Artículos

 

Delaying senescence of 'Ruby Red' grapefruit and 'Valencia' oranges by gibberellic acid applications*

 

Uso de ácido giberélico para retrasar la senescencia de toronja'Ruby Red' y naranja 'Valencia'

 

Martín Aluja^1§, Everardo Bigurra¹, Andrea Birke¹, Patrick Greany² and Roy McDonald³

 

¹ Instituto de Ecología. A. C. Xalapa, Veracruz, México. A. P. 63. C. P. 91000. ^§Corresponding author: martin.aluja@inecol.edu.mx.

² Formerly, USDA - ARS Center for Medical, Agricultural & Veterinary Entomology. 1700 SW 23^rd. Gainesville, Florida 32608, USA Retired.

³ Formerly, U. S. Horticultural Research Laboratory, ARS, USDA, 2120 Camden Road, Orlando Florida 32803, USA Retired.

 

* Recibido: agosto de 2010
Aceptado: febrero de 2011

 

Abstract

The demonstrate the effectiveness of gibberellic acid (GA₃) in delaying fruit senescence in 'Ruby Red' grapefruit and 'Valencia' oranges under sub-optimal orchard management conditions in tropical Veracruz, Mexico. For grapefruit, one or two applications of three different GA₃ doses (15, 20 and 40 mg L^-1) with or without surfactant (Silwet^® L77) at 0.035, 0.05 or 0.1%, were tested. For oranges, one or two applications of three different GA₃ doses (10, 15 and 20 mg L^-1) with or without surfactant at 0.05%, were tested. Pre-color break application of GA₃, at 10 mg L^-1 and 15 mg L^-1 with surfactant (0.05%), was sufficient to sustain peel firmness and delay colour development in oranges and grapefruit, respectively. GA₃ treatments with surfactant caused increased leaf drop in both citrus cultivars, although this was only noticed in trees treated with the highest surfactant dosages and mainly involved photosynthetically inactive leaves. One application of GA₃ at 15 mg L^-1 with surfactant (0.05%) significantly increased orange and grapefruit weights, resulting in yield increases of ca. 1.7 t ha^-1 for oranges and 1.87 t ha^-1 for grapefruit. A significant decrease in orange drop during the late harvest season in GA₃ treated trees, resulted in a harvest period that could be extended by up to six weeks. The market value of fruit harvested late in the season is up to eight times the value of fruit harvested when the market is saturated.

Keywords: Citrus, economic benefit, harvest delay, increase of fruit weight.

 

Resumen

Los datos demuestran la efectividad del ácido giberélico (AG₃) para retrasar la senescencia del fruto en toronja 'Ruby Red' y naranja 'Valencia', aún bajo condiciones sub-óptimas de manejo en la región tropical de Veracruz, México. En toronja, experimentamos con una o dos aplicaciones de tres dosis de AG₃ (15, 20 y 40 mg L^-1) con o sin surfactante (Silwet^® L77) a 0.035, 0.05 ó 0.1%. En el caso de la naranja, experimentamos con una o dos aplicaciones de tres dosis de AG₃ (10, 15 and 20 mg L^-1) con y sin surfactante a 0.05%. La aplicación de AG₃ en etapa de pre-coloración, a 10 mg L^-1 y 15 mg L^-1 con surfactante (0.05%), fue suficiente para mantener la firmeza del epicarpio (cáscara) y retrasar el desarrollo del color en naranjas y toronjas, respectivamente. El tratamiento de AG₃ con surfactante provocó un incremento en la caída de las hojas en ambos cítricos, aún cuando esto fue especialmente observado en árboles tratados con dosis elevadas de AG₃ y surfactante, principalmente en el caso de hojas con inactividad fotosintética. Una aplicación de AG₃ a 15 mg L^-1 con surfactante (0.05%), incrementó significativamente el peso de la naranja y la toronja, resultando a su vez en un incremento en la cosecha alrededor de 1.7 t ha^-1 de naranja y 1.9 t ha^-1 de toronja. Debido a la reducción significativa en la caída de la fruta de la naranja durante la última temporada de cosecha en los árboles tratados con AG₃, la cosecha podría extenderse por seis semanas adicionales. Los frutos cosechados tardíamente podrían alcanzar un valor de ocho veces mayor que aquellos cosechados cuando el mercado está saturado de producto.

