Factors that influence the retention of fungicide residues on plums

Export countries regulate traces of chemicals legally tolerated on fruit. Maximum residue limits (MRL’s) dictate the permissible residues. Non-compliance results in rejection of fruit upon arrival at export destinations, with concomitant financial loss. Variation in residues on fruit subjected to post-harvest fungicide application is sometimes experienced. To assist Growers and Packers, this study was initiated to determine the effect of cultivar, fungicide exposure time, fruit maturity, fruit temperature and fungicide solution temperature on residue retention, following post-harvest fungicide dip applications.

In 2018, Songold and Laetitia plums, each sampled at two harvest maturities were dipped in fungicide solutions with Fludioxonil the active ingredient. The variables included for testing, on the basis of resultant residue levels achieved, were: exposure time, fruit pulp temperature and fungicide solution temperature. In 2019, African Delight and Flavour Fall plums were also tested in addition to Songold and Laetitia

In 2018, plum cultivars reflected different residue levels following post-harvest fungicide dip applications. Residues were approximately 20% higher on Songold than Laetitia. Exposure times of 10 and 30 seconds, gave similar residues and these were significantly lower compared to 3 and 4 minutes exposure. Residues related to fruit and solution temperatures varied between cultivars. With Songold, temperatures influenced residues, but with Laetitia it had no effect. Similarly, harvest maturity affected residue levels on Laetitia, with higher residues on fruit of more advanced maturity. With Songold, harvest maturity influence on residue was inconclusive. In 2019, most of the parameters, namely cultivar, exposure time and fruit temperature had little or no effect on fungicide residues. Results associated with exposure times of 10 and 30 seconds were similar to 2018, and did not differ from each other at these durations. High fungicide solution temperatures resulted in higher residue levels

Special attention to cultivar, harvest maturity, fungicide exposure time and fruit temperature, within reason, is not required as the effect on residue loading on plums is minimal. High solution temperatures (± 40°C) may result in higher residues. While this research was conducted using dipping, it is postulated that with atomizers, other factors such as calibration and solution concentration are likely to have a bigger effect on residues than the other parameters tested in this study