A bacterial disease called Pseudomonas syringae pv actinidiae (Psa) has been plaguing New Zealand's kiwifruit industry. The bacteria were discovered in a Te Puke orchard in early November 2010.
Psa is usually spread by airborne spores so can move easily between plants and orchards in the right environmental conditions. It can also be spread by infected plant material, people who might inadvertently carry the bacteria on their clothing or shoes, animals and orchard equipment.
How Psa affects kiwifruit
The bacteria prefer cool and wet environments in a temperature range from 10°C to 20°C, with the disease naturally controlled over 25°C. It affects kiwifruit (Actinidia spp.) through openings on the plant, such as stomata or wounds. The most obvious symptoms are brown spots, often with a halo, on the otherwise green leaves, sometimes the edges of the leaves will also curl, and leaf buds can be discoloured brown. The canes, trunk and leaders can develop cankers (welts) that ooze with reddish orange or white sap. On infected plants, scraping the bark away will often reveal brown orange staining.
The disease can cause new shoots to whither and die as the bacteria invades and blocks vascular tissues. In worst case scenarios, the entire plant dies. It has been widely reported in the New Zealand media that the disease does not affect the fruit. However, there were several examples of ‘collapsed fruit’ in the Italian outbreak. The severity of impact differs between species of kiwifruit. Observations made in Italy suggested that damage is more severe on yellow-fleshed kiwifruit (A. chinensis) than on the green-fleshed cultivar (i.e. A. deliciosa cv. ‘Hayward’), which is commonly grown in New Zealand.
Psa poses no danger for people, animals or even other plants. Despite ongoing control measures, it continues to occur in Japan, where it was first identified in the 1980s, and also in Korea and parts of Europe, but had not been spotted here before. Psa has had a significant economic impact on the kiwifruit industry in badly affected areas.
How Psa affects kiwifruit markets
In 1992, Psa was discovered in northern Italy, but it remained sporadic with few cases for 15 years. However, in 2007/2008, conditions were ideal for the spread of the disease, and kiwifruit in the region of Lazio were decimated, including the destruction of a New Zealand-owned gold kiwifruit orchard. The disease is estimated to have cost Italy around 2 million Euros (NZ$3.5 million). This includes loss of trade as well as physical damage. The European and Mediterranean Plant Protection Organisation (EPPO) Secretariat has now added Psa to the EPPO Alert List.
In Japan, control using copper compounds and antibiotics has now led to resistant strains of the disease. Like the EPPO, MAF advocates that orchard hygiene and maintaining healthy plants is paramount in preventing and controlling the disease. This includes good fertilisation, avoidance of overhead irrigation, disinfection of pruning equipment, pruning and destruction of diseased material, regular inspections of orchards for symptoms of the disease, restriction of unnecessary access to the orchard and the use of healthy plant stock.
According to MAF’s annual report on the kiwifruit industry, New Zealand’s kiwifruit exports returned NZ$1.04 billion in the year to 31 March 2010 – the first time the industry has reached the billion dollar return mark. The current fruit crop is due to be harvested in autumn 2011. There are concerns that our trading partners will ban the import of New Zealand kiwifruit, with the US and Australia already having blocked the entry of plant roots (plant stock).
Scientists are working on how to combat Psa.
Plant & Food scientists are aiming to breed kiwifruit cultivars that show resistance to the pathogen Psa.