Palabras clave: Citrus, beneficio económico, incremento en peso del fruto, retraso en la cosecha.

 

INTRODUCTION

Citrus growers in Veracruz, Mexico, the largest citrus producing region of the country and one of the largest in the world, often attempt to keep their fruit on the tree as long as possible to obtain better prices and avoid oversupplied domestic markets. However, a large percentage of citrus production in Veracruz (25%) is lost to premature fruit drop caused in part by the attack of the Mexican fruit fly, Anastrepha ludens (Loew) (Ortíz-Moreno, 2009). Thus, products that can delay senescence, and at the same time decrease the fruit's susceptibility to fruit fly attack, are potentially important to growers (Aluja, 1994; 1999).

Plant growth regulators, including gibberellic acid (GA₃) and 2,4-dichlorophenoxyacetic acid (2,4-D), have been widely used to maintain rind firmness and peel colour and reduce fruit drop in California (Coggins, 1973), Florida (Ali Dinar et al., 1976; Ferguson et al., 1982; McDonald et al., 1987), and Australia (Considine and El-Zeftawi, 1971). These compounds have also been used to extend postharvest shelf life (El-Otmani and Coggins, 1991; El-Otmani et al., 2000; Ritenour et al., 2005; Davies and Zalman, 2007), due to reduced susceptibility to post-harvest bacterial and fungal attack in treated fruits (Lewis et al., 1967; Coggins and Hield, 1968; Coggins, 1973). Importantly, while fruit senescence is delayed and citrus peel remains firmer and greener for a longer period, the internal fruit ripening process is not halted (Coggins and Lewis, 1965; Lewis et al., 1967; Coggins, 1973; Ferguson et al., 1982; Birke et al., 2006).

Previous studies have also shown that GA₃ can reduce citrus fruit susceptibility to the Caribbean fruit fly, Anastrepha suspensa (Loew) (Greany et al., 1987; 1991; 1994; McDonald et al., 1988) and the Mediterranean fruit fly, Ceratitis capitata (Wiedemann) (Rössler and Greany, 1990). A similar effect was recently observed in the case of the Mexican fruit fly, A. ludens but only when fly populations were extremely low.

In Mexico, no prior use of plant growth regulators to reduce fruit drop, or delay the harvest period of oranges and grapefruit, has been formally documented. Based on the above, and given the demonstrated potential of GA3 to delay fruit senescence in other parts of the world, we decided to determine the efficiency of this approach in citrus groves in Veracruz, Mexico. Veracruz has the largest planted surface of citrus in the entire country (200 000 ha) and is one of the largest citrus growing regions in the world if one considers lime, tangerine in addition to various cultivars of oranges and grapefruit (SAGARPA, 2009).

Citrus production in Veracruz is concentrated over a short time period during which, due to oversupply, prices are low. But towards the end of the season, as prices increase, fruit rind has become senescent, and ripening fruit becomes highly susceptible to bacterial, fungal and fruit fly attack causing dramatic increases in fruit drop. Rössler and Greany (1990) have shown that GA₃ can also enhance citrus natural resistance (toxic citrus oils remain concentrated for several weeks) and fruit senescence is delayed. If GA₃ in Mexico shows to be as effective as in Florida, growers could be benefited by an extended harvest period, and concomitant access to higher prices.

Here, we report the results of a three-year study (1992-1995), conducted to determine the effect of GA₃ on specific physical characteristics of 'Ruby Red' grapefruit (Citrus paradisi MacFadyen) and 'Valencia' orange (Citrus sinensis L. Osbeck) trees. For oranges, we also evaluated the optimal dose rates for GA₃ and surfactant applications that could reduce fruit drop and extend the harvest period.

