How to control sour rot

Sour rot can be a devastating postharvest disease that can occur during storage.

It is characterised with a very soft watery appearance with a distinct margin between the decayed and healthy tissue (Figure 1).

There can be thin white fungal growth that develops in advanced rots. Sour rot becomes more common during long term storage and spreads by contact to healthy fruit after packing, creating nests of infected rotting fruit in the packed box.

It is also characterised with a unique vinegary smell which can be smelt throughout the box This disease is very distinctive and can cause major market losses.

While there has been some discussion on the correct scientific name for sour rot, to avoid mis-understandings, within this article we use the name Geotrichum candidum var. citri-aurantii as the fungus causing sour rot.

Occurrence

The transmission and incidence of sour rot is normally higher in growing areas with high rainfall, such as in Queensland and in coastal-growing areas.

This is due to the spores of sour rot commonly being found in the soil, where they can be blown by the wind or splash on the surface of the fruit and leaves.

However, with the recent frequent and high rainfall (which has also caused flooding) in most growing regions around Australia, sour rot has increased and has become a problem in many growing areas.

Management

The sour rot fungus infects the fruit through injury or damage to the skin, such as via insect stings or by mechanical damage to the fruit caused by rough picking and handling. It is therefore important to minimise any injuries to the fruit when harvesting and handling the fruit.

It is also critical to prevent any contact with the soil during harvest and handling. Always use clean picking bags and field bins to prevent any soil from coming into contact with the fruit. This not only minimises any potential sour rot but is also good practice for food safety.

Pre-sorting is an important measure to prevent damaged rotting fruit from coming into the packingshed / packingline and contaminating the packingline and wash water.

Control

It is important to sanitise the recirculating water in dips and drenches to kill the free sour rot spores. The effective use of sanitisers in the wash water is essential to reduce the risk of sour rot.

There are few postharvest fungicides registered for the control of sour rot. Many of the fungicides commonly used to control green and blue mould are not effective on sour rot but there are two fungicide options for sour rot control: (1) guazatine (trade names Zanoctine® or Panoctine® and (2) a combination of propiconazole and fludioxonil (Chairman®).

Chemical residues are a very big risk to market access, please be aware of MRLs for all postharvest fungicides in both domestic and export markets. For an up to date summary of the export MRLs, please consult with the Citrus Australia MRL listings on its website. These are regularly updated to ensure industry has the most up to date information to make informed decisions.

Guazatine is a member of the class of guanidines which belong to FRAC Group M7 and has multi-site activity. Guazatine is effective at controlling sour rot and has been used for many years in Australia.

We are fortunate that we still can still use guazatine for domestic trade and some international markets, as many overseas citrus producers (such as in Spain) do not have access to guazatine and have to rely on other methods such as sanitation to manage sour rot.

It is very important to note that in some large export markets such as Japan, guazatine is not a permitted food additive and it cannot be used for any exports into these markets.

If you use guazatine for sour rot control in any domestic or other export packing, it is critical to thoroughly clean the entire packingline to remove any potential guazatine residues and stop these from transferring to fruit destined to Japan and other sensitive markets.

Chairman® is a relatively new postharvest fungicide which contains the active ingredients propiconazole and fludioxonil (DMI and Phenylpyrrole

Group, FRAC Group 3 and Group 12) and is registered for use against sour rot.

Before using this fungicide please check the Citrus Australia MRL listings for market requirements.

This article was written by John Golding, research horticulturist, NSW DPI.