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Jan van Niekerk, Charl Kotze, Jade North and Paul Cronje

Phosphonate foliar applications in the period before harvest are routinely used in citrus (Citrus sp.) production for the control of phytophthora brown rot (Phytophthora sp.) control. However, several grower reports indicated that these applications caused phytotoxic damage on ‘Nadorcott’ mandarin (Citrus reticulata hybrid) fruit. To investigate this, trials were conducted over two seasons (2016 and 2017) in two climatically different citrus production areas of South Africa. These trials consisted of ammonium and potassium phosphite foliar applications (at full dose or half dose) at green, color break, or full color stages of fruit development. At commercial harvest, fruit was sampled from the different treatments and the incidence of the phytotoxic damage was documented as both percentage incidence and a phytotoxic index (PI). Results indicated that, regardless of the type of phosphonate or dosage applied, phytotoxic damage was observed at harvest if foliar applications were carried out at color break or full color stage of fruit development. The same results were observed in the different climatic areas, although the mean percentage of damaged fruit varied between the areas. Based on these results it is recommended that skirt pruning be used to mitigate phytophthora brown rot on ‘Nadorcott’ mandarin fruit.

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Ockert P.J. Stander, Jade North, Jan M. Van Niekerk, Tertia Van Wyk, Claire Love and Martin J. Gilbert

This study aimed to determine the effects of different types of nonpermanent netting (NPN) on foliar spray deposition, insect pest prevalence, and production and fruit quality of ‘Nadorcott’ mandarin (Citrus reticulata) trees in a commercial orchard at Citrusdal (lat. 32 32′31″S, long. 19 0′42″E), Western Cape, South Africa. The deposition quantity (FPC%) of foliar spray volumes of 3500, 7000, or 15,000 L·ha−1 was greater for leaves of control trees compared with leaves treated with NPN during summer (January) (8.8 vs. 6.1; P = 0.0055) and winter (June) (4.8 vs. 3.1; P = 0.0035). Deposition uniformity (CV%) was better for control leaves during summer (64.9 vs. 75.2; P = 0.0062) and winter (59.6 vs. 80.5; P = 0.0014), and deposition quality (ICD%) was better during winter (79.4 vs. 84.2; P = 0.0393). There were no differences in FPC%, CV%, and ICD% for fruit when foliar spray volumes of 3500 and 15,000 L·ha−1 were used for the control and NPN treatment groups during winter. However, with a foliar spray volume of 7500 L·ha−1, fruit from the control treatment group had greater FPC% (19.3 vs. 6.1; P = 0.0262), CV% (70.3 vs. 50.9; P = 0.0484), and ICD% (57.1 vs. 79.9; P = 0.0157). There were no differences in macronutrient concentrations between the leaves of trees subjected to control and NPN treatments, but leaf zinc (<81%; P = 0.0317) and iron (<78%; P = 0.0041) concentrations were lower with the NPN treatment. During short NPN treatments, fruit yield was reduced by ≈37% compared with that after control treatment, and longer NPN treatments had no effect on fruit yield. The reduction in fruit yield with NPN was not related to the effects of NPN on foliar spray deposition or to lower leaf micronutrient concentrations. The lower fruit yield during short NPN treatments was most likely caused by fruit drop that was exacerbated by the removal of the NPN. In the long NPN treatment group, fruit damage caused by sunburn was reduced by 17%, but the outer canopy fruit experienced increased wind damage or scarring. Except for the lower titratable acidity content with the shortest NPN treatment and the higher Brix°:TA ratio with two NPN treatments, NPN did not impact other fruit quality attributes. The use of NPN excluded male wild false codling moths (Thaumatotibia leucotreta) (FCM) males; however, it was still possible to capture a very small amount of mass-released sterile FCM and wild fruit flies under the NPN.