Decay control of tree fruit: Testing of different control options using fungicides, sanitizers, soft chemicals and biological agents, to identify methods for use on stone fruit

Post-harvest decay of stone fruit is a significant challenge in stone fruit production in South Africa. However, a limited number of fungicides are registered for decay control. Furthermore, restrictions are continually imposed on the application of active ingredients in fungicides on fruit, especially in export produce. This is done predominantly with human and environmental safety as the underlying incentive. Thus, an investigation into alternative decay control options is needed which could potentially support decay control in stone fruit production.

Studies on the efficacy of products for the control of brown rot (Monilinia laxa) and grey mold (Botrytis cinerea) were conducted over three consecutive years, in order to investigate the potential of such products.

Year 1 – 2013

A literature study on alternative decay control in fruit was conducted by Department Plant Pathology, Stellenbosch University. Potential products to investigate for decay control were chosen from this information. Products “Scholar®”, “Prodione® 500SC”, “Protector® 400SC” and Calcium hypochlorite were tested in vitro as chemical reference in decay control compared to a number of alternative products. Alternative products were Peracetic acid, Hydrogen peroxide, Sodium hypochlorite, Sodium bicarbonate (these products at three concentrations), “B-rus®” (bacterial a.i.),“Eco 77®” (fungal a.i.) and YIELD PLUS® (yeast a.i). Mixtures of the bacterial and fungal product with sodium bicarbonate were also tested. Generally, chemical compounds fully controlled both Botrytis cinerea and Monilinia laxa, with the exception of Sodium bicarbonate, Pyrimethanil and Hydrogen peroxide. In vitro, all three biological control agents clearly showed antagonism against both B. cinerea and M. laxa by suppressing fungal growth on Potato Dextrose Agar medium. Addition of sodium bicarbonate to these products generally did not enhance product performance.

Year 2 – 2014

This study was followed up by in vivo trials, testing decay control of products with potential on stone fruit. Trial one investigated the in vivo control of brown rot and grey mold by “B-rus®” (bacterial a.i.),“Eco 77®” (fungal a.i.) and YIELD PLUS® (yeast a.i.), with Fludioxonil as chemical control in plums. Trial two investigated decay control, only for grey mold, when biological control agents as mentioned and Scholar®”, “Rovral®”, Calcium hypochlorite, and a sanitizer containing peracetic acid and hydrogen peroxide was applied with an atomiser. Results on plums for B. cinerea and M. laxa indicated only Fludioxonil to be fungicidal. “B-rus®” exhibited some fungistasis against B. cinerea, whilst YIELD PLUS® exhibited fungistasis against M. laxa. When screening fungicidal effects of these products on nectarines, full grey mould control was obtained by both the fungicide active ingredients Fludioxonil and Iprodione. Decay control by chlorine hypochlorite was next best, and seemed to hold potential as a “new” product for

decay control, notably at a double dosage, for which fungistatic effects were also enhanced. Similarly, decay control for “B-rus®” – the biological control product, decay control potential was also enhanced at double the dosage at which it is currently registered for use on other crops.

Year 3 – 2015

In the final year of this project, studies were undertaken on the alternative products with best performance, as well as with T-tree oil, reported to have fungicidal properties in literature. This study was conducted on B. cinerea control on plums, using five previously tested products, namely “B-rus®”, chlorine hypochlorite, bio-steriliser (peracetic acid/hydrogen peroxide as actives), and Fludioxonil as chemical reference, as well as T-tree oil. The question arose if the extent of the fungal infection, the dosage of the product, and the addition of surfactants could play a role in product performance. Results indicate that other than for Fludioxonil, no decay control was evident from any of the products tested. The development of alternative methods for decay control in plums, ultimately, did not succeed.

Post-harvest decay of stone fruit is a recurrent challenge to South African stone fruit production. However, only a small number of fungicides are registered for decay control. Moreover, global emphasis on reduced use of pesticides, for concern of impacts on human and environmental health, threatens the number active ingredients which may be developed and registered for decay control. The study presented here follows on studies in two prior years, with the aim to test alternative products for the control of decay on stone fruit.