Nicotinic acid and nicotinamide on pear and apple flowers are not limiting factors for Erwinia amylovora growth when these chemicals are considered in relation to cultivar and flower age

Fire blight, caused by Erwinia amylovora, is a devastating disease of pear (Pyrus communis) and apple (Malus × domestica) in many areas of the world. The disease is often initiated by epiphytic populations that multiply on flowers and colonize the hypanthia. In vitro, E. amylovora requires nicotinic acid (NicAc) and/or nicotinamide (NicNH2) as essential growth factors. The amount of NicAc on pear hypanthia was positively correlated with the altitude of the growing site and was inversely correlated with the sum of the maximum temperatures in the 30 days before flowering. The sum of the amounts of NicAc and NicNH2 on the hypanthia was about 6 to 23 times higher in pear, and about 1.2 to 3.5 times higher in apple, than the amounts of NicAc or NicNH2 necessary to support maximum E. amylovora growth in vitro. No correlation was found between the amounts of NicAc and NicNH2 on the hypanthia of different pear and apple cultivars and at different growth stages and the growth of E. amylovora after experimental inoculation. In conclusion, NicAc and NicNH2 are essential for E. amylovora growth but the amounts of these chemicals on pear and apple flowers do not limit the establishment of the pathogen when competing bacteria are lacking