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

Post harvest decay in pome fruit remains an ongoing challenge to the pome fruit industry. The emergence of new regulations for maximum residue limits of agrochemicals on fruit, and pressure from consumers for fresh produce produced in an environmentally friendly manner obliges the pome fruit industry to search for alternatives to current decay control strategies.

Year 1 – 2013

In the first year of study, information on which alternative products should be tested, from the Department of Plant Pathology at Stellenbosch University, was delayed. Consequently, since chlorine based products are frequently used in pome fruit pack-houses for disinfecting flume water, the decision was made to measure the extent to which accurate use of chlorine in flumes could support decay control on pome fruit. Water was sampled at five sampling points in 12 different pome fruit pack-lines. These points were the source water (where pack-house water originates from), infeed water (where fruit first meets the flume water), a point pre-sorting, and a point pre- packing. Microbial survival within water was evaluated by plating the water onto Potato Dextrose Agar. To determine the potential of the water to control additional spore loads, spore loads of Botrytis cinerea and Penicillium expansum, respectively, were added to water samples before plating. Results indicated that the level of efficacy with which chlorine based products are used within flume water varied greatly between pack-houses. In best cases the chlorine levels in the pack-houses were sufficient to control both resident microbes, and the added pathogenic fungal spore load. In worst cases the chlorine levels were not sufficient to control microbes under any circumstance. Notably, in pack-houses where chlorine levels in flume water were regularly monitored and amended, microbial control was best. Such monitoring systems were recommended, since it supports most effective disinfection of flumes, which in turn can impact on the level of decay control experienced in cold storage.

Year 2 – 2014

The literature study on alternative decay control options in fruit was concluded by Department Plant Pathology, Stellenbosch University. Potential products, as listed in this study, had been investigated earlier as part of a similar, but different study on pome fruit. Results from this in vitro study suggested that two biological control agents with active antagonists Trichoderma harzianum and Bacillus subtillis, hold potential for suppressing fungal pathogens. As sterilisers, Chlorine and Bio-Steriliser were included, with Fludioxonil used as fungicide reference.

Three studies were undertaken:

Study 1: Product efficacy for controlling Penicillium expansum and Botrytis cinerea on Pink Lady apples

Study 2: Product efficacy for controlling P. expansum on Forelle pears

Study 3: Product efficacy for controlling B. cinerea on Cheeky pears.

Fruit were injured and inoculated with either P. expansum or B. cinerea. The inoculated fruit were dip- treated in the product solutions, as well as in water as an untreated control. Results indicated that decay control for both grey and blue mold is consistently achieved by the product Scholar (except for one instance with low decay incidence) on all fruit kinds and cultivars tested. Although lowered sporulation and or disease indices indicated that other products exerted a fungistatic effect on the pathogenic fungi, none of these products successfully controlled decay under the relatively high risk conditions of aggressive wounding and high spore inoculum of the pathogen.

Year 3 – 2015

In the final year of this project, studies were undertaken to determine if the alternative products could be useful in lower risk situations, where fruit were exposed to less fungal spores, and hence lower inoculum. Alternative products with good performance in the previous years, as well as a T-tree oil product, reported to have fungicidal properties in literature, were evaluated for B. cinerea and P. expansum control on Forelle pears. The decay control potential of chlorine hypochlorite and a bio-steriliser (peracetic acid/hydrogen peroxide as actives), and Fludioxonil (used throughout all the years) was also evaluated. 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. Subsequently, these factors were specifically evaluated. 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 pome fruit, ultimately did not succeed.