Planting strategies to extend pest resistance
While researching the cost-benefits of pre-emptive plant breeding to combat exotic grain pests, Queensland Department of Agriculture, Fisheries and Forestry scientist Dr Mandy Christopher has also found new on-farm strategies being developed in the US can help preserve genetic gains made.
Dr Christopher says the way genetic resistance is deployed can contribute to its longevity. For example, potential options include deployment of single genes or gene pyramids, in some cases in conjunction with chemical controls. This could be part of local or regional strategies that could also include deploying different resistance genes in different paddocks, or in different years, or the use of trap crops or multi-line (a mix of varieties) crops.
The need to maximise the return on investment in developing genetic resistance is a reflection of the high cost of such research.
“It means that large investments are only likely to be made in pre-breeding for resistance to a few of the highest-priority pests or pathogens,” says Dr Christopher, who works with the Plant Biosecurity Cooperative Research Centre.
“The difficultly is to know which pests or pathogens are most likely to occur, and also how much damage will be done. There are some pests that, economically, may not justify the investment in resistance breeding because these pests will have comparatively little impact on crops.” Added to this, she says, is the uncertainty of how long resistance traits will remain effective, as pests and pathogens adapt and new biotypes emerge.
During a recent visit to the US, Dr Christopher visited several plant breeding facilities including those developing Russian wheat aphid (RWA) resistant germplasm for use in Australian-adapted wheats and barleys in the event of an incursion. The trip was to assess the value of resistance investment in wheat and barley lines.
The US experience with RWA is providing a model for the potential costs that could be expected in the case of an Australian incursion. RWA first arrived in the US in 1986, and it took eight years for plant breeders to develop and deploy varieties with resistance to the pest. But within a few years, a second RWA biotype was discovered that had overcome the resistant varieties, sending plant breeders back to the laboratory.
Varieties with new sources of resistance are still in development and chemicals have become a greater part of control efforts. So far, the RWA incursion has cost the US about US$1 billion (A$1.1 billion) in lost production and investment in plant breeding and chemical control.
Dr Christopher says there is already some level of genetic resistance available to counter most of the exotic pests and pathogens identified as high-priority threats. These sources of resistance could be incorporated into locally adapted lines relatively quickly to combat pests including RWA, hessian fly and Sunn pest.
For pests and pathogens where plant resistance is identified as the most appropriate protection strategy, preserving resistance once it has been developed is an ongoing challenge.
In the case of hessian fly, in the US plant resistance has typically been deployed via breeding programs as a sequential series of single genes. However, modelling by researchers at North Carolina State University has suggested that using cultivars with two or more pyramided resistance genes, mixed with some totally susceptible plants (multi-line crops) generally offers the highest relative durability of resistance. The research predicted hessian fly resistance could last for more than 400 fly generations in multi-line crops.
Dr Christopher explains that the advantage of mixing resistant and susceptible plants is that it reduces the selection pressure on the fly population, which slows down the evolution of a fly population able to overcome the plants’ resistance.
Dr Christopher says such a strategy has a number of challenges. Pyramided resistance is more difficult to breed than single gene resistance, but is being helped by the use of DNA marker technologies. It is also more difficult to register a seed mix than a pure variety.
Also, resistant cultivars have to be purchased and planted well before a grower can assess the seasonal conditions and pest populations, so there is no option for varying the seed mix to match the circumstances.
Dr Mandy Christopher, 07 4639 8880, email@example.com
The Exotic Plant Pest Hotline is 1800 084 881.
GRDC Project Code NPB00008
Region North, Overseas