 

MATERIALS AND METHODS

In both grapefruit and oranges we assessed the effect of GA₃ on peel puncture resistance, coloration, leaf drop and fruit weight. In the case of oranges, we also evaluated the effect that GA₃ could have on reducing fruit drop and extending the harvest period. This could not be evaluated in grapefruit due to high fruit drop caused by increased fruit fly attack towards the end of the season (Birke et al., 2006).

Study sites. Experiments were conducted in two commercial citrus groves, Finca "Montecristo" ('Valencia' oranges), and Finca "La Florida" ('Ruby Red' grapefruit), located in Congregación de Cañadas, Martínez de la Torre, Veracruz, Mexico (400 masl; 96º 47' West Longitude; 19º 58 North Latitude). This area has a dry season that can last between four and five months from the middle of March to the middle of July. The mean annual rainfall is 1 600 mm and the average temperature is 22 °C (INEGI, 1984).

Application of GA₃ and surfactant in the grapefruit and orange orchards. The gibberellic acid used was ProGibb^® Plus 2X wettable powder (Abbott Laboratories, North Chicago, IL). The organosilicone surfactant used was Silwet^® L-77 (dimethyl polysiloxane modified with alkylene oxide) (Osi Specialties, Inc., Danbury, CT).

'Ruby Red' grapefruit orchard. Experiments were carried out from 1992 to 1995 using a random block design. During the 1992-1993 harvest seasons, 15 trees per treatment were used comprising five replicates of three trees each. During the following season (1993-1994), 162 trees per treatment were used comprising nine replicates of 18 trees each. For the 1994-1995 seasons, 30 trees per treatment were used comprising five replicates of 6 trees each. In all cases, approximately 30 litres of GA₃ solution were applied per treatment tree using a manual sprayer (JACTO AteSa S. A., Torreón, Mexico) (at 36 s L^-1) attached to a 1 500 litres tank pulled by a tractor. Control trees were sprayed with water using the same equipment described above. Fruit were always treated before colour break, on 03 August (first application) and 03 September (second application) in 1992, on 05 August in 1993, and on 09 August in 1994 (Table 1).

'Valencia' orange orchard. A random block design was used in the experimental plot. Experiments were conducted during two seasons. During the 1992-1993 seasons, 80 trees per treatment were used comprising 5 replicates of 16 trees each. For 1993-1994, a total of 440 trees were employed per treatment comprising 4 replicates of 110 trees each. GA₃ was applied during 1992-1993 with a manual sprayer (JACTO AteSa S. A. Torreón, Mexico) (at 36 s L^-1) attached to a 1 500 liter tank pulled by a tractor. During 1993-1994, a speed sprayer was used. Control trees were sprayed with water using the same equipment described above. The first year (1992), GA₃ was applied twice, on 15 October and 30 November, whereas in 1993, the compound it was applied only once on 19 October (Table 1).

Evaluation of peel firmness and fruit colour. Grapefruits were sampled monthly from November through February for 1992-1993 (N= 75 per treatment). Five fruits were sampled from each tree. For the 1993-1994 season, fruits were sampled from October through January (N= 164 per treatment), four fruits were sampled from 41 trees selected randomly. For the 1994-1995 season, fruits were also sampled from October through January (N= 20 per treatment); two fruits were sampled from 10 randomly selected trees.

Oranges were sampled monthly from December to March for 1992-93 (N= 80 per treatment); one fruit was harvested from each tree. For the 1993-1994 seasons (N= 144 per treatment) three fruit were harvested from 48 randomly sampled trees from December through May. Fruit harvested from the experimental plots was transported to the Instituto de Ecología, A. C. laboratories in Xalapa (3 h drive), to assess peel firmness and fruit colour. All measurements were taken within a 48 h period post-harvest.

