A team of researchers led by Dr Kerry Everett at Plant and Food Research is using diagnostic markers to quickly detect horticultural diseases in New Zealand, preventing their further spread and enabling New Zealand exporters to maintain market access overseas.
Diagnostic markers are short regions of DNA that are unique to any particular bacterial or fungal pathogen. Laboratory tests on plant samples determine if this unique region of DNA is present, which indicates whether the pathogen is present on the plant.
Kerry’s first work on diagnostic markers for horticultural diseases began in 2002 on citrus black spot (Guignardia citricarpa), which an historical record identified in New Zealand in 1983 based on morphological evidence. However, Kerry re-examined the record and confirmed that in fact the record contained a closely related strain that is not damaging but can only be distinguished from citrus black spot using molecular techniques. This finding confirmed that citrus black spot had actually never been found in New Zealand and helped to open market access for New Zealand citrus in Europe and the United States.
The second disease that Kerry and her team worked on was avocado scab, which is a superficial cosmetic disease of avocados caused by the fungus Sphaceloma perseae. Avocado scab was recorded as present in New Zealand in a 1991 record, again based on morphological evidence, but this was not validated by the New Zealand Phytosanitary Authorities and was therefore not on the avocado pest list. In 2006, Australian authorities found this 1991 record of the disease and immediately placed additional treatment and inspection requirements on the import of New Zealand avocados into Australia, as the disease is not present in Australia.
Kerry’s team was able to quickly re-examine the 1991 record using their DNA technology to show that in fact the culture upon which the record was based was not avocado scab but rather was an unrelated fungus that is common on grasses. This research enabled the scientists to conclude that the disease had not been found in New Zealand, thereby re-establishing the Australian market access for New Zealand avocado growers.
From this work, the avocado industry requested that Kerry develop diagnostic markers that would identify avocado scab. With B3 funding, Kerry developed these primers, which remain the only DNA tests available for avocado scab worldwide. In 2010, these markers became invaluable when a shipment of New Zealand avocados was intercepted in Hawaii with suspected avocado scab. Using the diagnostic markers and visual examination, Kerry was able to show that avocado scab was not present, which enabled the avocados to be released quickly and continue to market.
The third example of how Kerry has used diagnostic markers is her team’s research on Pseudomonas syringae pv. actinidiae (Psa), a disease that made headlines when it was first positively identified in kiwifruit leaves and flowers by Kerry and her team using these primers in a New Zealand commercial orchard in November 2010.
Kerry’s work on Psa dated back to 2004, when she compiled a list of the most potentially harmful pathogens on kiwifruit in order to begin designing primers for them. Psa was at the top on her list. Fortunately, this led Kerry to develop primers for Psa, so that when the disease emerged in 2010, a diagnostic tool was already in place. Kerry says, “The primers gave us six months lead time, because it gave us an instant diagnosis despite observing symptoms that we had not observed before.”
The team was thus able to detect the disease early and before it had spread to other regions. Furthermore, Kerry’s team did not identify Psa in any historical isolates from a collection made during another kiwifruit study by Kerry in 1991. This suggested that Psa was not widespread or may not have been present in New Zealand 19 years prior to the detection in 2010 and thus that it had been introduced more recently.
In all three of these examples, Kerry says, “The diagnostic markers turned out to be invaluable to a rapid original diagnosis, which with conventional technology could have taken weeks or even months.”