What was it all about? 

Beginning in mid-2014, this three-year project was tasked with studying bacterial diseases of onion crops in order to improve understanding of their introduction, spread and survival – and in turn help build the industry’s capacity to manage them. There was a particular focus on bacterial blight of leek, which affects onions and shallots and is caused by the bacteria Pseudomonas syringae pv. Porri, or ‘Psp’.

In its course, the project found…

• There is a strong link between temperature and disease symptoms caused by Psp – with warm, dry conditions not conducive to Psp infection and disease development, while cool, wet conditions are favourable to the disease.

• Thrips feedings and other mechanical wounding can increase the risk of infection where bacteria is present. Here, there was a relationship seen with free water – with cool wet weather or overhead irrigation shown to disseminate bacteria over the surface of plants, increasing the likelihood of bacterial presence at wound sites from thrips feeding. The researchers noted that thrips feeding damage in the absence of free water is unlikely to exacerbate disease, but appropriate control of thrips within onion crops, and consideration of irrigation regimes to minimise leaf wetness, is something to consider.

• The amount of Psp bacteria that infects plants affects the level of disease severity, but not necessarily disease incidence. In particular, higher bacterial concentrations were found to enhance the development of the yellow leaf symptom seen in Psp infection.

• Good news for growers – there can be consistent recovery of plants from infection and disease symptoms. The researchers found that when warming temperatures and/or a decrease in humidity and free water led to outer infected leaves senescing, pathogen infection of newer leaves was less likely, allowing plants to recover.

• No commercial red, brown or white onion varieties show resistance to Psp species, and all varieties showed similar susceptibility.

• The Psp populations studied were sensitive to copper. The researchers noted that while there are currently no products registered with the Australian Pesticides and Veterinary Medicines Authority specifically for the control of bacteria in onion crops, there are more than 40 copper-containing products registered for other uses in onions – with potential to expand the registered use of some of these products for the control of bacterial diseases. Further work is needed, however, to look at appropriate application methods, regimens and formulations for field control to be successful.

• Essential oils may also hold promise in regard to control, with clove oil showing good bactericidal activity in the lab. Again, further investigation would be needed in this area, with this research still in its infancy.

Disease surveys throughout the project didn’t actually detect Psp in Australian crops, and no diagnostic samples were submitted from any growing region during the project period either. The researchers did, however, look at samples of Psp collected in the early 2000s during outbreaks in leek crops in southern Australia, and from the 2011-12 outbreak in onion and shallot crops in the Lockyer Valley in Queensland. There were key differences observed, including aggressiveness on onion – indicating multiple introductions of Psp into Australia (and that spread from the leek outbreak in the early 2000s was very unlikely). Given Psp is spread in seed, the researchers noted a continued risk of further introductions of the pathogen into onion growing districts.

ACT NOW

This project was also tasked with enhancing preparedness for potential incursions of exotic diseases, such as Xanthomonas leaf blight of onion, caused by Xanthomonas axonopodispv. allii. To this end, the project team worked with Plant Health Australia to produce information on the disease. Download the Bacterial blight of onion fact sheet here.