The first occurrence of fire blight on thornless blackberries (Rubus spp.) was reported in Illinois in 1976 (7, 8). Infections appeared either at the cane tip and proceeded basipetally, at axillary buds, causing cane girdling, or on flower/fruit clusters. Diseased portions of the canes were necrotic, purplish-black, and the tips were curved. Lesions were characteristically water-soaked and produced abundant bacterial ooze. Infected fruits did not develop to maturity, became brown, dried, very hard, and remained attached to the fruit pedicel. Frequently entire fruit clusters were infected but generally 1 or 2 fruits in each cluster escaped (Fig. 1). Mummified berries were commonly present on the 4 commercial cultivars (‘Thornfree’, ‘Dirksen Thornless’, ‘Black Satin’, and ‘Smooth-stem’) as well as several unnamed selections of similar genetic background to these cultivars. One seedling selection (SIUS 68-6-15) exhibited about 65% fruit infection. A bacterium was isolated from these fruits and from infected stems. The organism was morphologically and physiologically identical to, but pathogenically different from Erwinia amylovora (Burr.) Winslow et al. isolates from apple and pear.
Freshly harvested seeds of 10 Rubus crosses were germinated in vitro on modified Lepoivre medium without growth regulators. Among several seed preparation treatments, the best germination and subsequent survival in soil was achieved with halved seeds. In vitro germination bypassed the need for cool moist stratification and resulted in 57% to 81% germination within 8 to 12 days. This system provides an alternative method to secure high germination when a breeder is working with limited seed number, as in some interspecific Rubus crosses.