Nicotinic acid and nicotinamide on pear and apple, analyzed in terms of cultivar and blossom age, are not limiting factors for Erwinia amylovora growth

Fire blight, caused by the enterobacterium Erwinia amylovora, is one of the major diseases of pear (Pyrus communis) and apple (Malus domestica) in many areas of the world. The disease is generally initiated by epiphytic bacterial populations that multiply on blossoms and colonize the hypanthia. E. amylovora requires nicotinic acid and/or nicotinamide as essential growth factors when cultured on laboratory minimal media. Our aim was to evaluate if the nicotinic acid (NiAc) or nicotinamide content in hypanthia is influenced by climate and/or cultivar. On pear hypanthia, the amount of NiAc was positively influenced by the altitude of the growing site and inversely correlated with the sum of the maximum temperatures for the 30 days before the blossom. No correlation was found between the growth of E. amylovora and the amount of nicotinic acid and nicotinamide on the hypanthia of different pear and apple cultivars and at different developmental stages. The sum of nicotinic acid and nicotinamide amounts of hypanthia was about 37 to 150 times higher in pear and about 8 to 23 times higher in apple hypanthia than the nicotinic acid or nicotinamide amounts necessary to support maximal E. amylovora growth in vitro. Nicotinic acid and nicotinamide are indispensable for E. amylovora growth and their amount in hypanthia are cultivar species dependent and related to temperature, however in no case is the amount detected on pear and apple blossoms limiting for the establishment of the pathogen in absence of bacterial competition