The role of sanitisers
“Sanitation prevents decay development by reducing or eliminating harmful organisms,” explains Fourie. “You can use registered sanitisers to reduce infections if the dwell time and target pathogens are correct.”
Dwell or contact time is the amount of time a sanitiser must be in contact with the pathogen to deactivate it. Dwell time usually reduces as sanitiser concentration increases. In practice, a sanitiser’s concentration will be guided by its registration.
Registered products have been tested against specific pathogens. Users should select products with proven efficacy against the pathogens that are a problem in their situation.
Sanitisers differ from fungicides. While fungicides are formulated to kill fungi, sanitisers deactivate a range of microorganisms. Sanitisers mainly target bacteria but can also be effective against specific viruses and fungi.
Both sanitisers and fungicides are surface-acting, but some fungicides also have localised systemic activity. In general, neither substance will successfully control infections once these have established themselves inside the fruit.
“In many cases, sanitisers don’t have a residual effect,” says Fourie. “That’s also the claim to fame of many sanitisers – that they break down completely. Whereas with fungicides, we do want a residue, to counter spores proactively.”
Previous work by other researchers has shown that the pre-harvest application of sanitisers, biofungicides, and biologicals can help to reduce postharvest decay. The incorporation of these products into an integrated decay management programme can lessen the grower’s reliance on fungicide applications, thereby lowering the risk of fungicide resistance developing and addressing the food-safety concerns of markets.
To assess the potential contribution of sanitisers to pre-harvest decay-control programmes, Fourie’s team conducted laboratory and field assessments of several commercially available products.
What products were tested?
In consultation with the table-grape industry, Fourie’s team identified and tested eight products with potential as pre-harvest sanitisers: one biofungicide, three biologicals, two sanitisers, one disinfectant, and one sanitiser-disinfectant combination.
Biofungicides are biologically derived substances that claim to kill fungi. In this study, the product tested was a mixture of ascorbic acid and bioflavonoids. The former activates natural plant defences, and the latter controls fungi.
Biologicals are living organisms that claim to control undesirable microbes. The biologicals tested in this study were two products containing bacteria (Bacillus spp.) and one containing a fungus (Trichoderma sp.).
Disinfectants are like sanitisers but target a wider range of microbes, killing up to 100% of bacterial or fungal populations. The active ingredient of the disinfectant assessed in this project was hypochlorous acid.
The active ingredients of the two sanitisers in the project were a combination of peracetic acid and hydrogen peroxide, and a combination of sodium and potassium persulfate. The formulation of the sanitiser-disinfectant combination was not disclosed.
Although the eight products fall into different categories, all will be referred to as sanitisers in the rest of this article.
Laboratory trials
For the first phase of the project, Fourie’s team tested the eight products against the three most common postharvest fungal pathogens (Alternaria alternata, Botrytis cinerea, and Penicillium expansum) in the laboratory. Three kinds of laboratory tests were conducted.
In the first, fungal spores were exposed to different concentrations (1x, 2x, and 4x the recommended dose) of the sanitisers. In the second, different fungal spore concentrations were exposed to the sanitisers at the recommended dose. In the third, spores were exposed to the sanitisers at the recommended dose for different contact periods (5, 10, 15, and 30 minutes).
Chlorine was used as a control for the laboratory trials.
When fungal spores were exposed to the recommended dose of the sanitisers, all products were 100% effective against B. cinerea. All products except the biofungicide, the persulphate combination, and the sanitiser-disinfectant combination were 100% effective against A. alternata and P. expansum.
As a result of their poor performance in the first trial, the biofungicide, the persulphate combination, and the sanitiser-disinfectant combination were excluded from the subsequent trials.
When the remaining five products were tested against different spore loads, the disinfectant was the only one that consistently achieved 100% efficacy against all three pathogens. A. alternata was most able to withstand the other sanitisers.
At all contact periods, the disinfectant, a Bacillus species, and chlorine were the only products that achieved 100% efficacy against all three pathogens.
Vineyard trials
Based on the laboratory results, four products were selected for vineyard spray applications: the disinfectant, the peracetic acid and hydrogen peroxide sanitiser, a Bacillus species, and a Trichoderma species. The sanitisers were incorporated into existing pre-harvest decay-control programmes.
In the first season, field trials were conducted in Crimson Seedless, IFG Ten, and Sugrathirtyfive vineyards. One sanitiser spray was applied one week before harvest in the IFG Ten vineyard, and two sanitiser sprays, two weeks before and at harvest, in the Crimson Seedless and Sugrathirtyfive vineyards. Visibly decayed berries were removed at the time of spraying.
In the second season, field trials were only conducted in Sugrathirtyfive. Three sanitiser applications were done, at five, two, and one week before harvest, but decayed berries were not removed.
The postharvest quality of the grapes was assessed after standard packing, six weeks of cold storage at minus 0.5 °C, and five days of shelf life at 7.5 °C. Grapes packed with and without SO2 sheets were compared.
“The results were a little disappointing,” says Fourie. “The laboratory trials showed that some products work very well. But as soon as you take them into the field, the results aren’t as good.”
Pre-harvest sanitiser applications didn’t reduce the occurrence or severity of postharvest decay in any of the cultivars. Some sanitiser applications were associated with a higher percentage of decay than the untreated control, possibly because the sanitiser applications created a wetter environment around the fruit.
The pre-harvest removal of decayed berries also didn’t reduce postharvest decay.
Despite these results, Fourie cautions against dismissing the importance of pre-harvest sanitation. “I have done other research that clearly showed that the physical removal of decaying particles reduces the spore load and affects decay,” he says.
