J. Gideon van Zyl and Paul H. Fourie
Citrus Research International | Department of Plant Pathology, Stellenbosch UniversityEwald G. Sieverding
Evonik Industries AG, Goldschmidtstraße, GermanyDavid J. Viljoen
Evonik Africa (Pty) Ltd, Somerset West
South African citrus fruit producers rely heavily on medium to high volume fungicide spray applications (Grout, 1997, 2003) to protect citrus fruit from challenging diseases such as Citrus black spot (CBS) (Phyllosticta citricarpa (McAlpine) van der Aa (syn. Guignardia citricarpa Kiely)) (Kotze, 1981, 2000; Schutte et al., 1997) and Alternaria brown spot (Alternaria alternata (Fr: Fr) Keissl., tangerine pathotype) (Schutte, 1996).
Citrus trees in South Africa are generally large and dense, with size depending on cultivar, rootstock, planting density and climatic region. Tree geometry and density complicates adequate deposition of fungicide or insecticide sprays on outer and the difficult-to-reach inner canopy susceptible leaves and fruit (Cunningham and Harden, 1998, 1999; Hoffmann and Salyani, 1996; Farooq and Salyani, 2002, 2004). Effective deposition of the active ingredient on target surfaces (leaves, twigs and fruit) is needed for effective disease control since disease control and spray deposition are closely related (Holownicki et al., 2002; Van Zyl et al., 2013). Hence medium to high volume fungicide spray applications ranging from 6000 to 16,000 l ha1 (with 8000 l ha1 being the norm) (Grout, 1997, 2003), which is almost double or triple the volumes used in other citrus producing counties such as Spain (Vicent et al., 2009) and Florida in the United States of America (Dewdney and Timmer, 2012). These application volumes do provide an acceptable balance between efficacy and efficiency based on existing economic considerations, especially considering the emphasis placed on effective CBS control given its quarantine status in certain export markets (EPPO, 2014). Most importantly, it serves as a “buffer” for loss of efficacy due to calibration and operator error and the use of inadequate spray machinery, equipment and technique. However, these high spray volumes are super-optimal, costly and not efficient in terms of time and input costs. Deposition is also not optimally efficient due to spray run-off and exo- and endo-drift (Salyani and Farooq, 2004; Fourie et al., 2009; Cunha et al., 2012; Schutte et al.,2012). Off-target deposition of fungicides is increased at these excessively high spray volumes (8000 l ha1 and higher) (van Zyl and Fourie, unpublished results), which in turn is an economical loss and an environmental pollution problem (De Jong et al., 2008; Furness et al., 2006a,b; Salyani and Farooq, 2004; Stover et al., 2002; Cunha et al., 2012). Reduced volume sprays have the potential to reduce the economic and environmental impact/cost of
fungal disease control and to be more effective (Cunningham and Harden, 1998, 1999).
Adjuvants can be used to potentially reduce spray volumes and as a result reduce application time and input costs and improve deposition parameters and disease control (Butler Ellis and Tuck, 1999; Green and Beestman, 2007; Gaskin et al., 2004; Gent et al., 2003; van Zyl et al., 2010a,b). Organomodified trisiloxanes or organosilicones as tank mix adjuvants are non-ionic surfactants that dramatically reduce surface tension and/or modify surface characteristics of hydrophobic leaves and/or fruit thereby improving wetting, spreading and dispersing effect of the sprayed mixture on the surface or interface (Hazen, 2000). The interaction between adjuvant concentration and spray volume and to some extent the effect it has on the biological efficacy of certain crop protection products (Greyson et al.,1995,1996) has been studied on fruit (Stevens, 1993), easy to wet foliage of potatoes (Greyson et al.,2006) and on difficult to wet foliage of wheat (Gaskin and Murray, 1997; Greyson et al., 1996).
In all cases, depending on the surface characteristics of the sprayed target, it was found that increased concentration of adjuvant or organosilicone at increased spray volumes led to increased spray run-off and reduced retention, whereas, increased concentration of organosilicone and decreased spray volume had the opposite effect (De Ruiter et al., 1990; Gaskin and Murray, 1997; Stevens et al., 1993). This interaction and its results are likely to influence the biological efficacy of crop protection products deposited.
In South Africa, these types of adjuvants are regularly used with fungicide and pesticide application, yet little literature exists on the effect of adjuvants, specifically organo tri-siloxane adjuvants at different application volumes in citrus canopies. Therefore, the objectives of this study were to evaluate the influence of two organo tri-siloxane surfactants at reduced application volumes in South African citrus orchards on different deposition parameters. In previous studies, an spray deposition assessment protocol, consisting of fluorometry, digital photomacrography and image analysis, was developed and improved (Brink et al., 2004, 2006; Fourie et al., 2009; van Zyl et al., 2010a,b, 2013) and was recently used to effectively determine the deposition quantity (the amount of active ingredient landed and retained on a target surface) and persistence (amount of product retained over time) of copper fungicides on orange leaves and fruit (Schutte et al., 2012). Recent improvements to the spray deposition assessment protocol also allows for determining deposition quality (uniformity of active ingredient distribution on the target surface) (J.G van Zyl; unpublished results). This deposition assessment protocol was used in this study.
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