Psa-V is surprisingly easy to kill in the lab. However, killing the infection without further damaging kiwifruit vines is another strategy altogether. Once the infection has progressed into the vascular system it is much more difficult. Common bactericides, like copper-based agrichemicals, will kill Psa on the surface as will acid and alkali solutions and most sterilants and cleansers. However, the issue is not around how easy it is to kill the bacteria but rather their ability to reproduce rapidly. Prokaryotes reproduce asexually through a process called binary fission. During binary fission, the single DNA molecule replicates and the original cell is divided into two identical cells.
This process, for Psa-V, may occur within an hour (after one hour two cells will emerge and after 24 hours there could be as many as eight million). So, while you might kill 95 per cent of the Psa-V bacteria in an orchard within a day, between what is replaced by air movements from outside inoculum and remaining unkilled bacteria, the population can quickly recover to original levels. This is why it is essential to provide continued protection.
Products used against Psa
Products can be grouped into one of five categories.
Application of protectant sprays is considered best practice in protecting against Psa-V. Currently, the most effective Psa-protectant sprays are copper based and their optimal use remains unknown. Regardless, they have proven to be proactive on plant leaf and cane surface by knocking back high Psa-V populations associated with the establishment of infection. It is also thought that they play a valuable role in reducing the production of spores from cankers. It is probable that Cu2+ ions are redistributed in rainfall and will accumulate where bacteria also redistribute—providing a confidence that copper sprays show good levels of resistance to being washed off by rain.
French post-flowering copper trial results
A draft report on French tirals comparing application of two different copper products post-flowering is now available on the link below. The treatments provided no indication of increased staining or russet with multiple applications of copper when compared with controls, on HOrt16A or Hayward.
KVH has also developed two observation resports on Hort16A orchards in Tauranga and Opotiki. These are also available on the links below.
Copper spray information
KVH has developed a copper spray information sheet for growers and industry which provides information on how copper works on Psa-V, basic facts on copper formulations, effects of copper on humans, organisms and on soil. Click on the link below to access the document.
Elicitors are products that induce the plants defence mechanisms allowing them to fight infection. In other crops they have been shown to provide control, similar to that described for biological sprays. Generally, there is a time delay between the spray application and the elicitor effect being activated. The effect is relatively short term requiring additional applications. Knowing beforehand when an infection will happen and applying the elicitor appropriately is likely to be the key to success. Elicitors have to be used carefully, as too much elicitor can induce the plants to shut down and resulting in decreased plant growth.
These types of products will kill Psa easily in vitro. However, they often are not persistent and fail to provide long-term protection. They also run the risk of removing all bacteria, creating an environment where harmful bacteria return rapidly.
4. Biological control agents
Biological control agents' mode of action is the same as copper sprays; acting as a protectant. They can provide this in a number of ways including:
• occupying the sites the pathogen would normally reside in;
• competing against the pathogen for food;
• producing anti-bacterial compounds that kill the pathogen.
There is a trade off with the use of streptomycin. On the one hand there is the ability to protect against disease and assist the health of the plant; while on the other hand there is a potential risk of bacteria building resistance. However, as part of the R&D programme this is being researched as the use of streptomycin overseas has proven to restrict and kill the growth of Psa. There will be controlled rules of use associated with best practice, following a better understanding of streptomycin in-field.
Considering all the options above copper-based agrichemicals will remain the mainstay protective spray against Psa. Not only do they kill bacteria but they are relatively persistent chemicals. They do not break down and are relatively ‘sticky', ie they are not easily washed off in the rain.