceptibility to chemical thinners

Summary Six experiments were performed to determine the effect of fruitlet diameter on the day of thinner application, fruitlet position within a fruiting cluster, and chemical thinners on retention of individual apple (Malus domestica Bork.) fruitlets, days of fruitlet growth, and days to fruitlet abscission following a thinner treatment. Fruitlet diameter at thinning time was used as a covariate in an analysis of covariance to account for differences in fruitlet diameter between king and side fruitlets within a fruit cluster. Fruitlet diameter, but not fruitlet position, consistently influenced the percentage of fruitlet retention. Carbaryl reduced fruitlet retention in four of six experiments and was most effective on fruitlets smaller than 14.0 mm in diameter. Accel was used in one experiment and was effective on small side fruitlets. Ethephon was used in three experiments and consistently reduced fruitlet retention regardless of fruitlet size (8.0 to 31.0 mm diameter) or position. Days from treatment to fruitlet growth cessation, but not days from treatment to fruitlet abscission or days from growth cessation to abscission, was positively related to fruitlet diameter on the day of treatment, but none of these response variables were consistently affected by fruitlet position. Ethephon was the only thinner that consistently reduced days from treatment to fruitlet growth cessation. In general, the number of days from treatment to growth cessation was 13–18 d on untreated trees and carbaryl-treated trees, and 8–9 d on ethephon-treated trees. The number of days from treatment to fruitlet abscission was 14–23 d on untreated and carbaryl-treated trees and 10–12 d on ethephon-treated trees.

