HORTSCIENCE 49(12):1498–1505. 2014. of the physiological fruit drop’’ when fruits have attained 10% or 30% of final fruit size, respectively. The premature fruit drop stages A New Approach for Analyzing and have been named invariably and there is also no common agreement on the number of drop Interpreting Data on Fruit Drop stages as well as the onset and duration of each. Dahshan and Habib (1985) originally in Mango described three distinct stages of premature fruit drop of mango and this classification was Michael H. Hagemann1, Malte G. Roemer, Julian Kofler, also used in the review of Singh et al. (2005). Martin Hegele, and Jens N. Wunsche€ The first stage is referred to as ‘‘post-setting Section of Crop Physiology of Specialty Crops, University of Hohenheim, drop’’ and ceases 60 d after ‘‘fruit set’’ (BBCH-scale 619). The second stage is Emil-Wolff-Str. 25, 70599 Stuttgart, Germany termed ‘‘mid-season drop,’’ characterized by Additional index words. fruit retention, intercropping, irrigation, Mangifera indica, plant a duration of 15 d with lesser intensity than growth regulator during the ‘‘post-setting drop.’’ The third stage is the ‘‘pre-harvest drop’’ with only moderate Abstract. Mango yields are frequently reduced by premature fruit drop, induced by plant losses. stresses during the fruit set period in response to unsuitable climatic or crop management These descriptions of premature fruit drop, conditions. There are varying strategies for assessing premature fruit drop, which render commonly found in the literature, represent the comparison and interpretation of published data difficult to draw general conclu- some considerable limitations. The onset of sions. Therefore, the objective was to provide a mathematical model that is generally each fruit drop stage is based on a ‘‘fixed’’ valid for describing fruit losses of mango. The model was tested and validated by number of days from ‘‘fruit set’’ as used for monitoring the fruit drop for the two local North Vietnamese cultivars, Hoi^ and Tro`n, in example in the review of Singh et al. (2005). different management systems over six consecutive growing seasons: 1) mango–maize Fruit set, however, is variably related to: intercropping and mango monocropping; 2) irrigation; and 3) plant growth regulator 1) time after all flowers have dried out at the applications with 10 ppm N-(2-chloro-4-pyridyl)-N#-phenylurea (CPPU), 40 ppm 1- end of bloom (Malik and Singh, 2003); 2) 14 d naphthaleneacetic acid (NAA), and 40 ppm gibberellins (GA3 and GA4+7). The timely after full bloom (DAFB) (Notodimedjo, pattern of fruit drop was best described with a sigmoid function (r2 = 0.85) and formed 2000); or 3) size of fruitlets (Lam et al., the basis for defining three distinct drop stages. The post-bloom drop, from full bloom to 1985). The flowering of mango is a very the maximum daily rate of fruit drop [FD(x)], had the highest fruit losses. The following prolonged and sometimes non-synchronized midseason drop stage ends at 1% FD(x), a threshold that is suggested after a compre- process, especially in the tropics where erratic hensive literature review. Thereafter, during the preharvest drop stage, treatment and flowering is common but also in the subtropics cultivar differences appear to remain constant despite continued fruit drop. In contrast flowering can occur from a few days of an to other mango intercropping studies, fruit loss was not greater in the mango–maize individual flower, 1 to 2 weeks within a panicle intercropping than in the mango monocropping. Irrigation resulted in approximately to up to 1 month within a tree canopy three times higher fruit retention compared with the non-irrigated control. A single (Goguey, 1997; Mukherjee, 1953; Verheij, application of NAA at marble fruit stage (BBCH-scale 701) resulted consistently in the 1986). Typically, panicles exhibit a hierarchi- highest fruit retention for both cultivars in midseason and at harvest. The model permits cal flowering pattern from distal to proximal the separation between the drop stages, thus allowing the evaluation of 1) natural with an overlapping continuum of flowering variation before treatment effects during post-bloom drop; 2) treatment efficacies during and fruit development (Mukherjee, 1953; midseason drop; and 3) yield forecasting at the beginning of the preharvest stage. Singh, 1954). The terms ‘‘full bloom’’ and ‘‘fruitlet size’’ are often subjectively assessed by scoring and variably defined and thus do The worldwide production of mango fruit (Mukherjee, 1953; Singh et al., 1966). not offer precise occurrences that justify (Mangifera indica L.) is frequently reduced Numerous abiotic and biotic factors reduce a valid comparison of published fruit set data. by severe losses of fruit numbers throughout pollen viability (Issarakraisila and Considine, The duration of each fruit drop stage is clearly the growing season, a phenomenon that is 1994), the fertilization process of the flower, dependent on seasonal, regional, and cultivar- referred to as premature fruit drop (Singh and embryo survival (Lakshminarayana and specific variability and therefore the ‘‘fixed et al., 2005). Aguilar, 1975), which are all commonly time after fruit set’’ definition might be useful Mango produces an abundance of male associated with extensive fruit drop in early for characterizing the annual drop pattern of and polygamous flowers, but only a small season (Singh et al., 2005). Fruit that remains a given cultivar in one location but is not proportion of the latter group is successfully attracts a greater share of the available tree appropriate when comparing multiple data pollinated and has the potential for setting resources for continued growth and develop- sets. Consequently, the main objective of this ment. Subsequent fruit drop is induced by any study is to provide a new approach for factor reducing carbohydrate availability and interpreting and evaluating fruit drop data, thus the demand of the growing fruit is not attempting to overcome or at least to alleviate Received for publication 14 July 2014. Accepted sufficiently matched by its supply (Wunsche€ for publication 12 Oct. 2014. the limitations described. Consequently, fruit Financial support provided by the Deutsche For- and Ferguson, 2005). This carbohydrate im- drop of two mango cultivars was monitored in schungsgemeinschaft (SFB 564) and the Fiat Panis balance can occur, for example, by air tem- largely different cropping and management Foundation is greatly appreciated. We thank Dr. peratures below 13 °C or exceeding 36 °Cas systems over six seasons in the Province So’n Juan Carlos Laso Bayas for statistical consulting and a result of heavily reduced leaf photosynthe- La in North Vietnam and data were tested and Dr. Pham T. Huong for her professional advice as sis rates (Issarakraisila and Considine, 1994; validated in a mathematical model. well as for facilitating good relationships with local Whiley et al., 1999; Yamada et al., 1996). Unfavorable environmental cues, particu- mango grower in the study area. We are grateful For mango, principal phenological larly when temperature extremes coincide to Dr. Patrick Winterhagen for valuable scientific growth stages are distinguished (Herna´ndez with severe drought conditions (Elsheery discussion and reading the manuscript. From a thesis submitted by Michael H. Hagemann Delgado et al., 2011) according to the et al., 2007), are thought to be key triggers as partial fulfillment of the requirements for the PhD general BBCH-scale (Biologische Bunde- for the extensive premature fruit drop pat- degree. sanstalt, Bundessortenamt und Chemische terns in this province. Huong (2010) further 1To whom reprint requests should be addressed; Industrie); however, fruit drop is only de- suggested poor orchard management, in par- e-mail [email protected]. scribed in Stage 7 with ‘‘beginning and end ticular insufficient pest management, as an 1498 HORTSCIENCE VOL. 49(12) DECEMBER 2014 CROP PRODUCTION additional cause of fruit drop for the local and offering more precise benchmarks for (PR2; Delta-t Devices Ltd., Cambridge, mango cultivars Hoi^ and Tro`n predominantly onset and duration of each fruit drop stage. U.K.). The measurements were carried out cultivated in this region. For a robust evaluation and validation of our at 10-cm increments from 10- and 40-cm Despite extremely low orchard productiv- model as well as the interpretation of the soil depth 50 cm apart from the trunk of five ity of 1t·ha–1 (Yeˆn Chaˆu, 2008, Statistical seasonal fruit drop continuum, fruit abscis- irrigated and non-irrigated trees, respec- Data of Yeˆn Chaˆu District, Statistical De- sion was monitored over several years in tively. The profile probe was connected to partment, unpublished data), mango trees are varying mango production systems. a handheld data-logging device (Moisture often planted either in monoculture or in meter HH2; Delta-t Devices Ltd.). intercropping systems with mainly maize in Material and Methods Experimental design and treatments. Sea- the mountainous Province of So’n La with sonal fruit drop was monitored in the same steep, sloping hillsides. Indeed, tree crops, Plant material and experimental sites. orchard where environmental parameters including mango, provide a more appropriate The experiments were conducted over six were monitored. Although the natural fruit and sustainable land use system for steep, consecutive growing seasons from 2007 to drop of ‘Hoi’^ was assessed in all years, fruit deforested slope sites (Roberts-Nkrumah, 2012 in the Tu´ Nang commune (lat. drop of both cultivars was also evaluated in 2000; Young, 1989), because the cultivation 20°37#0$ N, long. 106°4#60$ E) near the response to irrigation and PGR applications of annual crops in monocropping systems township Yeˆn Chaˆu, Province So’nLa,North in 2008 and 2009 (Table 2). under those topographical conditions will Vietnam. This is a mountainous region with The irrigation experiment was conducted lead to reduced soil fertility and severe soil prevailing monsoon seasonality and 1200 mm using 20 randomly selected trees per cultivar erosion (Clemens et al., 2010).