The effect of preharvest nutrition and crop load on fruit quality and postharvest disease in mango (mangifera indica l.)

Abstract

The aim of this thesis was to investigate how preharvest calcium (Ca) and nitrogen (N) nutrition, and low crop load during fruit development affect fruit quality and postharvest disease of Keitt mangoes (Mangifera indica L). Calcium, N and crop load were chosen because previous research and Gower reports implicate them as major factors in internal disorders, poor colour development, and postharvest disease in mangoes grown in the Mareeba district of Queensland. The symptoms and incidence of the internal fruit disorders seen in Keitt mangoes growing in Queensland were investigated to determine their similarity to other forms of internal disorders, and to develop a severity rating scale to assess fruit in later experiments. While the observed disorder in Keitt shared some symptoms with other named forms of disorders, not all symptoms were similar. The typical symptoms of patches of watery translucent flesh that breakdown and discharge cellular fluids in the more severe cases, are referred to as watery-pulp breakdown throughout this thesis. Watery-pulp breakdown-affected fruit had lower mesocarp Ca concentrations (0.05% dw) than healthy fruit (0.10% dw), but not N, K, P, and Mg concentrations. The severity of symptoms increased as maturity of the fruit increased, and early harvest may be a useful strategy to minimise the disorder. The first nutrition experiment investigated the effects of Ca, N and crop load on fruit quality in 6-year-old Keitt trees, between 1997 and 2000. Trees were fertilised with high (>1 kg Ca tree-1 and >300 g N tree-1) or nil concentrations of Ca and N, and natural or 70%-reduced crop loads. Fruit were assessed for watery-pulp breakdown, fruit colour, postharvest disease, fruit firmness, yield and average fruit weight. Although Ca was applied above the normal industry rates, there were no significant increases in fruit-Ca or leaf- Ca concentrations, or response in any of the fruit quality parameters measured. The factors influencing Ca concentrations in mesocarp were shown to be changes in dry matter accumulation and periodic reductions in Ca supply. Fruit from the high N treatment had significantly (p < 0.05) greater incidence and severity of postharvest side disease lesions (mainly anthracnose) but not stem lesions (mainly stem-end rot). The high N treatments also significantly (p < 0.05) reduced the percentage of yellow skin, and the percentage and intensity of red blush colour in ripe fruit. Watery-pulp breakdown, fruit firmness and average fruit weight were not significantly affected by the high N treatments. Reducing the crop load throughout fruit development significantly (p<0.05) increased the average fruit weights and the incidence of watery-pulp breakdown, but did not significantly affect the other fruit quality parameters measured. Further studies on Ca uptake and distribution were carried out in 3 additional experiments. The effects of Ca supply on fruit quality were investigated in a similar manner to the first experiment, by varying soil-applied Ca. Increased rates (1726 g tree-1 Ca) and more frequent applications (fortnightly) of gypsum were used to overcome leaching. Although soil Ca concentrations at harvest (0 - 30 cm deep) were increased by the Ca treatment, no significant effects were observed on fruit quality. The effect of leaf transpiration on leaf and fruit Ca concentrations was investigated by reducing leaf transpiration with an anti-transpirant throughout flowering and fruit development. Reduced leaf transpiration did not significantly affect mesocarp or leaf Ca, N, P, Mg, K, B concentrations, indicating the balance of transpiration mass flow between leaves and fruit is not a major factor in mango fruit Ca accumulation. Temporal changes in uptake and distribution of Ca during flowering and fruit development were studied using elemental strontium (Sr) as a Ca analogue. Sr and Ca concentrations were not similar during the experiment making prediction of Ca uptake and distribution based on Sr uptake and partitioning unsuitable. The effect of high N in increasing postharvest fruit disease in the first experiment was further studied to confirm the earlier results and to identify the mechanisms involved in this relationship. Fruit with a range of N concentrations were produced from trees fertilised with a range of N rates (0 417 g tree-1). The incidence and severity of postharvest anthracnose (C. gloeosporioides) increased as fruit-N increased, with significant relationships between anthracnose severity and exocarp-N concentration (anthracnose severity in sprung fruit = 163.18x2 151.85x + 36.987, r2 = 0.97**). High N had a similar effect on the incidence of Penicillium sp., Rhizopus sp., Guibnardia sp., or Epicoccum sp., but not Dothiorella sp. (stem-end rot). The antifungal compounds 5-n-pentadecyl resorcinol and 5-n-heptadecenyl resorcinol were influenced by exocarp-N in two ways. Resorcinol concentrations were lower in fruit with high exocarp-N than low-N fruit, and concentrations decreased faster in ripening fruit with high exocarp-N. The differences in resorcinol concentrations between high- and low-N fruit was strongest in the sprung stage of ripeness, and positive exponential relationships were found between exocarp-N and concentrations of 5-n-pentadecyl resorcinol (p=0.006. r2= 0.92) and 5-n-heptadecenyl resorcinol (p=0.016, r2=0.82). There were also positive relationships between anthracnose severity and 5-n-pentadecyl resorcinol (p=0.003, r2=0.86) and 5-n-heptadecenyl resorcinol (p=0.003, r2 =0.87). No significant differences were observed in exocarp morphology to explain the N effect on postharvest disease incidence or severity. The effect of timing of application of N on fruit quality and disease was investigated in a field experiment on 8-year-old Keitt mango trees on which N was applied at flowering, at mid fruit development, 2 weeks pre-harvest, or not applied (control). N applied at flowering and mid fruit development, significantly increased the severity of postharvest Colletotrichum sp. (p<0.05) Penicillium sp., Rhizopus sp., Guibnardia sp., and Epicoccum sp. (p<0.01) in ripe fruit. N applied 2 weeks prior to harvest did not significantly affect postharvest diseases, with fruit having similar anthracnose severity and incidence to the nil-N (control) treatment. Fruit Ca concentrations were not able to be manipulated by imposing field treatments. The inability to easily manipulate fruit Ca concentrations indicates it is unlikely to be an effective tool to manipulate fruit quality in mango. Nitrogen was shown to greatly influence fruit quality, having a major influence on postharvest disease, skin colour and sunburn susceptibility, but not watery-pulp breakdown. Increased N was also shown to reduce concentrations of natural antifungal resorcinol compounds as fruit ripened, making fruit more susceptible to postharvest rots. The sensitivity of fruit quality to N makes management of N a critical tool in managing fruit quality and disease susceptibility. Reduced crop load had a major effect on the incidence and severity of watery-pulp breakdown, indicating tree management practices to maximise crop load will also reduce the severity of watery-pulp breakdown.