Prediction of Bitter Pit in ‘Honeycrisp’ Apples and Best Management Implications

During a 3-year study of bitter pit in commercial ‘Honeycrisp’ apple (Malus 3domestica) orchards, incidence was associated with low calcium (Ca) levels in fruit peel; high ratios of nitrogen (N), potassium (K), and/or magnesium (Mg) to Ca in fruit peel; excessive terminal shoot length; and low crop load. Peel N and Mg concentrations were negatively correlated and peel Ca concentration positively correlated with crop density (CD). Shoot length (SL) was not consistently correlated with peel N, Mg, or phosphorus (P) and was negatively correlated with only Ca. A two-variable model that included SL and the ratio of N to Ca explained more than 65% of bitter pit incidence. The model has implications for best management of the cultivar in the field and during storage. The high susceptibility of ‘Honeycrisp’ to bitter pit is not well understood. Crassweller and Smith (2016) found levels of Ca in foliar tissue were lower in ‘Honeycrisp’ than in ‘Cameo’. Cheng (2016) reported lower fruit levels of Ca in ‘Honeycrisp’ compared with ‘Gala’. Fruit levels of K, Mg, and P were similar in the two cultivars, and he proposed the resulting nutrient imbalance predisposed ‘Honeycrisp’ to a deficiency of Ca and Carelated disorders. Research in New Zealand on mineral movement in bitter pit–prone cultivars indicated rapid early season uptake of Ca and poor to no late season uptake, whereas K andMg continued to increase over the course of the season (Ferguson, 2001; Ferguson and Watkins, 1989). Studies conducted on bitter pit development at the cellular level have improved the understanding of Ca localization in cells of pitted fruit. De Freitas et al. (2010) reported evidence of a connection between bitter pit and Ca binding to cell walls as well as Ca accumulating in storage organelles. Additional cytochemical research (De Freitas et al., 2015) demonstrated an association between higher levels of water-insoluble pectin Ca and bitter pit. Hocklin et al. (2016) proposed a possible role of apoplasmic calcium-pectin crosslinking. Bitter pit management in the orchard is central to disorder prevention but is not always effective, and the reasons are often unclear. Research conducted by Rosenberger et al. (2004) demonstrated that season-long Ca treatments were required for reducing bitter pit incidence in ‘Honeycrisp’ grown in New York. Bitter pit control was not enhanced by supplementing Ca sprays with trifloxystrobin fungicide, boron, or harpin protein treatments. Trials by Biggs and Peck (2015) showed that rates ranging as high as 26.3 kg·ha per season of elemental Ca were needed to significantly reduce bitter pit incidence in ‘Honeycrisp’ apples grown in Virginia and West Virginia orchards. Foliar Ca products were evaluated in both studies, and none were better than calcium chloride (CaCl2). Telias et al. (2006) reported that crop load had a more significant effect on bitter pit than Ca sprays, with bitter pit incidence being positively correlated to low yield and large fruit. Mitcham (2008) and Silveira et al. (2012) demonstrated that shoot growth suppression reduced bitter pit incidence. Research results reported by other investigators on the effects of Ca, crop load (CD), and shoot growth have at times been contradictory, and predictive tools are needed to assist producers in developing site-specific best management programs for managing bitter pit. Fruit mineral analysis has the potential to assist producers in managing nutrient imbalances in the orchard while also providing a possible predictive tool. In research by Ferguson et al. (1979), low Ca in ‘Cox’s Orange Pippin’ fruit sampled 3 weeks before harvest was associated with an increased risk of bitter pit development. Amarante et al. (2013), De Freitas et al. (2015), Dris et al. (1998), Ferguson and Watkins (1989), and Lanauskas and Kvikliene (2006) suggested high N, K, and/or Mg to Ca ratios in fruit of bitter pit–prone cultivars could improve the prediction of susceptibility to the disorder. Al Shoffe et al. (2014) reported significant correlations between bitter pit and levels of N, P, K, N/Ca, Mg, and (Mg + N)/Ca ratio in ‘Honeycrisp’ fruit. The fruit tissue sampling procedure affects the reliability of bitter pit prediction from mineral analysis, and Amarante et al. (2013) demonstrated tissue should be sampled from the calyx end of the fruit. The best tissue to sample from ‘Fuji’ was the peel, whereas the flesh was a better predictor for ‘Caterina’. Before the research reported in this article, the authors compared peel and flesh nutrient measurements for ‘Honeycrisp’ and found improved correlations to bitter pit with nutrients measured in peel tissues (Baugher et al., 2014). We also found peel tissues could be prepared by air-drying rather than freeze-drying, which made the technique more practical for commercial growers (unpublished data). The objectives of a 3-year study of ‘Honeycrisp’ grown at three crop densities in six commercial orchards were to 1. improve guidelines for balancing CD, terminal SL, and fruit nutrient levels to reduce bitter pit incidence in ‘Honeycrisp’ orchards and Received for publication 10 July 2017. Accepted for publication 21 Aug. 2017. This research was supported by the State Horticultural Association of Pennsylvania, the Pennsylvania Apple Program, and the Pennsylvania Department of Agriculture Research Program. We acknowledge the valuable contributions of Tom Jarvinen, Michael Basedow, Erin Dugan, Kristi Kraft, Danielle Ryan, Montserrat Fonseca Estrada, Alana Anderson, Ryan Hilton, Sladjana Prozo, and Gustavo Salazar (Penn State Extension); Tom Kon, Edwin Winzeler, and Melanie Schupp (Penn State Fruit Research and Extension Center); Dave and Jim Benner, Clint and Bill Lory, Ben and Joe Lerew, Chris Baugher, and Dave and John Wenk (grower cooperators); Lee Showalter, Leighton Rice, David Rice, Ben Rice (Rice Fruit Company); Ryan Hess (Hess Brothers Fruit Company); John Spargo, and Denyce R. Matlin (Penn State Agricultural Analytical Services Laboratory); and Jacqueline F. Nock and Yosef Al Shoffe (Cornell Apple Postharvest Physiology Laboratory). The mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product and does not imply its approval to the exclusion of other products or vendors that also may be suitable. Corresponding author. E-mail: tab36@psu.edu. 1368 HORTSCIENCE VOL. 52(10) OCTOBER 2017 2. develop predictive models for determining how to improve management and postharvest handling of ‘Honeycrisp’ apples. Packinghouses in major fruit growing regions use various fruit nutrient models to predict the potential for bitter pit in storage (Ferguson, 2001; Hanson, 2012). This investigation was designed to assess both field measurements and fruit nutrient measurements with the objective of developing a model that would guide both fruit producers and fruit packers. Materials and Methods Field trial design. During 2014 to 2016, uniform field trials were established in six high-density ‘Honeycrisp’ blocks in orchards with varying histories of bitter pit incidence. The studies included two orchard blocks each with histories of high, moderate, and low levels of bitter pit. The six blocks were the same each year with the exception of a change in 2016 because of two blocks (history of low bitter pit) receiving hail damage. At the start of the project, the trees ranged in age from 5to 8-year old, and no prohexadione-calcium was applied to suppress shoot growth in any of the blocks or years. Within each block, three trees each were selected with high, medium, and low crop loads. Individual trees were handled as replications, and the following data were collected: 1. Trunk diameter at a 20-cm height and the number of fruit at harvest for determinations of CD (fruit/cm trunk cross-sectional area); 2. Average SL calculated from 10 representative terminal shoots per tree after terminal bud set; 3. Fruit peel nutrient levels at 3 weeks before harvest (composite sample from 15 randomly selected fruit of similar size per tree); 4. Average fruit weight, soluble solids concentration, and flesh firmness at harvest (five representative fruit per tree); and 5. Bitter pit incidence at harvest and again following long-term storage (the percentage of 20 randomly selected fruit of similar size per tree with bitter pit symptoms). Fruit tissue preparation and analysis. Selection of the fruit tissue to sample was based on a 2012 preliminary study in which bitter pit incidence was more closely correlated to minerals in fruit peel than in fruit tissue (Baugher et al., 2014). Based on research by Amarante et al. (2013), 1-cm wide peel samples were taken from around the circumference at the calyx end of the fruit, using a potato peeler and exercising care to prevent removal of fruit flesh. Fruit peel samples were air-dried and then ground into a fine powder. Analyses for N, P, K, Ca, Mg, Mn, Fe, Cu, B, and Zn were conducted by the Penn State Agricultural Analytical Services Laboratory (procedures described at http:// agsci.psu.edu/aasl/plant-analysis/plant-methods; Penn State College of Agricultural Sciences, 2017). Bitter pit assessment. Fruit samples were collected at optimum maturity for long-term storage based on ground color and starch measurements (Blanpied and Silsby, 1992; Greene et al., 2015). The samples were stored at the Penn State Fruit Research and Extension Center, Biglerville, PA, in air storage maintained at 3.3 C. Fruit were assessed for the incidence of bitter pit after 4 months in storage plus 7 d at 20 C. Statistical analysis. Relationships between variables of the study were first evaluated as scatter plots with SAS’s PROC GPLOT and G3D before using PROC CORR (Freund and Littell, 2000) to verify the linear relationships between the response variable (percentage of fruit with bitter pit symptoms after storage) and 21 potential regressor variables [average SL (cm), CD (fruit/cm trunk cross-sectional

