Green mould on citrus

The project

Between May 2019 and June 2021, a series of trials were conducted to determine the protective control and sporulation inhibition effects of three fungicide active ingredients i.e. azoxystrobin, fludioxonil and pyrimethanil, in a shellac wax formulation.

During the pilot trial in 2019, untreated lemons were used to determine the effective concentration of azoxystrobin, fludioxonil and pyrimethanil needed to prevent green mould growth, while remaining within the maximum residue levels (MRLs).

The pilot trial was followed by two semi-commercial trials. During the first, each of the three active ingredients was applied individually, at the effective concentration, to untreated lemons, navels and mandarins in a commercial pack line. The researchers found that azoxystrobin provided the best protective control, and fludioxonil the best sporulation inhibition.

In a second semi-commercial trial, the alternative active ingredients were applied in combination with one another to untreated lemons, navels and mandarins in a commercial pack line. The results showed that the combination of azoxystrobin and fludioxonil had the best protective control and sporulation inhibition.

In addition to the three alternative active ingredients, two commercially available fungicides were also tested, namely Ortosol, which contains orthophenylphenol as an active ingredient, and Evolve, of which the active ingredients are azoxystrobin and fludioxonil.

In all instances a wax treatment was added to emphasise that protective control and anti-sporulant ability come from the active ingredients used, and not wax itself. A noteworthy finding is that when imazalil is applied within wax (instead of as a spray, drench or dip application), pathogen resistance appears to be less.

The final part of the study was meant to measure the resistance of propiconazole-resistant citrus sour rot and citrus green mould isolates to the three alternative active ingredients. However, disruptions and delays caused by the COVID-19 pandemic prevented completion. Trials are currently underway. This is an extremely important aspect of the overall project, given that azoxystrobin is registered for pre-harvest use as well, and that multiple applications (pre- and postharvest) on fruit need to be avoided in order to slow the development of resistance.

Alternatives prove their worth

Azoxystrobin emerged as the star performer. As a single treatment, it provided 98% protective control. In combination with fludioxonil, azoxystrobin was also the best at inhibiting sporulation. Fludioxonil on its own provided 83% sporulation inhibition.
Given the efficacy of its active ingredients, it was to be expected that the fungicide Evolve delivered excellent results. It provided a pooled average of 98.5% protective control over all fruit types and inhibited spores by up to 88% when added to wax.

The third alternative active ingredient, pyrimethanil, and the second product, Ortosol, proved to be ineffective against citrus green mould when added to wax.

Encouragingly, the research results showed that azoxystrobin and fludioxonil as active ingredients and Evolve as a product, delivered control that was at least on par with imazalil. In some instances, the alternatives outperformed imazalil.

Provided that the resistance risk can be managed, these alternatives bode well for the South African citrus export industry’s ability to maintain and even grow its international market share.

Featured Image: A single treatment of azoxystrobin provided 98% protective control. In combination with fludioxonil, azoxystrobin was also the best at inhibiting sporulation