In 2015, soft citrus (easy peelers) made up 9335 ha of the 68,000 ha within the South African citrus industry and is expected to significantly increase in the next 10 years. Within this group, late mandarin account for 6560 ha and has earned growers in 2015 a gross income of more than R11,000 (South African Rand) per tonne (South African Citrus Growers Association, 2016). Any disease that reduces yield and fruit quality can, therefore, greatly affect the profitability of this high value cultivar. In South Africa, brown rot of fruit is primarily incited by Phytophthora nicotianae or Phytophthora citrophthora (Meitz-Hopkins et al., 2013). It is a disease that can severely reduce yield, fruit quality, or both in the orchard or in the postharvest cold chain (Adaskaveg et al., 2015; Montenegro et al., 2008). The disease is especially severe in areas where rainfall occurs during the late stages of fruit development and maturation (Adaskaveg et al., 2015).
Propagules of the aforementioned two pathogens are present in most orchard soils, from where they are readily splashed onto low-hanging citrus fruit. Sporangia form on the low-hanging fruit from where they can be splash-dispersed to fruit higher up on the tree (Graham et al., 1998; Timmer et al., 2000). Brown rot epidemics are further promoted by periods of prolonged wetness (more than 7 d) and temperatures ranging between 23 and 32 °C (Graham et al., 1998; Timmer et al., 2000). These conditions are often prevalent in the cooler, winter rainfall citrus production areas of South Africa. ‘Nadorcott’ mandarin trees are known to bear heavily, resulting in branches bending down under the fruit weight, with numerous fruit often hanging close to the orchard floor (J. Joubert, personal communication). This characteristic and the fact that it matures during June–August, when rain often occurs in the aforementioned production areas, leads to the increased risk of severe brown rot epidemics occurring in mandarin orchards.
Properly timed foliar applications with phosphonates have been shown to be an excellent preventative control measure for brown rot of citrus fruit and root rot incited by Phytophthora species (Graham, 2011). For brown rot control in South Africa, it is specifically recommended to be applied 1 month or less before harvest (Van Zyl, 2017). Phosphonates are easily absorbed by the leaves of citrus trees from where they are translocated through the phloem to sinks, such as developing fruit and roots (Graham, 2011; Ouimette and Coffey, 1990). At the sites where they accumulate, they have been shown to have a direct fungistatic effect on invading pathogens and activating the plant’s own defense mechanisms (Afek and Sztejnberg, 1988, 1989; Fenn and Coffey, 1984, 1985; Smillie et al., 1989).
This direct and indirect control action combined with a maximum preharvest interval of 28 d makes late-season phosphonate applications an attractive option for citrus growers who are expecting rain close to harvest that could trigger a brown rot epidemic incited by P. nicotianae (warmer production areas) or P. citrophthora (cooler production areas) (Hardman and Hattingh, 2016). However, an increasing number of reports were made by growers in cooler, winter rainfall, production areas of South Africa that they are experiencing phytotoxic damage to mandarin fruit when they applied phosphonates at late fruit developmental stages, when color development is advanced. As this was the first of the reports of such damage on mandarin fruit, further investigation was warranted.
Le Roux (2000) reported that foliar sprays of phosphonates can cause phytotoxic damage to citrus leaves and rapidly developing fruit in the late season if the application rates are high, as well as if spraying is carried out at high ambient temperatures or if the treated trees are under drought stress. However, investigation of the reports from growers indicated that label recommendations regarding application conditions, timing and dosages were strictly adhered to, thereby, eliminating these as possible causes for the observed damage. However, Walker (1989) reported incidences of phytotoxic damage to leaves of small, nonfruiting mandarin trees treated with foliar phosphonate sprays and that the damage increased with increasing dosages. Furthermore, Manrakhan et al. (2015) found similar damage on ‘Nadorcott’ mandarin fruit when spinosad-based bait sprays were applied for the control of fruit flies (Ceratitis sp.). In this study, it was found that damage only occurred on fruit that were at the immature green and color break stage. These findings, therefore, indicate a possible change in susceptibly due to changes occurring during maturation of the rind.
As stated previously, phosphonate foliar applications in the period close to harvest are highly effective for the control of phytophthora brown rot (Graham, 2011). The aim of this study was to verify and quantify any possible phytotoxic damage to ‘Nadorcott’ mandarin fruit caused by phosphonate foliar applications, aimed at phytophthora brown rot control, at various fruit developmental stages, over two seasons (2016 and 2017) in two orchards, located in climatically diverse production areas.
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