Can GRAS (generally regarded as safe) compounds control citrus decay?

CONTACT DETAILS: +27 13 759 8033/ +27 82 335 7543/
DURATION: Two years
LEAD INSTITUTION: Citrus Research International
BENEFICIARY: The citrus industry

Green and blue moulds are severe citrus post-harvest fungal diseases, responsible for significant economic losses worldwide. Penicillium digitatum (green mould) and Penicillium italicum (blue mould) are typical wound pathogens that infect fruit at any stage during handling. The South African citrus industry uses fungicides like imazalil, thiabendazole, guazatine and sodium ortho-phenylphenate to control decay. However, pressure from health groups, export markets, political and regulatory bodies demand either a reduction in chemical compounds or that the use of fungicides is discontinued, to ensure lower residue levels.

Screening GRAS compounds for fungicidal properties

GRAS (generally regarded as safe) chemicals such as bicarbonates and carbonates have demonstrated efficient control of certain plant pathogens. GRAS compounds therefore show potential to control citrus post-harvest decay. In a Post-Harvest Innovation Programme project, led by Keith Lesar, Citrus Research International (CRI) screened GRAS compounds for possible fungicidal properties.

  • Healthy, untreated navel oranges, divided into three batches of twenty units per treatment, were washed, sterilised and dried.
  • Suspensions of Penicillium digitatum spores, sensitive or resistant to imazalil, were made up.
  • Fruit was wounded and inoculated with the pathogen, and left to incubate for eight hours at 23°C before further treatment.
  • The infected oranges were dipped in a hot water bath at 40°C to simulate the heated fungicide bath used in pack houses.
  • Imazalil sensitive and resistant spores were treated with water, 250 ppm (parts per million) and 500 ppm imazalil SO4, sodium bicarbonate at 1% and at 2%, with and without the addition of imazalil SO4.
  • After treatment, the fruit was incubated in paper packets at 23°C for about eight days to determine the percentage of green mould infection.
  • The pH readings of imazalil at 1%, 2% and in combination with sodium carbonate, as well as the titrated results, were recorded.

Outcome of study

  • Results of the simulated hot water bath trials did not show proper control of imazalil resistant or sensitive Penicillium digitatum strains at 1% and 2% sodium bicarbonate.
  • Sodium bicarbonate, used in combination with imazalil at standard or reduced rates, indicated better control of imazalil sensitive spores.
  • Overall, 1% and 2% sodium bicarbonate did not show efficient control of post-harvest infections.
  • Other research findings have shown that certain GRAS compounds can control citrus post-harvest decay efficiently. More research on the use of GRAS compounds in combination with post-harvest fungicides is necessary and trials should be repeated.