Rind firmness was determined using a 1 mm flat-tip, metal probe (4 equatorial punctures per fruit) connected to a force gauge (Accuforce gauge III, model AF3010CE, Ametek, Mansfeld & Green Division, Largo, FL) on a motorized test stand (model 4665, Ametek, Mansfield & Green Division, Largo, FL).

Evaluation of leaf drop. For grapefruit trees, leaf drop was assessed by placing two plastic baskets (1520 cm) underneath tree canopies (N= 10 baskets per treatment) and monthly counts the number of fallen leaves inside the containers from October to December. For orange trees, leaf drop was assessed during the 1992-1993 season, one month after the final GA₃ application. Two wooden squares (50 cm²) were randomly thrown underneath the canopy of 20 orange trees per treatment and the total number of fallen leaves inside the squares was counted.

Determination of fruit weight. To determine whether GA₃ had an effect on fruit weight, groups of 14 grapefruit from 41 trees per treatment (N= 41 groups of fruit per treatment), and groups of three oranges per tree from 48 randomly selected trees per treatment (N= 48 groups of fruit per treatment) were sampled from the experimental orchard during the 1993-1994 season and were transported to the laboratories in Xalapa. Fruit were weighed using an electronic digital precision balance (OHAUS^® Precision plus TP4KD, OHAUS Corporation, Florham Park, N. J. USA).

Quantification of fruit drop. Orange drop was quantified by counting and removing fallen fruits beneath tree canopies each month. During 1992-1993, we assessed fruit drop regularly from December through March. After March, fruit drop was counted daily until a 13 fruit per tree "harvest threshold" was reached. Grapefruit drop was not assessed because late in the season, the few remaining fruit on the tree were heavily infested by A. ludens larvae and therefore tended to drop prematurely. The fruit fly population was extremely high during this season (Birke et al., 2006).

Determination of optimal orange harvest period. The effect that GA₃ had on the length of the orange harvest period was assessed during the 1992-1993 season in an experimental plot in which fruit was not harvested to determine whether GA₃ treatments would allow trees to retain fruit past the conventional harvest period in the region. The harvest threshold was set by local growers at 13 fallen fruits per tree after 31 March which is when the harvest season normally ends (i.e., all ripe fruit still on the tree are usually harvested by this date). Growers considered that more than 13 fallen fruit per tree resulted in an economic loss (Bigurra, 1995).

Statistical analysis. All results were rank-transformed to ranks (Zar, 1999) and were subjected to two-way analyses of variance (ANOVA) (SAS, 1998), with the exception of leaf drop and fruit drop that were not transformed prior to ANOVA. Posthoc comparisons were performed using Scheffe tests (p< 0.05).

 

RESULTS

Various GA₃ and surfactant doses proved to be effective in maintaining fruit firmness and delaying fruit color change in both 'Valencia' oranges and 'Ruby Red' grapefruit. However, GA3 treatments with surfactant also increased leaf drop within weeks of application. Importantly, a dose of 15 mg L^-1 GA₃ and surfactant significantly increased fruit weight in both oranges and grapefruit. Finally, and in the case of oranges, GA₃ in combination with a surfactant significantly diminished fruit drop late in the harvest season for a period of up to six weeks.

GA₃ effect on peel firmness and peel coloration. GA₃ significantly delayed peel fruit softness and skin coloration (Figure 1). During the 1992-1993 season, grapefruit peel was firmer (two-way ANOVA, F_{3, 1681}= 213.18, p< 0.0001 months; F_{5, 1681}= 127.39, p< 0.0001 treatment; F_{15, 1681}= 2.26, p= 0.0038 monthstreatment). Fruit treated with GA₃ plus surfactant was firmer than those treated only with GA₃, while control fruit was the least hard. Peel coloration was also dose-dependent, (two-way ANOVA, F_{3, 1681}= 618.73, p< 0.0001 months; F_{5, 1681}= 99.23, p< 0.0001 treatment; F_{15, 1681}= 14.67, p< 0.0001 monthstreatment), fruit treated with two applications of 20 mg L^-1 + 0.1% L77 maintained greenness for a longer period than fruit treated with other doses.

