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Response of citrus physiology to phosphorus acid and silicon as elicitors of induced disease resistance

Mkhize, N., Bower, J.P., Bertling, I., Mathaba, N.
Acta horticulturae 2013 no.1007 pp. 135-141
Citrus, Penicillium digitatum, air drying, benomyl, benzimidazole, chemical composition, control methods, cultivars, disease control, disease resistance, elicitors, farms, fungicide resistance, fungicides, lemons, oranges, pathogens, petrolatum, phosphoric acid, phosphorous acid, phosphorus, physiology, postharvest diseases, postharvest treatment, preharvest treatment, quality of life, shelf life, shrinkage, silicon, spores, synergism, thiabendazole, transportation, tree trunk, trees
Although technological advances have greatly improved the storage life and quality of citrus, postharvest decay is still a major problem. Penicillium digitatum (green mold) and P. italicum (blue mold) are, economically, the most important postharvest pathogens. Over the years fungicides belonging to the benzimidazole, thiabendazole (TBZ) benomyl and imidazol (IMZ) groups have been used extensively to control these diseases. The development of fungicide resistant strains, together with the withdrawal of effective chemicals from the market, haveled to the search for more integrated methods of disease control. Silicon and phosphorus acid seem to trigger a systemic response that enhances the fruit’s resistance to pathogen attack. The aim of this research was, therefore, to ascertain the changes in the fruit’s biochemical composition after the application of these two chemicals in order to improve the understanding of the mechanisms involved. ‘Valencia’ and ‘Navel’ oranges, togetherwith the lemon cultivar, ‘Eureka’, were harvested from Ukulinga Farm and treated, both pre- and postharvest, with three different concentrations of silicon (3.35, 10.7 and 21.4 ml/L) and one concentration (5 ml/L) of phosphorus acid. As preharvest treatment, individual trees were drenched around the base of the tree trunk with 5 L treatment solution. Fruit were left on the trees for 21 days after treatment to allow for uptake and transportation of the silicon and phosphorous acid to the fruit and then harvested. As postharvest treatment, fruit were immersed in treatment solutions and then air-dried. After treatment, fruit were inoculated with a 1×10-4 spore suspension of Penicillium digitatum. Disease progress was monitored for 30 days postharvest. Different levels of Penicillium control were achieved with the different treatments used. Petroleum jelly applied onto the surface of the fruit was able to provide sufficient cover to prevent fruit shrinkage and thus allow for continuous sampling over the 30 day period. Although some treatment combinations did not seem to have a synergistic effect on disease suppression they seemed to delay disease onset. Further research is necessary to fully understand the biochemical changes induced by silicon and phosphoric acid.