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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 and Paul J.R. Cronjé

Hand thinning is not often applied as a commercial cultural practice in citriculture due to the practice’s reliance on costly manual labor. However, hand thinning could provide unique benefits such as treatment selectivity and easier control over thinning intensity, as opposed to foliar sprays of chemical thinning agents. In “on-year” ‘Nadorcott’ mandarin (Citrus reticulata) trees, summer (January) and autumn (April) hand-thinning treatments of removal of all fruit <20 and <40 mm diameter were evaluated for effects on leaf carbohydrates and fruit growth rate. Other factors assessed included treatments’ effects on tree total fruit yield, fruit quality, and fruit size distribution. In addition, two different summer hand-thinning treatments (removal of all fruit <20 and <25 mm) were evaluated for effects on fruit size distribution and fruit yield over two seasons to determine their potential financial implications. Summer hand thinning reduced the numbers of small fruit and increased initial growth rate of prevailing fruit. This resulted in larger numbers of premium-sized fruit per tree, without treatments reducing total fruit yield and fruit quality. Additional labor was eliminated by quicker picking of fruit during harvest, and in season 2, fruit yield was higher for summer hand-thinning treatments compared with control. A higher potential income resulted from change in fruit size distribution and the breaking of alternate bearing over the 2-year period. The results provide producers of late mandarin cultivars with an alternative to chemical thinning agents to manage crop load and improve fruit size in individual “on-year” trees.

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Paul J.R. Cronje, Graham H. Barry, and Marius Huysamer

Because photosynthesis provides the required carbohydrates for fruit development and respiration releases the stored energy from these carbon compounds, interalia during postharvest storage, it is therefore important that fruit tissues have an adequate carbohydrate concentration at the start of the postharvest period to ensure optimal storage life. In addition to photosynthate supply from leaves, the chlorophyll-containing flavedo of citrus (Citrus sp.) fruit (outer, colored part of the rind) has the ability to fix CO2 through its own photosynthetic system. In this experiment, spanning three seasons, the three main sugars (sucrose, glucose, and fructose) were quantified in the flavedo of ‘Nules Clementine’ mandarin (Citrus reticulata) fruit during Stages II and III of fruit development. Flavedo was sampled from fruit borne on the inside (low light intensity) or outside (high light intensity) of the tree’s canopy. In one season, the photosynthetic and respiration rates of fruit borne in the two canopy positions were measured pre- and post-color break (March and April, respectively). Sucrose concentration increased constantly from initial sampling in February until harvest (May), whereas glucose and fructose concentrations increased significantly only during the last month of fruit development. The flavedo of inside fruit, developing under low-light conditions, was less well colored (higher hue angle) and had a lower sugar concentration compared with outside fruit developing under conditions of high light levels. This response could be attributed to the higher pigment concentration leading to a higher photosynthetic rate as well as greater sink strength of the outside fruit. The inside fruit had an increased susceptibility to the progressive postharvest physiological disorder, rind breakdown. The lower carbohydrate and pigment concentrations of the rind from fruit borne inside the canopy compared with those from the outside of the canopy could be indicative of a weaker rind condition at the time of harvest.

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Paul J.R. Cronjé, Gerard Jacobs, and Nigel C. Cook

Two-year-old apple branches, ≈50 cm long, were selected from a commercial `Royal Gala' orchard in the Ceres (Koue Bokkeveld) region of the Western Cape, South Africa [33 °S, 945 m, 1500 Utah model chilling units (CU)]. In 2000, the branches received either cold storage at 5 to 7 °C or natural chilling in the field. In 2001, the trial was repeated, but only with field chilling. The branches received five dormant pruning treatments: control (not pruned); pruning back to the fourth lateral shoot (heading) before or after chilling; and removal of the second and third lateral shoots (thinning) before or after chilling. After pruning and chilling, the branches were removed from the orchard or cold room every 2 weeks and forced in a growth chamber at 25 °C. The rate of budburst (1/days to budburst) was determined for the terminal buds of the lateral shoots. Lateral shoots on the 2-year-old branches were categorized according to position: the most distal extension shoot, and all other laterals grouped. Removing distal tissue by pruning (heading more than thinning) enhanced the effect of chilling on the terminal buds on the lateral shoots and promoted budburst. Pruning was more effective before than after chilling. Pruning enhanced the growth potential of the terminal buds on proximal shoots on 2-year-old branches.