Control fruit senesced faster than GA3-treated fruit. In the 1994-1995 season, peel firmness also differed significantly (two-way ANOVA, F_{3, 228}= 124.33, p< 0.0001 months; F_{2, 228}= 26.16, p< 0.0001 treatment; F_{6, 173}= 1.58, p< 0.154 monthstreatment). Fruit coloration during this year was also significantly different between months and treatments (two-way ANOVA, F_{3, 228}= 82.82, p< 0.0001 months; F_{2, 228}= 34.12, p< 0.0001 treatment; F6, 228= 2.43, p= 0.027 monthstreatment); all treated fruit was greener than control fruit.

In 1992-1993 season, oranges were firmer when treated with GA₃ (two-way ANOVA, F_{3, 1896}= 121.11, p< 0.0001 months; F_{5, 1896}= 25.65, p< 0.0001 treatment; F_{15, 1896}= 1.19, p= 0.27 monthstreatment) (Figure 2). Fruit coloration also showed significant differences between treatments (two-way ANOVA, F_{3, 1894}= 286.87, p< 0.0001 months; F_{5, 1894}= 206.21, p< 0.0001 treatment; F_{15, 1894}= 9.44, p< 0.0001 monthstreatment). In 1993-1994 season, a similar effect was observed in both peel firmness (two-way ANOVA, F_{5, 1716}= 70.91, p< 0.0001 months; F_{1, 1716}= 250.31, p< 0.0001 treatment; F_{5, 1716}= 9.44, p< 0.0067 monthstreatment) and peel coloration (two-way ANOVA, F_{5, 1715}= 287.66, p< 0.0001 months; F_{1, 1715}= 801.6, p< 0.0001 treatment; F_{5, 1715}= 801.6, p< 0.000007 monthstreatment) (Figure 2). Dependent effects on peel firmness and color were detected in relation to GA₃, surfactant concentrations.

GA₃ effect on leaf drop. During our three-year study, treatments with GA₃ plus surfactant caused significant increases in leaf drop shortly after application in grapefruit (Figure 3A) (two-way ANOVA, F_{2, 81}= 108.92, p< 0.0001 months; F_2,81= 19.42, p< 0.0001 treatment; F_{4, 81}= 24.19, p< 0.0001 months treatment). In the case of grapefruit, GA₃-treatments caused an increase in leaf drop compared to control trees. For oranges (Figure 3B) leaf drop also increased in trees treated with GA₃ and surfactant (two-way ANOVA, F_{1, 228}= 113.48, p< 0.0001 months; F_{5, 228}= 22.98, p< 0.0001 treatment; F_{5, 228}= 35.65, p< 0.0001 monthstreatment).

GA₃ effect on fruit weight. From a grower's perspective, a highly positive effect of GA₃ and surfactant applications was a significant increase in grapefruit weight during the 1993-94 seasons (two-way ANOVA, F_{3, 320}= 50.4, p< 0.0001 months; F_{1, 320}= 31.376, p< 0.0001 treatment; F_{3, 320}= 2.263, p< 0.081 monthstreatment) (Figure 4A). On average, grapefruit weight increased 53 g per fruit, equivalent approximately 1.87 t ha^-1. Treated orange (same dose as above) also weighed significantly more than untreated ones (two-way ANOVA, F_{5, 564}= 1.178, p< 0.015 months; F_{1, 564}= 4.636, p< 0.0001 treatment; F_{5, 564}= 2.439, p< 0.0001 monthstreatment) (Figure. 4B). On average, orange weight increased 34 g per fruit, equivalent approximately 1.7 t ha^-1.

GA₃ effect on fruit drop. Throughout the orange harvest period, from December through March, fruit drop was reduced in treated trees at the end of the harvest season, when control trees did not retain fruit (two-way ANOVA, F_3,1896= 39.6, p<0.0001 months; F_5,1896=11.13, p<0.0001 treatment; F_15,1896=1.63, p=0.06 months*treatment) (Figure 5A).

