CSIRO PUBLISHING www.publish. csiro.au/journals/ajea A u s tra l i an J ou rn a l of E x p er i mental A gri cul ture, 2007, 41, 63 5-644 Ammonium thiosulfate and 6-benzyladenine improve the crop load and fruit quality of 'Delicious'apples S. A. BoundA,B and S. J. Wilson^ ATasmanian lnstitute of Agricultural Research, University of Tasmania, 1 3 St Johns Avenue, New Town, Tas. 7008, Australia. ECorresponding author. Email: sally.bound@dpiwtas.gov.au Abstract. Four trials were conducted over 3 years to assess the effect of ammonium thiosulfate (ATS) as a blossom thinner of 'Delicious' apple trees. In addition to determining the optimal concentration of AIS and the effectiveness of multiple applications, the effect on fiuit quality was assessed. A program using ATS at bloom and 6-benzyladenine (BA) applied postbloom was examined. Multiple applications of AIS reduced crop load more than a single application. A concentration of l.}Yo (v/v, 5800 mgA) applied twice during the flowering period is recommended, with the first application at2lo/obloom and the second at 80% bloom. Fruit firmness and sugar content were increased by ATS, and a positive relationship between fruit weight and sugar content and between fruit weight and firmness was demonstrated. ATS can also be effectively combined in a program with BA with the added benefits of increases in fruit size, firmness and sugar content; however, caution is advised to prevent overthinning. Additional keywords: desiccant, fruit set, postbloom thinner. Introduction When selecting bioregulators for a chemical thinning program, issues that need consideration include efficacy and reliabiliry environmental impact, and availability of alternative chemicals. The efficacy of naphthalene acetic acid and ethephon, both currently in use as blossom thinners in Australia, and the postbloom thinner 6-benzyladenine (BA) is dependent on weather conditions, particularly temperature, both at the time of application and the period following application (Jones and Koen 1985; Stover 1992; Boundet aI.1997). Although it can be difficult to achieve consistency with these bioregulators, particularly given the unpredictable spring weather in most Australian apple-growing regions, the loss of any of these bioregulators would cause problems similar to those in the USA following the removal of dinitrocresol from the market (Williams et al. 1995). Hence, it is important to have a range of bioregulators available for thinning, giving growers several options to fit different circumstances. The desiccant ammonium thiosulfate (ATS) is an effective blossom thinner of'Delicious' (Bound and Jones 2004) and 'Elstar', 'Idared', 'Jonagold' and 'Golden Delicious' apples (Schroder and Bangerth 2005); however, there is little information available on the impact of AIS on fruit qualiry or on how ATS interacts with postbloom thinners. Interactions between chemicals used in a spray program can have deleterious effects on crop load and/or fruit quality. Spray programs including both Cytolin (registered as Promalin in the USA) and naphthalene acetic acid can result in either severe over-thinning or excessive production ofpygmy fruit (Bound e/ al.l99la). Use of paclobutrazol on 'Delicious' within 7 days of carbaryl also results in excessive fruit thinning (Jones et al. 1991). There has been little reported on the effect of desiccant-t1pe thinning agents on internal fruit quality parameters such as soluble solids content and firmness, or their impact on seeds. Byers (1 997) discussed the impact ofendothal, pelargonic acid YI-1066, Wilthin and ATS on fruit skin finish, and Ferree and Schmid (2001) also examined the effect of endothal on skin finish. Bound and Jones (1997) found that endothal increased both soluble solids content and firmness of 'Delicious' and Bound (2001) reported that the greater the number of applications of endothal, the greater the increase in fruit firmness and sugar content. In addition to determining the optimal concentration and number ofapplications forAIS as an apple thinner, the present study was aimed at assessing the impact of ATS on fruit qualiry and examining any interactions between AIS and the postbloom thinner BA (Cylex formulation produced by Abbott Laboratories, Chicago, IL, containing 2%BA). Materials and methods Four trials were conducted in the Huon Valley in southern Tasmania over three consecutive seasons on mature regular-bearing'Delicious'apple hees. Trial 1 was conducted on ll-year-old Hi-Early 'Delicious' hees at Cygnet. The following season, two trials were conducted, one on lO-year-old Oregon Spur 'Delicious'trees at Grove (trial 2) and the other on 9-year- old Hi-Early 'Delicious' ftees at Cradoc (trial 3). Trial 4 was conducted in the third season on 1 O-year-old Hi-Early 'Delicious' trees at Cradoc. All trial ftees were on MM106 rootstocks. O CSIRO 2OO7 t0.l07l/EA05217 0816-1089/071050635