First discovered in the 1980s in Japan, PSA is a kind of pseudomonas syringae actinidiae. In Asian orchards, this bacterial strain has been successfully treated and controlled. It was discovered in northern Italy in 1992.
Due to the Kiwifruit PSA, there were significant economic losses in 2007/2008 with the emergence of a more virulent strain (PSA V). In 2010, it was discovered in kiwifruit farms in New Zealand’s Bay of Plenty region of the North Island.
Specific to yellow-fleshed cultivars, this vulnerability was more pronounced. Other than kiwifruit vines, Psa is not harmful to humans or animals. PSA is assumed to be dispersed through wind, rain, and plant matter. Additional sources of transmission include footwear, vehicles, and orchard equipment
How PSA virus affects kiwifruit
The kiwifruit vine canker Pseudomonas syringae PV actinidiae (Psa) was found by kiwifruit growers in November 2010. In the Te Puke area, many kiwifruit orchards were quarantined.
A temperature range of 10°C to 20°C is ideal for the bacteria, with the sickness naturally regulated above 25°C. KIWIFRUIT (Actinidia spp.) is infected through stomata or wound holes. However, there are more subtle symptoms such as discoloured leaf buds as well as brown spots on otherwise green leaves with a halo around them. Some of the canes and leaders may develop reddish-orange or white sap-filled cankers (welts). When removing the bark from affected plants, brown-orange discoloration is often visible.
By blocking circulatory tissues, germs can cause young shoots to wilt and die. It is possible that the entire plant will die. In the New Zealand media, it has been widely stated that the disease has little effect on the fruit. Though few cases of “collapsed fruit” were reported during the outbreak in Italy. Each kiwifruit species has different levels of influence. Researchers in Italy found that yellow-fleshed kiwifruit (A. Chinensis) is more susceptible to damage than the green-fleshed cultivar (A. deliciosa cv. ‘Hayward’), which is more widespread in New Zealand.
People, animals, and even other plants are not at risk from Psa. There have been reports of this virus in Japan, where it was initially discovered in the 1980s, and also in Korea and parts of Europe, but it has never been reported in the United States. Kiwifruit production in hard-hit areas has been severely damaged by Psa.
How the Kiwifruit PSA virus affected the markets
For 15 years, Psa remained sporadic with few occurrences in northern Italy after its discovery in 1992. When the illness spread in 2007/2008, conditions were favourable, and kiwifruit in Lazio was decimated, including a New Zealand-owned gold kiwifruit orchard that was destroyed. It is believed that the sickness has cost Italy roughly 2 million Euros (NZ$3.5 million) in medical expenses. This involves both monetary and physical damage to a business. European and Mediterranean Plant Protection Organization (EPPO) Secretariat has added Psa to the EPPO Alert List, a list of pests that should be taken seriously.
As a result of the use of copper compounds and antibiotics in Japan, the disease has developed resistance. EPO and MAF agree that good sanitation and a healthy environment are essential to controlling and avoiding the disease. Good fertilisation, avoiding overhead watering and disinfecting pruning equipment are all part of the plan, as are regular inspections for disease symptoms, restricting unnecessary access to the orchard and using healthy plant stock.
Exports of New Zealand kiwifruit hit NZ$1.04 billion in the year ended March 31, 2010, according to MAF’s annual report on the business. This was the first time the industry has exceeded the $1 billion mark. This year’s fruit crop was expected to be gathered in the fall. With the US and Australia already blocking the entry of plant roots, there was danger that trading partners would limit the import of Kiwifruit from New Zealand (plant stock).
Where is the PSA virus?
- It can either live on the plant’s surface as an epiphyte or
- It can penetrate the vine through natural openings in the plant or man-made wounds and cause severe infection.
If the bacteria are allowed to grow outside or inside the vines, it can cause leaf spotting, cane/leader death, and also in severe situations, vine death with the release of exudates.
Symptoms of the Kiwifruit PSA virus
- Spot with surrounding halo
- Spot on a strung cane with halo
- Dark, irregular spots filling inter vein areas
- Spotted leafs starting to curl
- Spot with no halo
- Infected flower bud
EXUDATE Symptoms
- Red/orange exudate
- White exudate
DIEBACK Symptoms
- Shoot dieback
- Cane collapse
- Dieback with shriveled fruit
Kiwifruit PSA Strains
Five closely related, but genetically distinct Pseudomonas syringae lineages have been implicated in different PSA outbreaks, and it’s likely that many more occur in wild kiwifruit populations.
Special attention should be paid to Japanese and Korean strains of the disease, which are more aggressive against Hayward cultivars than the New Zealand strain Psa-V.
In addition to the potential for horizontal gene transfer, new kiwifruit strains may have an adverse effect on existing or new kiwifruit cultivars.
We can predict the emergence of new kiwifruit Psa strains in New Zealand, but we don’t know their features or pathogenicity towards new and existing kiwifruit cultivars yet. In order to avoid the spread of any novel strains between orchards and growing regions, good biosecurity techniques are essential.
