Fighting bacteria with bacteria: a novel way of protecting kiwifruit

Symptoms of Psa on kiwifruit leaves. Image source: ccaankara. “Pseudomonas syringae pv. actinidae”. July 14, 2016 Flickr online image. All creative commons

When people mention brown, round and hairy, kiwifruit aren’t the first thing to appear in the mind’s eye. They are more commonly associated with a vibrant green colour, deliciousness and the final touches on top of a traditional kiwi pavlova. Unfortunately, the recent establishment of a disease novel to New Zealand means that threatened, should now be added to the list.

Kiwifruit has become a staple fruit in many New Zealand households. Kiwifruit is also a significant export crop, earning New Zealand $1 billion annually, and is ranked as the second most important export crop by value. In 2010, the kiwifruit disease Pseudomonas syringae pv. (Psa) was accidently introduced into New Zealand, and this has brought the kiwifruit industry to its knees. While traditional control for Psa (spray the heck out of it) can be effective, there are many social and environmental ramifications associated with pesticide applications on food crops.  Researchers believed that there was a more environmentally friendly and long term solution to the disease. This led to the search for a Psa biological control agent.

Image source: f99aq8ove. Kiwifruit. 10 August 2010. Flickr online image. All creative commons

Psa is the causal agent of bacterial canker in kiwifruit. Infected vines have significantly lower yields and in extreme cases die. In an early and unsuccessful attempt to control the spread of Psa, vines that were considered heavily infected were destroyed. This equated to 2000ha, roughly 2411 rugby fields, of kiwifruit vines being ripped up, and destroyed. At a removal cost of $5000-7000 ha, not to mention the cost of lost income and time to re-establish healthy vines, it doesn’t take a high flying BCom student to understand the economic punch this microorganism can throw.

The kiwifruit industry exemplified effective incursion and disease response with the rapid development of best practice information and management options for growers. Alongside agricultural practices which make orchards less favourable to Psa, chemical controls were highlighted as a key management option. Psa can be controlled with streptomycin or copper sprays, but these are toxic to plants, promote chemical resistance in Psa, and can result in chemical residues in fruit. These are not good results.

Biological control is a method of disease control which doesn’t suffer from the same drawbacks as chemical control. Biological control has been widely used to control pest species throughout the world and under the right circumstances, can be highly successful. It works on the principle that ‘the enemy of my enemy are my friends’.

In other words, organisms that are known to be detrimental to the pest species are released at sites where a crop has pest problems, in the hope that they will lower pest populations. With the invasion of Psa in 2011, research began into potential biological control agents. Wicaksono, Jones, Casonato, Monk & Ridgway (2017), working the Department of Pest Management and Conservation at Lincoln University,  evaluated the potential of endophytic bacteria from Mānuka (Leptospermum scoparium) a native New Zealand shrub

From the endophytes (bacteria which lives within a plants without causing disease) of Mānuka, five species of bacteria were identified as hostile to Psa in the lab. The shortlisted agents were all in the genus Pseudomonas, that is the genus that Psa belongs to, meaning researchers were looking at a potential family squabble to help them. Of these five, three were transferable to kiwifruit vines. Formulations containing each of the potential agents were applied to artificial wounding on the plants and then given four weeks to establish. Wounds are natural entry points for Psa and so this was done to mimic natural infection opportunities. Psa was then applied to the plants. Disease severity was quantified by measuring lesion size, the amount of bacterial ooze and the number of collapsed stems.

Pseudomonas spp. under a helium ion microscope. Image source: ZEISS Microscopy. May 3, 2012. “Single Pseudomonas bacteria attaching itself to a lung fibroblast cell”. Flickr online image. All creative commons

Inoculations of all three endophytic bacteria significantly reduced amount of Psa compared to non-inoculated vines. The exact mechanism of inhibition is unclear but the production of antibacterial compounds, induced natural plant resistance and nutrient competition are suggested to be likely mechanisms of control.

This study showed that there is the potential for the biological control of Psa in kiwifruit orchards. This itself is an important milestone but it should be noted that a commercially available biological control product is a long way away. Extensive testing is required to improve the effectiveness of the selected agent, then find a way of converting live populations of bacteria into a product that can be practically applied by growers.

This is the first step in a long, convoluted process that marries science and economics. The ideal outcome from this research is a commercial biological control product that would replace chemical control. This product would be effective, environmentally safe, economic and compatible with current practices and technologies on farm.

While there are undeniably are raft of complications that must be overcome to achieve something like this, it is a positive first step in the biological control of Psa. Ultimately, it gives hope to the idea that sustainable science can be used to ensure kiwifruit remain the favourite topping of the iconic kiwi pavlova.

The author Adam Ross is a postgraduate student in the Master of Science taught at Lincoln University. He wrote this article as part of his assessment for ECOL 608 Research Methods in Ecology.

Leave a Reply

Your email address will not be published. Required fields are marked *