Root and mycorrhizal fungal foraging responses to fruit removal in apple trees

Background and aimsRoot and mycorrhizal fungal foraging in nutrient-rich patches is an energy-intensive process, and shifts in carbon (C) availability may affect foraging strategies. We hypothesize that when trees are C limited, they will prioritize root and mycorrhizal hyphal growth in nutrient-rich soil patches.MethodsApple (Malus domestica Borkh.) trees with fruit were compared to trees with fruit removed to investigate the effect of reproductive effort and associated shifts in belowground C availability on root and arbuscular mycorrhizal (AM) fungal growth in unfertilized soil and localized nitrogen (N)-rich patches (containing inorganic or organic nitrogen).ResultsAcross nutrient treatments, fruit removal enhanced root production compared to fruiting trees. In fruiting trees, about four times more roots proliferated in the inorganic-N patch than in unfertilized soil or the organic-N patch. However, in trees with fruit removal, root proliferation was similar among nutrient treatments. Arbuscular mycorrhizal extramatrical-hyphal biomass was not affected by fruit removal but was greater in the organic-N patch than the inorganic-N patch or unfertilized soil. Fruit removal and N addition had modest effects on AM fungal colonization of apple roots and no effect on non-mycorrhizal fungal colonization.ConclusionsRoot and AM foraging for nutrients should be considered in the context of C availability. Apple trees may manipulate root foraging more than AM fungal foraging when C belowground is constrained.

Sensitivity of Eleven Milkweed (Asclepias) Species to Ozone

Abstract We exposed 11 milkweed species to ozone within continuous stirred-tank reactor (CSTR) chambers in a greenhouse to determine species sensitivity and potential use as bioindicators to detect phytotoxic levels of ambient ozone. Asclepias syriaca (Common Milkweed), A. ovalifolia (Oval-leaf Milkweed), A. sullivantii (Prairie Milkweed), A. speciosa (Showy Milkweed), A. asperula (Spider Milkweed), A. incarnata (Swamp Milkweed), A exaltata (Tall Milkweed), and A. curassavica (Tropical Milkweed) developed typical ozone-induced dark stipple on the adaxial surface of older leaves. Tropical Milkweed also exhibited significant premature defoliation (accelerated leaf senescence). Asclepias tuberosa (Butterfly Milkweed), A. hirtella (Green Milkweed), and A. verticillata (Whorled Milkweed) were tolerant to ozone. Foliar stipple on Common Milkweed increased with ozone concentration and time. In addition to Common Milkweed, a bioindicator commonly used to detect phytotoxic levels of ozone, the other 7 ozone-sensitive milkweed species should be evaluated further as potential ozone bioindicators.