GA₃ effect on orange harvest delay. Results obtained during the 1992-1993 season, indicated that GA₃ significantly delayed the time at which the predetermined harvest threshold was reached i. e., 13 fallen fruit under the canopy of a tree after 31 March. Treatments that combined GA₃ with a surfactant allowed the grower to harvest fruit up to six weeks later than control trees and 15 days later than trees treated with GA₃ alone (without surfactant). Treatments that retained fruit on the tree for the longest time were 10 mg L^-1 GA₃ plus 0.05% L-77 (1 application) and 20 mg L^-1 GA₃ plus 0.05% L-77 (1 application) (Figure 5B).

 

DISCUSSION

An economic perspective on the results of the application of AG₃ in citric orchards in the State of Veracruz. Believe that results have important practical implications in terms of orchard management and economics. First, GA₃ applications significantly increased the average weight of fruit. On average, weight increased 34 g and 29 g per fruit, for oranges and grapefruit, respectively. We highlight the fact that the weight increase alone can be potentially important in terms of economic gains. Considering a mean average yield of 50 000 oranges ha^-1 for a small producer with a limited amount of capital (Aluja et al., 1996), 34 g per fruit would entail an increase in total yield of approximately 1.7 t ha^-1. Considering a mean size of 50 ha per citrus grove, there is a potential 85 tons yield increase, which represents thousands of US dollars at current prices. We note further, that the significant increase in fruit weight, could be achieved with a relatively low GA₃ dose of 15 mg L^-1 + 0.05% L-77.

Another positive effect of GA₃ and surfactant treatment was that it reduced orange drop, particularly at the end of the harvest season (Figure 5A). This effect, plus the fact that significantly more GA₃-treated fruit maintained market quality (i. e., heavier, tougher skin and better coloration) after the peak harvesting period was over, opens up a possibility for the Veracruz orange growers to extend the harvest season into the month of May. The most effective doses were 10 mg L^-1 GA₃ + 0.05% L77 (1 application) and 20 mg L^-1 GA₃ + 0.05% L-77 (1 application). In both of these cases, the surfactant Silwet L-77 appeared to enhance the effectiveness of GA₃ treatments (Figure 5B).

According to an economic study conducted by us (Aluja et al., 1996), delaying the harvest could help growers substantially raise their profits. In Mexico, prices increase up to eight-fold late in the season (May through August). For example, an extra gain of $190 US dollars ha^-1 could be obtained if fruit were offered to the national market in May. This, added to the fact that yield increases of up to 8.5 t ha^-1 can be obtained by the increase in treated fruit weight when compared with untreated plots (7 t ha^-1) represents an average increase of 17% in overall yield. In our opinion, such a scenario would render the GA₃ applications highly profitable.

In addition to the above, GA₃ (applied with or without a surfactant) is not toxic and does not harm beneficial insects (Greany et al., 1994). Furthermore, GA₃ has been proven to be an effective means of control against A. suspensa when fly populations are very low (Greany et al., 1994). In the case of the Mexican fly of the fruit, A. ludens, the effect of the GA₃ is much less effective, had that the females manage to evade the toxic barrier of the fruit being deposited their less egg far from the same (Birke et al., 2006).

Effect of GA₃ on fruit characteristics and leaf drop. This study confirms the usefulness of GA₃ in helping sustain early season properties (e. g., peel firmness) of both 'Ruby Red' grapefruit and 'Valencia' oranges. The effect of GA₃ on peel firmness and peel coloration was most apparent when GA₃ was applied in conjunction with a surfactant. Treating grapefruit with two applications of 20 mg L^-1 GA₃ and 0.1% Silwet L-77 yielded the hardest and greenest fruit, while treating oranges with a single application of 20 mg L^-1 GA₃ and 0.05% Silwet L-77 yielded the hardest and greenest oranges (Figures 1 and 2). The observed dose-dependent effect found is also consistent with other reports of GA₃ combined with different concentrations of Silwet L-77 in delaying 'Marsh' grapefruit peel softening and colour change (Greany et al., 1987; McDonald et al., 1987).

