Improvement of fire blight resistance in apple and pear

Fire blight caused by the bacterium Erwinia amylovora is known to incite substantial damage in pomefruit production. The disease originated in North America, from where it slowly spread around the world. Hosts of E. amylovora are members of the Maloidae, such as apple, pear and quince, and ornamentals, such as Cotoneaster, Crataegus and Mespilus. The disease can affect all tree parts such as blossoms, shoot tips and rootstock crowns. The name of the disease resembles the main symptom in host plants, i.e. the black necrosis of shoots and browning of leaves. Antibiotics provide effective control, but these are increasingly banned due to ecological considerations and the emergence of antibiotic resistant strains of E. amylovora. Alternative control strategies are based on copper ions and the application of antagonists. The use of fire blight resistant cultivars is another approach to prevent fire blight epidemics. Fire blight resistance is present in the cultivated apple, but high levels of resistance generally are found in wild species of apple and pear. The introduction of wild species’ traits into cultivars is a slow process in fruit tree breeding. Understanding the genetics of the disease resistance, biotechnology and genetic engineering can promote and accelerate classical breeding. This review focuses on crop improvement for fire blight resistance. We describe the origin of the disease, its spread and the infection tools of the pathogen, summarize the genetic resources available to breeders and strategies to improve apple and pear for fire blight resistance.