How important is Russian wheat aphid resistance in South African wheat cultivars?

DR ASTRID JANKIELSOHN, ARC-SMALL GRAIN INSTITUTE, AN INSTITUTE OF THE ARC-FIELD CROPS DIVISION, BETHLEHEM

Russian wheat aphid resistant cultivars have been bred throughout the world for a long time and are numerous among commercially available wheat cultivars. When choosing a wheat cultivar, there are many factors to consider. One such factor is whether the cultivar you have chosen has sufficient Russian wheat aphid resistance. The question is how important is this factor to consider.

What is resistance?

In nature, plants are attacked by a variety of different insects, which necessitates some kind of strategy to defend them against these attacks. One of the strategies that plants developed to survive is resistance against insects. Insect biotypes develop as a result of an arms race between insects and plants. When insects feed on plants, there is a selection pressure by the insects on the plants. The plants need to develop some sort of strategy to survive and this result in the development of resistant characters in the plant. Consequently the survival of the insects is reduced and this again results in the development of counter-resistance in the insects. By breeding resistant lines, plant breeders are speeding up the natural process in favour of the plant, but nature will follow its course and when breeders release a resistant crop, counter-resistance will eventually follow.

Why is resistance necessary?

Increased genetic diversity in wheat breeding is desirable for dealing with present and future challenges caused by the need to adapt to changing climate and change in pest dynamics as a result. Of the several constraints towards realising the potential wheat yield, losses incurred due to biotic and abiotic stresses are enormous. Therefore breeding for resistance in wheat takes the centre stage in any breeding programme with the end goal of breeding high yielding varieties.

The advantage of Russian wheat aphid resistant cultivars for producers is a direct economic benefit – a reduced cost of pesticides and fuel due to a reduced need for applications. Deploying genetic resources for resistance also assists in achieving yield stability without resorting to harmful chemicals in addition to preventing environmental degradation and benefiting the resource-poor producers who cannot afford the use of costly chemicals.

These benefits are of greater importance when cereal prices are low, because low prices make the application of pesticides less cost-efficient. While the main benefits of Russian wheat aphid resistant cultivars are for producers, society as a whole has a lot to gain from them as they contribute to a general reduction in pesticide use, which is recognised as a significant and worthwhile public objective because of the environmental and health problems linked to the use of pesticides.

How is durable resistance achieved?

New Russian wheat aphid resistant genes need to be incorporated into the breeding programmes all the time to keep ahead in the race. The most important step in any breeding programme is the availability of genetic diversity for the objectives of the breeding programme. Resistance traits governed by major dominant genes are not stable over time, while those governed by several genes are more durable. The strategy of building up a combination of genes, each of which contributes only a partial degree of resistance, would in the long term offer more effective
management.

Despite the common and lengthy experience in breeding for Russian wheat aphid resistance, the evolution of new biotypes after the cultivar occupies the wider cultivation, makes breeding for resistance an ever continuous process. There are, however, some cultivars that have remained resistant for fairly long periods of time, giving credence to the fact that resistance conferring durability can be achieved. The true test of resistance is how well a particular genotype develops and yields in the presence of aphid in the field, under a range of environmental conditions. Scientists’ efforts over many years resulted in availability of resistance in wheat and became related to wheat sustainability over decades and consequently contributed to food security. For sustainable, long term management of Russian wheat aphid biotypes, planting a cultivar with Russian wheat aphid resistance will therefore be an important factor to consider.

For more information, contact Dr Astrid Jankielsohn at the ARC-Small Grain Institute, Bethlehem, at jankielsohna@arc.agric.za or 058 307 3431.