Although GA₃ delayed senescence, it also increased leaf drop. Similarly, Coggins et al. (1965) working with various citrus cultivars in California, USA; observed an increase in leaf drop when GA₃ was applied at high dosages in combination with a surfactant. In the case this study, leaf drop in grapefruit was highest one month after the final application and then levelled off (Figure 3A); it is possible that trees become stressed shortly after GA₃ applications.

For oranges, leaf drop accelerated one month after treatments in both control and trees treated with 20 mg L^-1 GA₃ + 0.05% L-77 and 10 mg L^-1 GA₃ + 0.05% L-77 (Figure 3B). Higher doses of GA₃ with surfactant enhanced leaf drop compared to control trees for the month of November (Figure 3B). In the case of grapefruit, treatments of 15 mg L^-1 GA₃ + 0.035% L-77 and 15 mg L^-1 GA₃ + 0.05% L-77 presented the highest leaf drop compared to control trees.

A delicate balance needs to be established for surfactant use. In both grapefruit and oranges, a high surfactant dose increased leaf drop. Total defoliation of the tree is obviously undesirable. However in this case, the leaves that fell from both citrus cultivars were for the most part, old and photosynthetically inactive. This can generate a rapid defoliation of the tree with a concomitant positive effect on future yields. Nevertheless, the question is whether a tree would be able to sustain such stress over consecutive seasons and whether this would eventually reduce the productive life of the tree? Further, it needs to be determined if other surfactant can act more efficiently or if local weather and orchard microclimatic conditions also play a role in the leaf-drop phenomenon observed in this study.

Based on all the above, there is a lesson to be learned when attempts are made at transferring and applying novel technologies in orchards that are sub-optimally managed. There are potential dangers that can result in costs to the grower. Citrus trees in Veracruz are, for the most part, under severe nutrient, water balance and climatic stress. Applying a plant growth regulator to these types of trees with the goal of maximizing productivity, could backfire in the long run, such as the question of long-term effects of severe defoliation, previously mentioned. We therefore caution that GA₃ and surfactant should be applied at the lowest possible doses to minimize collateral effects.

The results are encouraging as they open up the possibility of combining GA₃ treatments with applications of the synthetic host marking pheromone of Anastrepha ludens (Aluja et al., 2009). The synergy of these two biorational management mechanisms will undoubtedly foster the development of more environmentally-friendly fruit fly management schemes, particularly for economically important fruit flies (Aluja and Mangan, 2008).

 

CONCLUSIONS

The GA₃ application in conjunction with a surfactant delayed citrus senescence, increased fruit weight and extended the harvest period, all of which are likely to result in economic benefits for citrus growers. Further studies testing other surfactants are necessary to reduce secondary effects such as defoliation.

 

ACKNOWLEDGMENTS

The Bigurra-Armida family allowed us to work in their citrus grove. A. Zuñiga, I. Jácome, M. López, J. Piñero, A. Vázquez, E. Piedra, O. Díaz, C. Ruiz and A. Diego provided technical assistance, and R. Macías-Ordóñez and J. Piñero advised on statistical analyses. D. Pérez-Staples, F. Díaz-Fleischer, J. Piñero, C. Fowler, J. Sivinski, J. Rull, T. Williams, and tow anonymous reviewers provided valuable comments on earlier drafts. N. Righini, A. Anzurez-Dadda and S. Tamez-Cruz format the manuscript and the figures. USDA-ARS (Specific Cooperative Agreement No. 58-661563-006), ABBOTT Laboratories, Fondo de Estudios e Investigaciones Ricardo J. Zevada, and the Mexican Campaña Nacional Contra Moscas de la Fruta provided financial support.

 

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