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Integrated management of Penicillium digitatum in citrus fruit using preharvest silicon applications, plus postharvest hot water treatments

Iona Basdew: Discipline of Plant Pathology: University of KwaZulu-Natal

<div>South Africa is the second largest exporter of fresh citrus in the world, after Spain. Postharvest damage by <em>Penicillium</em> <em>digitatum</em> often exceeds 40% in fruit destined for export markets. Studies were conducted to investigate whether an integrated system combining the effects of preharvest silicon treatments on citrus trees, followed by postharvest hot water treatment (HWT) and a biological control agent, could be used to control green mould of citrus fruit. Fruit were further assessed for the expression of plant protective compounds associated with disease pressure. Navel and Valencia trees were treated with two silicon preparations. Fruit were picked, inoculated with <em>P. digitatum</em> spores, and exposed to HWT at 60°C, 62°C and 64°C for 10s, 15s and 20s, respectively. This was followed by fruit coating with a biological control yeast, B13. Fruit peels were quantitatively assessed for antioxidant, flavonoid and phenolic levels. Electron dispersive x-ray spectroscopy showed that silicon is indeed taken up by the tree and transported to fruit. Silicon pretreatments coupled with HWT+B13, at 62°C and 64°C (15s and 20s exposure time), provided 98-100% control of green mould with zero heat damage. Controls (field treatment: water or potassium sulphate) provided 88-93% protection. Coating fruit with the biological control agent provided long-term disease protection for 6 weeks post-treatment, manifested by zero disease development. A two-fold increase in the levels of antioxidants, flavonoids and phenolics in fruit peels was found, compared to controls. The combined use of silicon and HWT, produced a faster and stronger resistance reaction than either treatment alone.</div>