Fungal degradation of wooden pallets in citrus export

In the past, the fumigant and pesticide methyl bromide (MeBr) was used to keep wood-attacking fungi at bay. However, due to its toxicity and damaging impact on the ozone layer, as well as danger to humans and animals when used incorrectly, the use of MeBr was banned.

Its replacement, 2-ortho-phenyl-phenol (OPP), has proven disappointing. Apart from not being particularly effective against fungicides on pallet wood, OPP muddies the allowable-residues waters. Residues of the chemical have been detected in Europe on fruit that had been declared either chem-free or organically produced. Although the OPP was discovered at levels close to the limit of detection (LOD = 0,01 ppm), any organically produced or chem-free fruit should have zero residues. Furthermore, while OPP residues may be allowed on conventionally produced fruit, they add another chemical residue to an already very limited number of residues allowed by some retailers.

Pallet bases are mostly made of pine and are susceptible to degradation – a great concern in terms of SA’s distant export markets, such as the Far East and Canada.

Given that OPP poses an unnecessary risk and that fungal degradation outbreaks on wooden pallet bases have been on the increase since the banning of MeBr, alternative actives are urgently needed. The matter is further complicated by the fact that the source of fungal contamination on pallets is unknown, making it difficult to treat.

Between late 2018 and early 2020, I led a study to find a solution to fungal contamination in pallet bases. Although the study was focused on the citrus industry at the outset, similar challenges in other fresh fruit sectors resulted in cooperation from the pome and stone fruit, and table grape industries.

Working with me on this study that received funding from the Postharvest Innovation Programme, were Bernard Henning from the Perishable Products Export Control Board (PPECB) and Elaine Basson, also from CRI.

Project objectives and methodology

In designing the study, I knew that the problem needed to be defined properly before solutions could be investigated. One of the main questions to answer was what the source of the fungal contamination was.

Six objectives were set, divided into two research phases.

Phase 1

1. Determine the occurrence and spread of the problem, including industries other than citrus.

2. Optimise a self-inoculation technique of wood used for pallet base manufacturing.

Phase 1 also entailed simulating fungal growth on wood used in the manufacturing of export pallet bases.

Phase 2

3. Optimise methods to isolate fungi from collected wood samples.

4. Identify the fungal specimens isolated from wood pallets.

5. Investigate SOPP, as well as alternative products not currently in use for pallet preservation.

6. Determine the extent to which storage practices contribute to the occurrence of fungal contamination on palletising wood.

To determine the extent of the problem, the study team visited pallet manufacturers and liaised with fruit exporters and fresh fruit industries other than citrus, both in person and through questionnaires.

In the second phase, once fungi had been successfully isolated from the wood samples collected at the pallet manufacturers, PCR and ITS sequence comparison in GenBank was carried out to confirm the identities of the isolates. A method to culture wood mould in a controlled environment was developed – a critical step that allowed researchers to test the efficacy of different pallet preservation products.

Questionnaires were sent to citrus packhouses to determine the impact of storage conditions on the development of fungal contamination on pallet bases. The focus was on quantifying moisture build-up and the extent to which pallets are exposed to free water in containers.

Key results

The questionnaires revealed that storage conditions in the packhouses have no influence on the development of mould on the wood. Instead, the pallets themselves built from wet off-saw wood, were the source of the contamination.

The researchers found that the problem is widespread in other fruit industries as well, and that recent changes in export conditions contributed to the seriousness of the issue.

In terms of solutions, it was found that while pallets built from pine presented with the problem, eucalyptus wood was not affected by mould. And, while using a different wood source might appear to be the solution, the availability of pine for pallet manufacturing far exceeds that of eucalyptus. Finding a suitable treatment for wood is, therefore, the only feasible option.

The volume of fruit exported by the South African citrus industry used 1 841 243 pallet bases in 2020, while the export volumes for pome and stone fruit, and table grapes required approximately 1,5 million pallet bases.

As far as treating pine pallets is concerned, three products returned successful results in the controlled testing environment. A product based on stabilised glutaraldehyde is a particularly promising prospect, as it not only controls fungal growth but has a sealing action too.

Coronavirus travel and movement restrictions made it difficult to obtain the base materials used for the active ingredients in the tested products. Despite this, two of the products are already fully commercialised, with actives that are food grade and have food safety acceptance.

The issue of OPP residues was addressed in parallel to the fungal contamination study. Following a meeting of several stakeholders, the PPECB became closely involved and a plan of action was put in place to investigate the products currently used for container hygiene and pallet wood treatment. All affected industries have since implemented management policies and guidelines to deal with the issue.

The successful outcomes of the research study and the cooperation between the citrus, pome and stone fruit, as well as table grape industries, have resulted in significant benefits. It is reasonable to assume that the gains will extend to all fresh fruit and vegetable industries in SA.

Featured Image: The fungus Penicillium as seen under a microscope