Plant Breeding for Disease Resistance

Fungal, bacterial and viral pathogens attack a wide range of cultivated crops, and the damage they cause is especially severe in tropical climates. Crop losses from disease can be significant — typically 20–30%, and occasionally total. Breeding cultivars that resist disease is therefore an important way of enhancing food production, and it also reduces how much a farmer needs to rely on fungicides and bactericides.

Resistance is the host plant's ability to prevent a pathogen from causing disease, and it is determined by the plant's genetic make-up. Before breeding for resistance can begin, breeders need to know the causative organism and how it spreads. Some examples of crop diseases:

• Fungal — rusts such as brown rust of wheat, red rot of sugarcane, late blight of potato
• Bacterial — black rot of crucifers
• Viral — tobacco mosaic, turnip mosaic

Methods of breeding for disease resistance

The conventional method is hybridisation and selection, following essentially the same sequence used for breeding any other agronomic trait: screening the germplasm for sources of resistance, hybridising selected parents, evaluating and selecting the hybrids, and finally testing and releasing new varieties. Several crop varieties developed this way, resistant to fungal, bacterial and viral diseases, are listed below.

Conventional breeding is limited by how many resistance genes are already known and identified within existing crop varieties or their wild relatives. Mutation breeding offers a way around this: mutations — changes in the base sequence of genes — are induced artificially using chemicals or radiation such as gamma rays, and the resulting plant population is screened for individuals that have gained useful resistance. Plants showing the desirable trait can then be multiplied directly or used in further breeding. In mung bean, for instance, resistance to yellow mosaic virus and powdery mildew has been induced through mutation. Other approaches include selecting among somaclonal variants and using genetic engineering. …