Essential oils could help control post-harvest disease

Monilinia fructicola (brown rot) is a costly cause of disease in stonefruit.
Monilinia fructicola (brown rot) is a costly cause of disease in stonefruit.

NSW Department of Primary Industries (DPI) post-harvest researchers are evaluating the antimicrobial properties of essential oils and their potential to control post-harvest diseases on fresh produce.

Post-harvest diseases – caused by fungi such as brown rot, green and blue moulds and soft rots - cause the loss of between 10 to 50 per cent of fruit and vegetables worldwide.

Chemicals have been widely used to control post-harvest diseases. Although they can be effective, many are no longer used because of economic, environmental, or health concerns, or because diseases have become resistant.

Few chemicals are still available for post-harvest treatments, making alternative approaches more attractive.

Dr Elena Lazar and Dr Vivian Ku from DPI at Gosford are testing several essential oils to see if they prevent fungal growth and spore germination. Essential oils are highly fragrant natural oils extracted from plants which may be used in flavourings and perfumery.

Dr Lazar said that many essential oils can inhibit diseases before they become a problem.

Four essential oils were tested in laboratory trials: lemon myrtle, cinnamon bark, oregano and thyme oil.

Dr Lazar said the oils would all stop fungal growth in Monilinia fructicola (brown rot), one of the most common diseases of stone fruit.

They also reduced spore germination. Lemon myrtle was the most effective, killing 100 per cent of spores.

The first round of trials involved mixing the oils at strengths of 1000 parts per million (0.1%).

Lower concentrations of lemon myrtle were also tested against M. fructicola. These concentrations of lemon myrtle delayed rather than stopped fungal growth and spore germination.

Trials were conducted on 300 nectarines deliberately infected with M. fructicola. Lemon myrtle oil was applied to the wound site.

Dr Lazar said this had varying effectiveness, depending on the time of application. Treatment with lemon myrtle oil was more effective if applied before the fruit was infected with M. fructicola. This suggests that the oil had prevented disease rather than cured it.

She said the lemon myrtle oil used in the trials did not damage the fruit skin.

Dr Lazar said that essential oils are extremely complex, so diseases are unlikely to develop resistance against them.

However, in order to meet industry expectations, she says large scale trials - conducted under commercial and semicommercial conditions - are required for different products. These would need to consider all cost, legal, and safety issues.