First Report of Fire Blight on Indian Hawthorn Cultivar Olivia in Louisiana

Twig and branch death were observed on Indian hawthorn (Raphiolepis indica (L.) Lindl.) cv. Olivia in an experimental planting in May 1998. Symptoms resembled those of fire blight caused by Erwinia amylovora and included the release of large numbers of bacteria from stem sections placed in water drops. Small pieces of wood from surface-disinfected cv. Olivia twigs were crushed in drops of sterile water, dilutions made and streaked on yeast extract-dextrose-CaCO agar, and single colony cultures established. Pathogenicity tests were performed on immature pear (Pyrus communis L. 'Kieffer') and cherry laurel (Prunus caroliniana (Mill.) Aiton) fruits by injecting them with bacterial suspensions containing 10 CFU/ml. Twigs of cv. Olivia were inoculated with a needle that had been dipped in the bacterial cultures. Control inoculations were done with sterile water. Inoculated fruits (nine pear and eight cherry laurel) turned black and oozed bacteria after 5 to 10 days and seven of nine inoculated twigs developed leaf chlorosis followed by browning and death of foliage and twigs after 7 days. Control inoculations were negative. The bacterium was reisolated and identified as E. amylovora based on positive pathogenicity tests and results from Biolog Microplate tests. Although R. umbellata (Thunb.) Makino is a natural host of E. amylovora (1), R. indica was reported only as an experimental host (2) but is now generally recognized as a natural host of E. amylovora. Among nine Indian hawthorn cultivars. (total of 48 plants) in the Louisiana planting, only cv. Olivia was infected with fire blight and all six plants died by late summer. References: (1) E. M. Hildebrand. Phytopathology 44:192, 1954. (2) H. E. Thomas and H. E. Thomas. Phytopathology 21:425, 1931.