AUSVEG has been made aware of confirmation of a new biotype of Nasonovia ribisnigri (currant lettuce aphid) in Victoria that is able to break resistance on Nasonovia-resistant lettuce varieties.

Nasonovia ribisnigri is primarily a pest of lettuce, chicory, endive and radicchio where aphids colonise lettuce hearts and rosettes. Until now, only the Nasonovia Nr:0 biotype was found in Australia, for which the Nr:0 resistance gene provides protection in a large range of lettuce varieties from a number of breeding companies. Recently a new biotype, named Nr:1, was discovered in Australia. It is able to break the Nr:0 resistance gene and therefore feed and multiply on Nr:0 resistant lettuce varieties.

The ability of aphids to evolve new biotypes is well documented. As aphids can be prolific, Nasonovia populations can often reach high levels with a lifecycle that is short, therefore increasing the probability for natural mutation to occur and result in the evolution of a new biotype. Growers will be familiar with lettuce downy mildew as an example of resistance-breaking biotypes developing independently in locations across the globe.

For more information on Nasonovia ribisnigri, including host range and management options, please click here for an information sheet produced by Rijk Zwaan.

Background

Most lettuce varieties grown in Australia have a single dominant gene that confers resistance to the currant-lettuce aphid Nasonovia ribisnigri biotype Nr:0 (based on the Nr:0 gene). This species of aphid has been present in Australia since 2004 and is now considered to be widespread.

Tests conducted in the Netherlands on live specimens of the aphid collected in lettuce (Lactuca spp.) crops in Victoria have confirmed the presence of a new biotype of this aphid (designated Nr:1) that is able to overcome resistance to Nr:0. As a result, growers will be advised that they can no longer rely on Nr:0 resistance to manage this aphid.

When was the new biotype confirmed and where has it been found?

Rijk Zwaan sampled aphids from lettuce crops in Victoria in early spring 2017 for identification and testing in response to reports from crop consultants and growers who suspected that lettuce crops with Nr:0 resistance were no longer resistant to aphids.

The presence of the resistance breaking biotype was tested in the Netherlands using a bioassay that requires live aphids.

The presence of the new resistant biotype was confirmed in December 2017. The finding was then reported to Agriculture Victoria.

Additional work

Work is also being carried out in an Australian lab by cesar, initiated by Stuart Grigg Ag-Hort Consulting and RMCG, to identify the biotypes of several other Nasonovia ribisnigri populations collected at different locations in Victoria. Results can be expected in the coming weeks and will be communicated to growers.

How can I differentiate this biotype from other biotypes/other pests?

The only difference between the two Nasonovia biotypes is their ability to either reproduce or not reproduce on varieties with genetic resistance. Otherwise they are indistinguishable.

To perform this test, viable aphids are required, and a rigorous bioassay under controlled conditions is needed to distinguish the biotypes.

For more information please contact your local agronomist or sales rep.

Growers who suspect that they may have currant-lettuce aphid in their crop can contact Dr Siobhan de Little from cesar (details below) to discuss options for identification of the biotype on their properties. Instructions on proper aphid collection can be found here.

Dr Siobhan de Little
Senior Consultant – Sustainable Agriculture
cesar
Phone: +61 3 9349 4723
Email: sdelittle@cesaraustralia.com

Where can I find more information?

For more information on chemical control of Nasonovia ribisnigri please speak with your local agronomist or sales rep. You can also speak to Dr Siobhan de Little from cesar on 03 9349 4723.

More information can also be found on the Rijk Zwaan information sheet or on Plant Health Australia’s website at http://www.planthealthaustralia.com.au/wp-content/uploads/2013/01/Currant-lettuce-aphid-FS.pdf

This post appeared in the AUSVEG Weekly Update published 14 February 2018.