Use of preharvest ethephon applications to reduce breba crop load.

Figs, F. carica, are harvested worldwide on 419,000 hectares, with an annual production of over 1 million tonnes. In the United States, ranked sixth in world production (FAO, 2007), figs are primarily grown in California (Stover et al., 2007). Fig trees, depending on the variety, may have one or two crops per year (Soby, 1997). The first crop of the season is called the breba crop and the second crop is called the fig crop. The breba crop is produced from overwintering fruit buds in one-year-old shoots from the previous season (Petrucci and Crane, 1950), while the fig crop is produced from fruit buds that differentiate in the current season shoots (Petrucci and Crane, 1950). The commercial crop is the fig crop, while the breba crop yields are low or nonexistent (Dominguez, 1990), with less flavorful and lower quality fruit (Doster and Michailides, 2007). The fig industry has been affected by an increase in labor cost, and as a consequence, in some cases, the harvest of the breba crop becomes economically unviable (Crisosto et al., 2007). If breba fruit are not harvested, they will persist on the tree or abscise, and eventually decay. These decayed breba are potential sites for fungal pathogens and may serve as insect attractants. In addition, spores produced on infected breba fruits can infect the commercial crop (Doster and Michailides, 2007). Consequently, reduction of the breba crop load would be beneficial to reduce decay and/or pest attacks, as well as to reduce nutritional competition with the second crop, which is the commercial crop. Ethephon (2-chloroethylphosphonic acid) is a systemic plant growth regulator that, in solutions of pH 4 or higher, decomposes to ethylene, phosphate, and chloride ions (R.S.C., 2007). In Crane et al. (1970), application of 500 ppm ethephon to ‘Mission’ figs during period I (first stage of fruit development, characterized by rapid growth) inhibited fruit growth and triggered fruit abscission in 5-6 days. Only minimal vegetative responses, such as some leaf epinasty and leaf abscission, took place occasionally (Crane et al., 1970). Fall applications of 200 ppm and 500 ppm ethephon (during leaf abscission) have been shown in peach to reduce crop load (Crisosto et al., 1990), and to induce severe injure to flower buds as well as preventing flowers from opening (Anderson and Seeley, 1993; Sloam and Matta, 1996), respectively. Based on the work done by Crane et al. (1970), application of ethephon to breba crop during period I would be a good strategy to reduce breba crop load. However, that study was only performed on 15 branches, and the effect on the whole tree was not evaluated. Fall ethephon applications, although not yet tested on figs, would be another approach to further reduce breba crop. Therefore, the objective of this study is to evaluate the effect of spring and fall preharvest ethephon applications, at different concentrations and times of application, on the total breba crop and on the following fig crop. This study was carried out for three seasons (2005-2006, 2006-2007, and 2007-2008) in a commercial fig orchard in Madera County, CA, and in the last season two more sites were added (another field in Madera and a field in Chowchilla). Fall and spring ethephon treatments (1501,000 ppm) were tested on ‘Conadria’ figs at different times during leaf abscission and during period I of breba fruit growth, respectively. The summer following fall and spring applications, at breba commercial maturity, the breba fruit were harvested and counted. The results were expressed as the number of breba harvested per tree and the number of breba harvested per trunk cross-sectional unit area (fruit cm). Breba weight and soluble solids concentration (SSC) were measured in 2007-2008 and 2007, respectively. Each season, the percentage of bud break (number of new shoots per total nodes) was measured. Four branches per tree, one in each quadrant of the tree at the middle height of the canopy, were used for bud break measurements. In 2007 and 2008, at fig commercial maturity, fig crop load and fig fruit weight were also measured. In 2007, breba fruit from trees sprayed in fall with 1,000 ppm ethephon, figs from trees sprayed in fall with 500 ppm ethephon, and figs from trees sprayed in spring with 500 ppm ethephon were analyzed for ethephon residues by the method GLC of ethephon and Fenoprop in apples, with a detection limit of 0.10 ppm. Fall applications of 500-1,000 ppm significantly reduced breba crop load. These concentrations represented a 61 and 65 % reduction in the number of breba harvested per tree and per trunk cross-sectional unit area, respectively. Time of application during leaf abscission did not have a significant effect on breba crop load. Spring application of 250-1,000 ppm ethephon significantly reduced breba crop load. These ethephon concentrations represented a 91.6 and 94.6 % reduction in the number of breba harvested per tree and a 91.7 and 93.8 % reduction in the number of breba harvested per trunk cross-sectional unit area in 2006 (early ethephon application) and 2007, respectively (figure 1). In spring 2006, early ethephon applications (breba fruit and leaves starting to develop) were more effective in reducing breba crop load. In spring 2007, time of application did not have a significant effect on breba crop load, which could be due to the low breba crop load in 2007.