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Ockert P.J. Stander, Karen I. Theron, and Paul J.R. Cronjé

Various mandarin (Citrus reticulata) cultivars are prone to fruit splitting, a physiological disorder that entails cracking of the rind, starting from the stylar end of the fruit, with eventual splitting of the endocarp and abscission of the fruit. On two mandarin cultivars, Marisol and Mor, foliar applications of 2,4-dichlorophenoxyacetic acid (2,4-D), calcium (Ca), and potassium (K) were evaluated over two growing seasons for efficacy to reduce fruit splitting in Paarl, South Africa (lat. 33°69′S, long. 18°95′E). Foliar treatment of 10 mg·L−1 2,4-D directly after physiological fruit drop (APFD) compared with later dates in January and February, either alone or in combination with K, increased rind thickness and reduced fruit splitting of ‘Marisol’ and ‘Mor’ by up to 50%, without negatively affecting internal fruit quality. Treatments increased rind thickness and rind strength throughout fruit development in addition to fruit diameter, length, and growth rate, with no significant effect on rind coarseness. There was a slight reduction in juice content and titratable acidity (TA), but no effect on the total soluble solids (TSS). Application of 2,4-D APFD is thought to increase rind integrity due to a direct strengthening effect on the rind during early stages of fruit development.

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Ockert P.J. Stander, Graham H. Barry, and Paul J.R. Cronjé

The objectives of this study were to improve the understanding of the mechanism of alternate bearing and the role of carbohydrates in ‘Nadorcott’ mandarin (Citrus reticulata) trees. Selected phenological responses were measured in natural heavy- (“on”) and low-fruiting (“off”) ‘Nadorcott’ mandarin trees grown under commercial South African production conditions. The relationships with seasonal leaf and root carbohydrate concentrations were evaluated at the shoot-, branch- and tree level over two seasons. Fruit load [R 2 = (−)0.80 and R 2 = (−)0.73 in seasons 1 and 2, respectively; (P < 0.01)] and the number of newly developed vegetative shoots [R 2 = 0.81 and R 2 = 0.78 in seasons 1 and 2, respectively; (P < 0.01)] were the most important determinants of return bloom. Sprouting of a higher number of new vegetative shoots from “off” trees compared with “on” trees (“off” = 863 and 1439 vs. “on” = 306 and 766) was not related to leaf carbohydrate concentration. Root sugar concentration peaked during full bloom and higher root growth activity was observed before a higher number of new vegetative shoots developing in “off” trees during summer. The root sugar concentration early in the season was ≈3-fold lower, and root and shoot growth were absent, or lower in “on” trees compared with “off” trees. These results concur with previous research and confirm that fruit load in “on” trees inhibits summer vegetative shoot development, which manifests in poor flowering and an “off” year. This study shows that fruit are the major carbohydrate sink and probably disturb the balance between vegetative shoot development and root growth by limiting carbohydrate allocation to roots.

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Ockert P.J. Stander, Graham H. Barry, and Paul J.R. Cronjé

The significance of macronutrients nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) in leaves was studied in relation with their possible roles in alternate bearing of ‘Nadorcott’ mandarin (Citrus reticulata) trees over a period of three seasons. Fruit load (“on,” a heavy fruit load, vs. “off,” a light fruit load) affected the leaf macronutrient concentrations, and the amount of macronutrients removed through the harvest of fruit, i.e., the crop removal factor (g·kg−1), was consistent in both seasons. The crop removal factors were higher for each macronutrient in “off” trees—harvest of 1 kg fruit removed ≈2.3 g·kg−1 N, 0.3 g·kg−1 P, 3.1 g·kg−1 K, 1.0 g·kg−1 Ca, and 0.4 g·kg−1 Mg, compared with 1.3 g·kg−1 N, 0.2 g·kg−1 P, 1.7 g·kg−1 K, 0.6 g·kg−1 Ca, and 0.2 g·kg−1 Mg in “on” trees. Fruit load per tree (kg/tree) of 84, 110, and 52 kg/tree in “on” trees, however, removed ≈217 g/tree N, 28 g/tree P, 296 g/tree K, 100 g/tree Ca, and 35 g/tree Mg, which was 1.5–6 times more than that of fruit loads of 14, 71, and 16 kg/tree in “off” trees. In “off” trees, N, P, and K, and in “on” trees, Ca accumulated in leaves to between 20% and 30% higher concentrations in season 1, but the higher macronutrient status did not manifest in or consistently correlate with intensity of summer vegetative shoot development in the current season, or intensity of flowering in the next season, the two main determinants of fruit load in ‘Nadorcott’ mandarin. Apart from some anomalies, the concentrations of macronutrients in leaves were unaffected by de-fruiting and foliar spray applications of N and K to “on” trees, and showed no consistent relationship with treatment effects on parameters of vegetative shoot development and flowering. Leaf macronutrients in alternate bearing ‘Nadorcott’ mandarin trees, fertilized according to grower standard practice, are not related to differences in flowering and vegetative shoot development, and appear to be a consequence of fruit load and not a determinant thereof.