"We know that phage-based sprays work well in tomato and capsicum on Pseudomonas syringae bacteria and these phage-based sprays are being used commercially in the United States.
"The phage used on tomato are very specific to the bacteria that attacks tomatoes and phage specific to Psa-V have now been identified.
"Although the phage which attacks Psa-V has been found, we don't know yet how effective it will be on kiwifruit to control Psa-V."
The phage specific to Psa-V was identified from trillions of phage screened from various sources, including drain water, sea water and soil samples.
"Finding a Psa-killing phage isn't difficult. Basically you put trillions of phage on a plate with Psa-V bacteria and watch for an area of dead bacterial cells which will indicate where the specific phage has attached and killed the Psa."
What is more complex is developing a formulation to apply to plants and that was why Zespri had approached Omnilytics, which is a world leader in this field.
"We didn't want to waste time reinventing what had already been done. Minimising time to deliver a solution to growers is critical," Mr Parkes said.
Phage were probably the most common organism on the planet.
"They are found everywhere, on a computer key board, on your skin, on plants, in the food we eat and the water we drink.
"As each phage strain is specific to the bacteria they attack, and only infect bacteria, they are of no threat to humans. In fact, in some cases, phage are sprayed on to food to kill bacteria that cause food poisoning."
The phage kills bacteria by injecting their DNA into its cell and taking control of the cell's "mechanics".
"It tells the bacteria to stop growing and to start making copies of the phage. The bacteria makes over a hundred copies of the phage and then the bacteria splits open, releasing copies of the phage.
These new copies then float off to kill the next bacteria. This cycle happens in less than one hour, so phage can chew through a lot of Psa when they come in contact with the bacteria."
If they prove effective against the disease, bacteriophage are likely to be applied to kiwifruit vines as a spray. Mr Parkes said it was possible phage could be carried inside the plant by Psa-V, or could be injected into the plant, but that was being still tested to see which would kill Psa-V inside the plant as well.
Zespri is also working with the University of Otago to look for phages that can infect Psa-V and a commonly found bacteria that naturally grows on the vine.
"The theory is by having two hosts, the phage will be maintained and amplified on the surface of the kiwifruit vine by the commonly found bacteria. When Psa-V lands on the plant, the phage will kill the Psa-V. It's a good idea, but it is probably going to be difficult to find a phage that infects two different types of bacteria.
"The phage may not be as robust as a chemical solution but its upside is that it is a biological control and has no withholding period, so can be used year round to protect the vines."
